Introduction

The skin is the largest organ in the body, coveringits entire external surface.The skinhas3 layersthe epidermis, dermis, and hypodermis,which have different anatomical structures and functions (seeImage.Cross Section, Layers of the Skin). The skin's structure comprises an intricate network that serves as the body's initial barrier against pathogens, ultraviolet (UV) light, chemicals, and mechanical injury.This organ also regulates temperature and the amount of water released into the environment.

Skin thicknessvariesby body region and isinfluenced by the thickness of the epidermal and dermal layers. Hairless skin in the palms of the hands and soles of the feet is the thickest due to the presence ofthe stratum lucidum, an extra layer in the epidermis.Regions lacking this extra layer are considered thin skin. Of these regions, the back has the thickest skin because it has a thick epidermis.[1][2][3]The skin's barrier function makes it susceptible to various inflammatory and infectious conditions. In addition, wound healing, sensory changes, and cosmesis are significant surgical concerns. Understanding the skin's anatomy and function is crucial for managing conditions across all medical fields.

Epidermis

The epidermis, the skin's outermost layer, is composed ofseveral strata and various cell types crucial for its function.

Layers of the epidermis:From the deepest to the most superficial, the epidermal layers are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale,also known asstratum germinativum, is separated from the dermis by the basement membrane (basal lamina) and attached to it by hemidesmosomes. The cells in this layer are cuboidal to columnar, mitotically active stem cells that constantly produce keratinocytes. This layer also contains melanocytes. The stratum spinosum, comprising 8to 10 cell layers, isalsocalled the prickle cell layer. This layer contains irregular, polyhedral cells with cytoplasmic processes, sometimes called spines,that extend outward and contact neighboring cells by desmosomes. Dendritic cells can be found in this layer.[4][5]

The stratum granulosum has 3to 5 cell layers and containsdiamond-shaped cells with keratohyalin and lamellar granules. Keratohyalin granules contain keratin precursors that aggregate, cross-link, and form bundles. The lamellar granules contain the glycolipids secreted to the cell surfaces, functioning as anadhesiveto maintain cellular cohesion. The stratum lucidumcomprises 2 to 3 cell layers and ispresent in thicker skin on the palms and soles. This thin and clear layer consists of eleidin, a transformation product of keratohyalin. The stratum corneum has 20to 30 cell layers and occupies the uppermost epidermal layer. The stratum corneumis composed of keratin and dead keratinocytes (anucleate squamous cells) that form horny scales. This layer has the most variable thickness, especially in callused skin. Dead keratinocytesrelease defensins within this layer, which are part of our first line of immune defense mechanisms.[6][7]

Cells of the epidermis:The epidermal cells include keratinocytes, melanocytes, and Langerhans and Merkel cells(seeImage.Cells of the Epidermis). Keratinocytes are the predominant cells of the epidermis,originating from the basal layer. These cells produce keratin and lipids essentialforformingthe epidermal water barrier. Keratinocytes also contribute to calcium regulation by enablingUVB light absorption in the skin,which iscriticalfor vitamin D activation. Melanocytes derive from neural crest cells and primarilysynthesize melanin, the main skin pigment component. These cells are found between stratum basale cells. UVB light stimulates melanin secretion, protecting against further UV radiation exposure and acting as a built-in sunscreen. Melaninforms during the conversion of tyrosine to dihydroxyphenylalanine by the enzyme tyrosinase. Melanin then travels from cell to cell, relying on the long processesconnecting the melanocytes to the neighboring epidermal cells. Melanin granules from melanocytestransit through the lengthy processes to the cytoplasm of basal keratinocytes. This transfer occurs through cytocrine secretion, where keratinocytes phagocytose the tips of melanocyte processes.

Langerhanscellsare dendritic cells that act as the skin's first-line cellular immune defenders andare crucialfor antigen presentation. Special stains allow visualization of these cells in the stratum spinosum. Langerhans cells are of mesenchymal origin, derived from CD34-positive bone marrow stem cells, and are part of the mononuclear phagocytic system.These cells contain Birbeck granules and tennis racket-shaped cytoplasmic organelles. Langerhans cells express major histocompatibility complex (MHC) I and MHC II molecules, uptake antigens in the skin, and transport them to the lymph nodes. Merkel cells are oval-shaped modified epidermal cells found in the stratum basale, directly above the basement membrane. These cells serve as mechanoreceptors for light touch and are found in the palms, soles, and oral and genital mucosa, with the highest concentration in the fingertips. Merkel cells bind toadjoining keratinocytes through desmosomes and contain intermediate keratin filaments. The cell membranes of Merkel cells interact with free nerve endings in the skin.

Dermis

The dermis is connected to the epidermisby the basement membrane.The dermis consists of 2 connective tissue layers, papillary and reticular, which merge without clear demarcation. Thepapillary layeris the upper dermal layer,which isthinner and composed of loose connective tissue that contacts the epidermis. Thereticular layeris the deeper layer, which is thicker and less cellular. This layer consists of dense connective tissuecomposedof collagen fiber bundles. The dermis houses the sweat glands, hair, hair follicles, muscles, sensory neurons, and blood vessels.

Hypodermis

The hypodermis, also known as the subcutaneous fascia, is located beneath the dermis.This layer is the deepest skin layer and contains adipose lobules,sensory neurons, blood vessels, and scanty skin appendages, such as hair follicles.

Functions

The skin's comprehensive roles highlight its complexity and importance in maintaining overall health and well-being.These roles are discussed below.[8][9]

Barrier function:The skin has multiple protective roles, acting as a barrier against various external threats. The skinshieldsthe body fromexcessive water loss or absorption, invasion by microorganisms, mechanical and chemical trauma, and UV light damage. The cell envelope establishes the epidermal water barrier, a layer of insoluble proteins on the inner surface of the plasma membrane. This barrier is formed through thecross-linkingof small proline-rich proteins. Larger proteins such as cystatin, desmoplakin, and filaggrincontribute to the barrier's robust mechanics. The lipid envelope is a hydrophobic layer attached to the outer surface of the plasma membrane. Keratinocytes in the stratum spinosum produce keratohyalin granules and lamellar bodies containing a mixture of glycosphingolipids, phospholipids, and ceramides assembled within Golgi bodies. The contents of lamellar bodies are then secreted through exocytosis into the extracellular spaces between the stratum granulosum and corneum.

Immunological defense:The skin plays a crucial role in both adaptive and innate immunity. In adaptive immunity, antigen-presenting cells initiate T-cell responses, leading to increased levels of helper T cells, such as TH1, TH2, or TH17. In innate immunity, the skin produces various peptides with antibacterial and antifungal properties. The skin-associated lymphoid tissue is a significant component of the immune system,aiding in preventing infections, as even minor skin breaks can lead to infection. Langerhans cellsare part of the adaptive immune system, presenting foreign antigens encountered in the skin to T cells.

Regulation of homeostasis:The skin plays a vital role in maintaining body temperature and water balance. This organregulates heat exchange with the environment, particularly through the blood vessels and sweat glands. The skin managesthe rate and amount of water evaporation and absorption.

Endocrine and exocrine functions:Keratinocytes produce vitamin D by converting 7-dehydrocholesterol under UV light exposure. These cells also express the vitamin D receptor and contain enzymes that activate vitamin D, essential for the proliferation and differentiation of keratinocytes. The skin's exocrine functions include temperature control by perspiration and skin protection by sebum production. Sweat and sebaceous glands are crucial to these functions.

Sensory functions:The skin is equipped with nociceptors that allow for the sensation of touch, heat, cold, and pain, facilitating interaction with the environment. The skin's sensory roles are essential for an individual's movement, protection, andinteraction with the environment.

Diagnostic indicator:Skin characteristics such aspigmentation, smoothness, elasticity, and turgor provide insights into an individual's overall health status. Skin assessment is often a crucialpartof a person's physical examination.[10][11]

Cell division, desquamation, and sheddingin the skin:Cell division occurs in the stratum basale. Basal cells (young keratinocytes)begin the synthesis of keratinous tonofilaments, whichare grouped into bundles called tonofibrils. Older keratinocytes are then pushed into the stratum spinosum after mitosis.Skincellsbegin toproduce keratohyalin granules with intermediate-associated proteins, filaggrin, and trichohyalin in the upper part of the spinous layer. Thisprocess helps aggregate keratin filaments and convert granular cells into cornified cells, known as keratinization. Cells also produce lamellar bodies during this stage.

Keratinocytescontinue to move into the stratum granulosum afterward, where they become flattened and diamond-shaped. The cells accumulate keratohyalin granules mixed between tonofibrils.Keratinocytesthen continue to the stratum corneum, flattening and losing organelles and nuclei. The keratohyalin granules turn tonofibrils into a homogenous keratin matrix.Cornified cells reach the surface and are desquamated when desmosomes disintegrate. The proteinase activity of kallikrein-related serine peptidase is triggered by lowered pH near the surface. The processes ofskinshedding and desquamation vary slightly by body region.Hairless skincomprisesmore layers, withtheadditionof thestratum lucidum. Thus, keratinocytes in body regions with hairless skin go through more layers before reaching the surface.[12][13]

The epidermisis derived from ectodermal tissue. The dermis and hypodermisare derived from mesodermal tissue from somites. The mesoderm is also responsible for the formation of Langerhans cells. Neural crest cells, responsible for specialized sensory nerve endings and melanocyte formation, migrate into the epidermis during epidermal development.[14][15]

Blood vessels and lymphatic vessels are found in the skin's dermal layer.Blood supply to the skin comprises 2 plexusesonebetween the papillary and reticular dermal layers and another between the dermis and subcutaneous tissues.Blood supply to the epidermis is through the superficial arteriovenous plexus (subepidermal/papillary plexus). These vessels are important for temperature regulation. The body regulates temperature by increasing blood flowto the skin, transferring heatfrom the bodyto the environment.The autonomic nervous system controls the changes in blood flow.Sympathetic stimulation results in vasoconstriction, resulting in heat retention.Conversely, vasodilationleads to heat loss. Vasodilation is the body's response to increased body temperature,resulting from inhibiting the sympathetic centers in the posterior hypothalamus. In contrast, decreased body temperature causes vasoconstriction.[16][17]

Nerves of the skin include both somatic and autonomic nerves. The somatic sensory systemtransmitspain (nociception), temperature, light touch, discriminative touch, vibration, pressure, and proprioception sensations to the central nervous system. Specialized cutaneous receptors and end organs mediate perception, including Merkel disks and Pacinian, Meissner, and Ruffini corpuscles. Autonomic innervation controls vasculature tone, hair root pilomotor stimulation, and sweating. The free nerve endings extend into the epidermis and are responsible for sensing pain, heat, and cold. These sensory structures are most numerous in the stratum granulosum layer andaround most hair follicles. Merkel disks sense light touch andreaches the stratum basale layer. The other nerve endings are found in the deeper portions of the skin and include the Pacinian, Meissner, and Ruffini corpuscles. The Paciniancorpusclessense deep pressure. The Meissner corpuscles sense low-frequency stimulation at the level of the dermal papillae.TheRuffini corpuscles sense pressure.[18][19][20]

The arrector pili muscles are bundles of smooth muscle fibers attached to the connective tissue sheath of hair follicles. Contraction of these muscles pulls the hair follicle outward, erecting the hair. The arrector pili also compress the sebaceous glands, facilitating sebum secretion. Hair does not exit perpendicularly but at an angle. The erection of hair, known as piloerection, produces goosebumps, giving the skin a bumpy appearance when exposed to cold temperatures.[21]Studies show that piloerection contributes to thermoregulation and stem cell growth.[22]

Langer lines, also known as cleavage lines, are topological lines used to defineskin tension.Theselines correspond to the alignment of collagen and elastic fibers in the reticular dermis. Less scarring occurs ifsurgical incisions are made along these lines.[23]

The skin's clinical significance spans all medical disciplines. A few are discussed below.

Dermatomes

Dermatomes areskin segments divided based on afferent nerve distribution, numbered according to spinal vertebral levels. Spinal nerves comprise8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygealnerve. Diseasessuch as shingles caused by varicella-zoster infection manifest pain and rashes in dermatomal patterns. Dermatomes also aid inlocalizing spinal injuries.

Squamous Cell Carcinoma

Squamous cell carcinoma is amalignancy arising from mutated keratinocytes,typically due to UV damage in individuals with type I or II skin types. These individuals typically have light skin, blue or green eyes, and red or blonde hair and burn without tanning.The lesions oftenappearasscaly, flaky, thick red patches that may bleed.Somesquamous cell carcinoma tumors resemble warts.This type of skin cancer can metastasize. Squamous cell carcinoma often arises from actinic keratosespremalignant lesions with cutaneous hornsdeveloping fromchronicUV damage.[24]

Basal Cell Carcinoma

Basal cell carcinoma is a malignant neoplasm of the basal layers of the epidermis. Unlikesquamous cell carcinoma, itis much less likely to metastasize.This type of skin cancer is more common in sun-exposed areas, often appearing as pearly papules on the face, with telangiectasias and a great tendency to ulcerate.

Melanoma

Melanoma is a highly invasive malignant melanocyte tumor that is fatal but rarer than skinsquamous cell carcinoma and basal cell carcinoma. This neoplasm's high metastatic potential is significantly mediated bylesiondepth.Melanoma can be found anywhere on the body and is typically irregularly pigmented but can be amelanotic.[25]

Langerhans Cell Histiocytosis

Langerhans cell histiocytosis is a type of cancer in which Langerhans cells accumulate in the body andformgranulomas, often in the bones, causing bone pain.These granulomas can also appear in the skin, producing rashes, erythematous papules, or blisters (seeImage.Histology, Trichodysplasia Spinulosa). Notably, Langerhans cellhistiocytosis can affect the pituitary gland, leading to diabetes insipidus, infertility, or other endocrine disorders due to hormone deficiencies. Pancytopenia is apotentially fatal Langerhans cellhistiocytosis complication, manifesting with anemia, thrombocytopenia, and leukocytopenia, caused by overcrowding of Langerhans cells in the bone marrow.[26]

Merkel Cell Carcinoma

Merkel cell carcinoma is an uncommon cancer of the Merkel cells. This tumor is categorized as a neuroendocrine small cell carcinoma. Clinically,Merkelcell carcinoma often presents as a painless, solitary cutaneous or subcutaneous nodule, sometimes with a cystic appearance. The nodule can be red, pink, violet, blue, or skin-colored. Lesions may ulcerate or have satellite lesions.Merkel cell carcinoma is typically smaller than 20 mm at diagnosis but shows rapid tumor growth over a few months.[27]

Pemphigus Vulgaris

Pemphigus vulgaris is an autoimmune disease that targets the desmosomes, the intercellular proteins connecting keratinocytes. Desmosome degradation results in acantholysis and the formation ofeasily ruptured blisters within the epidermis. The disease is characterized by a positive Nikolsky sign, where the epidermis peels away upon rubbing.

Bullous Pemphigoid

Bullous pemphigoid is a blistering disease that affects older adults, causing tense subepidermal blisters.The conditionis causedby antibodies targeting hemidesmosomes, which connect the epidermis to the dermis at the basement membrane. This condition is not acantholytic and does not show a positive Nikolsky sign.[28]

ScaldedSkinSyndrome

Scalded skin syndromearises fromthe effects of the exfoliative toxin released byStaphylococcal aureus. The condition manifests as generalized skin exfoliationwith apositiveNikolsky sign, a severely burned (intensely red) appearance, and fever.[29][30]

Drug Reactions

Various drug reactions manifest in the skin, including erythema multiforme and the syndromes of drug reaction with eosinophilia and systemic symptoms, Stevens-Johnson, and toxic epidermal necrolysis. These conditions are often associated with certain medications, including sulfa-containing drugs,nonsteroidal anti-inflammatory drugs, and antiepileptics.[31][32]

The epidermis contains much of our normal flora,with the microbiome varying by body region. The microorganisms inhabiting our skin surfaces are nonpathogenic and can be commensal or mutualistic. The bacteria that tend to predominate are Staphylococcusepidermidis andS aureus, Cutibacterium acnes, Corynebacterium, Streptococcus, Candida, and Clostridium perfringens. However, infections may occur when the protective skin barrier is altered or breached.[33]

Histology, Trichodysplasia Spinulosa. The left column shows hematoxylin and eosin staining of healthy skin (A1) and trichodysplasia spinulosa lesional skin (B1) at low power. At high power, healthy (A2) and trichodysplasia spinulosa (B2) epidermis and (more...)

Cross Section, Layers of the Skin. This is a cross-section view of the hair follicles, hair roots and shafts, sweat glands, pores, epidermis, dermis, and hypodermis. The papillary and reticular layers are also included. The eccrine sweat gland is located (more...)

Cells of the Epidermis. The image shows stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, stratum basale, and dermis. Contributed by C Rowe

Disclosure: Hani Yousef declares no relevant financial relationships with ineligible companies.

Disclosure: Mandy Alhajj declares no relevant financial relationships with ineligible companies.

Disclosure: Adegbenro Fakoya declares no relevant financial relationships with ineligible companies.

Disclosure: Sandeep Sharma declares no relevant financial relationships with ineligible companies.

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Anatomy, Skin (Integument), Epidermis - StatPearls - NCBI Bookshelf

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The weather is getting cooler and many of us are turning to hot showers and baths to warm up and wind down.

But what actually happens to your skin when you have really hot showers or baths?

Your skin is your largest organ, and has two distinct parts: the epidermis on the outside, and the dermis on the inside.

The epidermis is made up of billions of cells that lay in four layers in thin skin (such as on your eyelids) and five layers in thick skin (such as the on sole of your foot).

The cells (keratinocytes) in the deeper layers are held together by tight junctions. These cellular bridges make waterproof joins between neighbouring cells.

The cells on the outside of the epidermis have lost these cellular bridges and slough off at a rate of about 1,000 cells per one centimetre squared of skin per hour. For an average adult, thats 17 million cells per hour, every day.

Under the epidermis is the dermis, where we have blood vessels, nerves, hair follicles, pain receptors, pressure receptors and sweat glands.

Together, the epidermis and dermis (the skin):

So, your skin is important and worth looking after.

Washing daily can help prevent disease, and really hot baths often feel lovely and can help you relax. That said, there are some potential downsides.

Normally we have lots of healthy organisms called Staphyloccocus epidermis on the skin. These help increase the integrity of our skin layers (they make the bonds between cells stronger) and stimulate production of anti-microbial proteins.

These little critters like an acidic environment, such as the skins normal pH of between 4-6.

If the skin pH increases to around 7 (neutral), Staphyloccocus epidermis nasty cousin Staphyloccocus aureus also known as golden staph will try to take over and cause infections.

Having a hot shower or bath can increase your skins pH, which may ultimately benefit golden staph.

Being immersed in really hot water also pulls a lot of moisture from your dermis, and makes you lose water via sweat.

This makes your skin drier, and causes your kidneys to excrete more water, making more urine.

Staying in a hot bath for a long time can reduce your blood pressure, but increase your heart rate. People with low blood pressure or heart problems should speak to their doctor before having a long hot shower or bath.

Heat from the shower or bath can activate the release of cytokines (inflammatory molecules), histamines (which are involved in allergic reactions), and increase the number of sensory nerves. All of this can lead to itchiness after a very hot shower or bath.

Some people can get hives (itchy raised bumps that look red on lighter skin and brown or purple on darker skin) after hot showers or baths, which is a form of chronic inducible urticaria. Its fairly rare and is usually managed with antihistamines.

People with sensitive skin or chronic skin conditions such as urticaria, dermatitis, eczema, rosacea, psoriasis or acne should avoid really hot showers or baths. They dry out the skin and leave these people more prone to flare ups.

The skin on your hands or feet is least sensitive to hot and cold, so always use your wrist, not your hands, to test water temperature if youre bathing a child, older person, or a disabled person.

The skin on your buttocks is the most sensitive to hot and cold. This is why sometimes you think the bath is OK when you first step in, but once you sit down it burns your bum.

You might have heard women like hotter water temperature than men but thats not really supported by the research evidence. However, across your own body you have highly variable areas of thermal sensitivity, and everyone is highly variable, regardless of sex.

Moisturising after a hot bath or shower can help, but check if your moisturiser is up to the task.

To improve the skin barrier, your moisturiser needs to contain a mix of:

Not all moisturisers are actually good at reducing the moisture loss from your skin. You still might experience dryness and itchiness as your skin recovers if youve been having a lot of really hot showers and baths.

If youre itching after a hot shower or bath, try taking cooler, shorter showers and avoid reusing sponges, loofahs, or washcloths (which may harbour bacteria).

You can also try patting your skin dry, instead of rubbing it with a towel. Applying a hypoallergenic moisturising cream, like sorbolene, to damp skin can also help.

If your symptoms dont improve, see your doctor.

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Bone Basics and Bone Anatomy

Have you ever seen fossil remains of dinosaur and ancient human bones in textbooks, television, or in person at a museum? It's easy to look at these and think of bones as dry, dead sticks in your body, but this couldn't be further from the truth. Bones are made of active, living cells that are busy growing, repairing themselves, and communicating with other parts of the body. Lets take a closer look at what your bones do and how they do it.

The skeleton of an adult human is made up of 206 bones of many different shapes and sizes. Added together, your bonesmake up about 15% of your body weight. Newborn babies are actually born with many more bonesthan this (around 300),but many bones grow together, orfuse, as babiesbecome older. Some bones are long and thick, like your thigh bones. Others are thin, flat, and wide, like your shoulder blades.

Support: Like a house is built around a supportive frame,a strong skeleton is required to support the rest of the human body. Without bones, it would be difficult for your body to keep its shape andto stand upright.

Protection: Bones form astrong layer around some of the organs in your body, helping tokeep them safe when you fall down or get hurt. Your rib cage, for example, acts like a shield around your chest to protect important organs inside such as your lungs and heart. Your brain is another organ that needs a lot of protection. The thick bone layer of your skull protects your brain. For this purpose, being "thick-headed" is a very good thing.

Movement: Many of your bones fit togetherlike the pieces of a puzzle. Eachbone has a very specific shape which often matches up with neighboring bones. The place where two bones meet to allow your body to bend is called a joint.

How many different ways can you move your joints? Some bones, like your elbow, fit together like a hingethat lets you bend your arm in one specific direction. Other bones fit together like a ball and socket, such as the joint between your shoulder and arm. This type of jointlets you rotate your shoulder in many directions, or swing it all the way around in a circle like softball pitchersdo.

The movement of our bodies is possible because of both joints and muscles. Muscles often attach to two different bones, so that when the muscle flexes and shortens, thebones move. This allows youto bend your elbows and knees, or pick up objects. A skeleton has plenty of joints, but without muscles, there is nothing to pull the bones in different directions. More than half of the bones in your body are actually located in your hands and feet. These bones are attached to many little muscles that give you very exact control over how you move your fingers and feet.

Blood Cell Formation: Did you know that most of the red and white blood cells in your body were created inside of your bones? This is done by a special group of cells called stem cells that are found mostly in the bone marrow, which is the innermost layerof your bones.

Storage: Bones are like a warehousethat storesfat and many important minerals so they are available when your body needs them. These minerals are continuously being recycled through your bones--deposited and then taken out and moved through the bloodstream to get to other parts of your body where they are needed.

Now that you know what bones do, let's take a look at what they're made of and their anatomy.

Each bone in your body is made up of three main types of bone material: compact bone, spongy bone, and bone marrow.

Compact Bone

Compact bone is the heaviest, hardest type of bone. It needs to be very strong as it supports your body and muscles as you walk, run, and move throughout the day. About 80% of the bone in your body is compact. It makes up the outer layer of the bone and also helps protect the more fragile layers inside.

If you were to look at a piece of compact bone without the help of a microscope, it would seem to be completely solid all the way through. If you looked at it through a microscope, however, you would see that it's actually filled with many very tiny passages,or canals,for nerves and blood vessels. Compact bone is made of special cells called osteocytes. These cells arelined up inrings around the canals. Together, a canal and the osteocytes that surround it are called osteons. Osteons are like thick tubes all going the same direction inside the bone, similar to a bundle of straws with blood vessels, veins, and nerves in the center.

Spongy Bone

Spongy bone is found mostly at the ends of bones and joints. About 20% of the bone in your body is spongy. Unlike compact bone that is mostly solid, spongy bone is full of open sections called pores. If you were to look at it in under a microscope, it would look a lot like your kitchen sponge. Pores are filled with marrow, nerves, and blood vessels that carry cells and nutrients in and out of the bone.Though spongy bone may remind you of a kitchen sponge,this bone is quite solid and hard, and is not squishy at all.

Bone Marrow

The inside of your bones are filled with a soft tissue called marrow. There are two types of bone marrow: red and yellow. Red bone marrow is where all new red blood cells, white blood cells, and platelets aremade. Platelets are small pieces of cells that help you stop bleeding when you get acut.Red bone marrow isfound in the center of flat bones such as your shoulder blades and ribs. Yellow marrow is made mostly of fat and is found in the hollow centers of long bones, such as the thigh bones. It does not make blood cells or platelets. Both yellow and red bone marrow have many small and large blood vessels and veins running through them to let nutrients and waste in and out of the bone.

When you were born, all of the marrow in your body was red marrow, whichmade lots and lots of blood cells and plateletsto helpyour body grow bigger. As you got older, more and more of the red marrow was replaced with yellow marrow. The bone marrow of full grown adults is about half red and half yellow.

The Inside Story

Bones are made of four main kinds of cells: osteoclasts, osteoblasts, osteocytes, and lining cells. Notice that three of these cell type names start with 'osteo.' This is the Greek word for bone. When you see 'osteo' as part of a word, it lets you know that the word has something to do with bones.

Osteoblasts are responsible for making new bone as your body grows. They also rebuild existing bones when they are broken. The second part of the word,'blast,' comes froma Greek word that means 'growth.' To make new bone, many osteoblasts come together in one spot then begin making a flexible material called osteoid. Minerals are then added to osteoid, making it strong and hard. When osteoblasts are finished making bone, they become either lining cells or osteocytes.

Osteocytes are star shaped bone cells most commonly found in compact bone. They areactually old osteoblasts that have stopped making new bone. As osteoblasts build bone, they pile it up around themselves, then get stuck in the center. At this point, they are called osteocytes.Osteocytes have long, branching arms that connect them to neighboring osteocytes. This lets them exchange minerals and communicate with other cells in the area.

Lining cells are very flat bone cells. These cover the outside surface of all bones and are also formed from osteoblasts that have finished creating bone material. These cells play an important role in controlling the movement of molecules in and out of the bone.

Osteoclasts break down and reabsorb existing bone. The second part of the word, 'clast,' comes from the Greek word for 'break,' meaning these cells break down bone material. Osteoclasts are very big and often contain more than one nucleus, which happens when two or more cells get fused together. These cells work as a team with osteoblasts to reshape bones. This might happen for a number of reasons:

It's not completely understood how bone cells in your body are able to work together and stay organized, but pressure and stress on the bone might have something to do with it.

The smallest bone in the human body is called the stirrup bone, located deep inside the ear. It's only about 3 millimeterslong in an adult.

The longest bone in the human is called the femur, or thigh bone. It's the bone in your leg that goes from your hip to your knee. In an average adult, it's about 20 inches long.

References:

Marieb. E.N. (1989) Human Anatomy and Physiology, CA: Benjamin/Cummings Publishing Company, Inc

Heller, H.C., Orians, G.H., Purves, W.K., Sadava, D. (2003) Life: The Science of Biology, 7th Edition. Sunderland, MA: Sinauer Associates, Inc. & W. H. Freeman and Company

Skeleton Image: By Lady of Hats - Mariana Ruiz Villarreal, via Wikimedia Commons.

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Bone Anatomy | Ask A Biologist

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Bones make up the skeletal system of the human body. The adult human has two hundred and six bones. There are several types of bones that are grouped together due to their general features, such as shape, placement and additional properties. They are usually classified into five types of bones that include the flat, long, short, irregular, and sesamoid bones.

The human bones have a number of important functions in the body. Most importantly, they are responsible for somatic rigidity,structural outline, erect posture and movement (e.g. bipedal gait). Due to their rigidity, bones are the main 'protectors' of the internal organs and other structures found in the body.

This article will describe all theanatomical and important histological facts about the bones.

A bone is a somatic structure that is composed of calcified connective tissue. Ground substance and collagen fibers create a matrix that contains osteocytes. These cells are the most common cell found in mature bone and responsible for maintaining bone growth and density. Within the bone matrix both calcium and phosphate are abundantly stored, strengthening and densifying the structure.

Each bone is connected with one or more bones and are united via a joint (only exception: hyoid bone). With the attached tendons and musculature, the skeleton acts as a lever that drives the force of movement. The inner core of bones (medulla) contains either red bone marrow (primary site of hematopoiesis) or is filled with yellow bone marrow filled with adipose tissue.

The main outcomes of bone development (e.g. skull bones development)are endochondral and membranous forms. This particular characteristic along with the general shape of the bone are used to classify the skeletal system. The bones are mainly classified into five types that include:

These bones develop via endochondral ossification, a process in which the hyaline cartilage plate is slowly replaced. A shaft, or diaphysis, connects the two ends known as the epiphyses (plural for epiphysis). The marrow cavity is enclosed by the diaphysis which is thick, compact bone. The epiphysis is mainly spongy bone and is covered by a thin layer of compact bone; the articular ends participate in the joints.

The metaphysis is situated on the border of the diaphysis and the epiphysis at the neck of the bone and is the place of growth during development.

Some examples of this type of bones include:

The short bones are usually as long as they are wide. They are usually found in the carpus of the hand and tarsus of the foot.

In the short bones, a thin external layer of compact bone covers vast spongy bone and marrow, making a shape that is more or less cuboid.

The main function of the short bones is to provide stability and some degree of movement.

Some examples of these bones are:

In flat bones, the two layers of compact bone cover both spongy bone and bone marrow space. They grow by replacing connective tissue. Fibrocartilage covers their articular surfaces. This group includes the following bones:

The prime function of flat bones is to protect internal organs such as the brain, heart, and pelvic organs. Also, due to their flat shape, these bones provide large areas for muscle attachments.

Due to their variable and irregular shape and structure, the irregular bones do not fit into any other category. In irregular bones, the thin layer of compact bone covers a mass of mostly spongy bone.

The complex shape of these bones help them to protect internal structures. For example, the irregular pelvic bones protect the contents of the pelvis.

Some examples of these types of bones include:

Sesamoid bones are embedded within tendons. These bones are usually small and oval-shaped.

The sesamoid bones are found at the end of long bones in the upper and lower limbs, where the tendons cross.

Some examples of the sesamoid bones are the patella bone in the kneeor the pisiform bone of the carpus.

The main function of the sesamoid bone is to protect the tendons from excess stress and wear byreducing friction.

Learn the basics of the skeletal system with this interactive quiz.

The bones mainly provide structural stability to the human body. Due to the development of the complex bony structures(e.g. spine) the humans are able to maintain erect posture, to walk on two feet (bipedal gait)and for all sorts of other activities not seen in animals.

Due to their rigid structure, bones are key in the protection of internal organs and other internal structures. Some bones protect other structures by reducing stress and friction (e.g. sesamoid bones) while some bones join together to form more complex structures to surround vital organs and protect them (e.g. skull, thoracic cage, pelvis).

Bones also harbor bone marrow which is crucial in production of blood cells in adults. In addition, the bone tissue can act as a storage for blood cells and minerals.

Common bone diseases often affect the bone density, e.g. in young children due to malnutrition. For example, rickets is a bone deformity seen in young children who lack vitamin D. Their legs are disfigured and they have trouble walking. The damage is irreversible though surgery may help. Osteomalacia and osteoporosis are diseases seen mainly in adulthood.

Osteomalacia is the improper mineralization of bone due to a lack of available calcium and phosphate. The bone density decreases and the bones become soft. Osteoporosis has been noted in all ages but mostly in postmenopausal and elderly women. A progressive decrease in bone density increases the risk of fracture. Patients who are on long-term steroid medication are in particular risk.

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Bone Marrow: Functions, Disorders, and Treatments  Metropolis Healthcare

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Before we talk about bone formation, we need to discuss how cartilage turns into bone. When you're floating around in the womb, your developing body is just beginning to take its shape, and it's creating cartilage to do so. Cartilage is a tissue that isn't as hard as bone, but much more flexible and, in some ways, more functional. Cartilage is pretty good stuff to use if you're going to mold a human good enough for the finer work, especially, such as your nose or your ear.

A large amount of that fetus cartilage begins transforming into bone, a process called ossification. When ossification occurs, the cartilage begins to calcify; that is, layers of calcium and phosphate salts begin to accumulate on the cartilage cells. These cells, surrounded by minerals, die off. This leaves small pockets of separation in the soon-to-be-bone cartilage, and tiny blood vessels grow into these cavities.

Specialized cells called osteoblasts begin traveling into the developing bone by way of these blood vessels. These cells produce a substance consisting of collagen fibers, and they also aid in the collection of calcium, which is deposited along this fibrous substance.

Eventually, the osteoblasts become part of the mix, turning into lower-functioning osteocytes. This osteocyte network helps form the spongelike lattice of cancellous bone. Cancellous bone isn't soft, but it does look spongy. Its spaces help transfer the stress of external pressures throughout the bone, and these spaces also contain marrow. Little channels called canaliculi run all throughout the calcified portions of the bone, enabling nutrients, gases and waste to make their way through.

Before turning into osteocytes, osteoblasts produce cortical bone. One way to imagine this process is to picture a bricklayer trapping himself inside a man-sized brick chamber of his own construction. After forming the hard shell (cortical bone), the bricklayer himself fills the chamber. Air makes its way through the brick and decays the bricklayer.

In bone, this part of the process is accomplished by osteoclasts, which make their way into the calcifying cartilage and take bone out of the middle of the shaft, leaving room for marrow to form. Osteoclasts do this by engulfing and digesting the bone matrix using acids and hydrolytic enzymes. So, our bricklayer (osteoblast) made the tomb (cortical bone), died inside the tomb (became an osteocyte), decayed over time (dissolved by osteoclasts) and left behind his remains that formed a network of mass and space inside the brick tomb.

Eventually, all the cartilage has turned to bone, except for the cartilage on the end of the bone (articular cartilage) and growth plates, which connect the bone shaft on each side to the bone ends. These cartilage layers help the bone expand and finally calcify by adulthood.

So, right now in your body, there are osteoclasts hard at work absorbing old bone cells and osteoblasts helping to build new bone in its place. This cycle is called remodeling. When you're young, your osteoblasts (the builders) are more numerous than the osteoclasts, resulting in bone gain. When you age, the osteoblasts can't keep up with the osteoclasts, which are still efficiently removing bone cells, and this leads to loss of bone mass (and a condition called osteoporosis, which we'll discuss shortly).

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Young Male Stem Cell Donors Could Be the Miracles Countless Patients Need Today  Good Things Guy

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How bone marrow transplants are changing the outlook for rare blood diseases?  Healthcare Radius

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Newborn with bubble boy disease now thriving, thanks to Singapores early detection programme  The Straits Times

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In human anatomy, the spinal canal, vertebral canal or spinal cavity is an elongated body cavity enclosed within the dorsal bony arches of the vertebral column, which contains the spinal cord, spinal roots and dorsal root ganglia. It is a process of the dorsal body cavity formed by alignment of the vertebral foramina. Under the vertebral arches, the spinal canal is also covered anteriorly by the posterior longitudinal ligament and posteriorly by the ligamentum flavum. The potential space between these ligaments and the dura mater covering the spinal cord is known as the epidural space. Spinal nerves exit the spinal canal via the intervertebral foramina under the corresponding vertebral pedicles.

In humans, the spinal cord gets outgrown by the vertebral column during development into adulthood, and the lower section of the spinal canal is occupied by the filum terminale and a bundle of spinal nerves known as the cauda equina instead of the actual spinal cord, which finishes at the L1/L2 level.

The vertebral canal is enclosed anteriorly by the vertebral bodies, intervertebral discs, and the posterior longitudinal ligament; it is enclosed posteriorly by the vertebral laminae and the ligamenta flava; laterally, it is incompletely enclosed by the pedicles with the interval between two adjacent pedicles on either side creating an intervertebral foramen (allowing the passage of the spinal nerves and radicular blood vessels).[1]

The vertebral canal progressively narrows inferiorly.[1] It is wider in the cervical region to accommodate the cervical enlargement of the spinal cord.[2][3]

The outermost layer of the meninges, the dura mater, is closely associated with the arachnoid mater which in turn is loosely connected to the innermost layer, the pia mater. The meninges divide the spinal canal into the epidural space and the subarachnoid space. The pia mater is closely attached to the spinal cord. A subdural space is generally only present due to trauma and/or pathological situations. The subarachnoid space is filled with cerebrospinal fluid and contains the vessels that supply the spinal cord, namely the anterior spinal artery and the paired posterior spinal arteries, accompanied by corresponding spinal veins. The anterior and posterior spinal arteries form anastomoses known as the vasocorona of the spinal cord and these supply nutrients to the canal. The epidural space contains loose fatty tissue, and a network of large, thin-walled blood vessels called the internal vertebral venous plexuses.[citation needed]

Spinal stenosis is a narrowing of the canal which can occur in any region of the spine and can be caused by a number of factors. It may result in cervical myelopathy[4] if the narrowed canal impinges on the spinal cord itself.

Spinal canal endoscopy can be used to investigate the epidural space, and is an important spinal diagnostic technique.[5][6]

The spinal canal was first described by Jean Fernel.[citation needed]

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Overview

Spinal stenosis happens when the space inside the backbone is too small. This can put pressure on the spinal cord and nerves that travel through the spine. Spinal stenosis happens most often in the lower back and the neck.

Some people with spinal stenosis have no symptoms. Others may experience pain, tingling, numbness and muscle weakness. Symptoms can get worse over time.

The most common cause of spinal stenosis is wear-and-tear damage in the spine related to arthritis. People who have serious spinal stenosis may need surgery.

Surgery can create more space inside the spine. This can ease the symptoms caused by pressure on the spinal cord or nerves. But surgery can't cure arthritis, so arthritis pain in the spine may continue.

Spinal stenosis often causes no symptoms. When symptoms do happen, they start slowly and get worse over time. Symptoms depend on which part of the spine is affected.

Spinal stenosis in the lower back can cause pain or cramping in one or both legs. This happens when you stand for a long time or when you walk. Symptoms get better when you bend forward or sit. Some people also have back pain.

Spinal stenosis in the neck can cause:

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As the spine ages, bone spurs or herniated disks are more likely to happen. These problems can shrink the amount of space available for the spinal cord and the nerves that branch off of it.

Spinal bones are stacked in a column from the skull to the tailbone. They protect the spinal cord, which runs through an opening called the spinal canal.

Some people are born with a small spinal canal. But most spinal stenosis occurs when something happens to reduce the amount of open space within the spine. Causes of spinal stenosis include:

Most people with spinal stenosis are over age 50. Younger people may be at higher risk of spinal stenosis if they have scoliosis or other spinal problems.

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The spinal cord is a long, fragile tubelike structure that begins at the end of the brain stem and continues down almost to the bottom of the spine. The spinal cord consists of bundles of nerve axons forming pathways that carry incoming and outgoing messages between the brain and the rest of the body. The spinal cord contains nerve cell circuits that control coordinated movements such as walking and swimming, as well as urinating. It is also the center for reflexes, such as the knee jerk reflex (see figure Reflex Arc: A No-Brainer).

Like the brain, the spinal cord is covered by 3 layers of tissue (meninges). The spinal cord and meninges are contained in the spinal canal, which runs through the center of the spine. In most adults, the spine is composed of 33 individual back bones (vertebrae). Just as the skull protects the brain, vertebrae protect the spinal cord. The vertebrae are separated by disks made of cartilage, which act as cushions, reducing the forces on the spine generated by movements such as walking and jumping. The vertebrae and disks of cartilage extend the length of the spine and together form the vertebral (spinal) column.

How the Spine Is Organized

Like the brain, the spinal cord consists of gray and white matter.

The gray matter forms a butterfly-shaped center in the cord. The front wings (called anterior or ventral horns) contain motor nerve cells (neurons) which transmit information from the brain or spinal cord to muscles, stimulating movement. The back part of the butterfly wing (called posterior or dorsal horns) contains sensory nerve cells, which transmit sensory information from other parts of the body through the spinal cord to the brain.

The surrounding white matter contains columns of nerve fibers (axon bundles) that carry sensory information to the brain from the rest of the body (ascending tracts) and columns that carry motor impulses from the brain to the muscles (descending tracts).

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Paralysis Ends Now: Revolutionary Cell Therapy That Repairs Severed Spinal Cords Enters Trials and Begins Restoring Human Mobility  Rude Baguette

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In this section, we will cover the anatomical structure of the spinal cord and vertebral column, spinal nerve organisation, blood supply, motor and sensory pathways, clinical examination principles, myotomes and dermatomes, localisation of spinal cord lesions, and common spinal cord syndromes.

The spinal column encases and protects the spinal cord, which serves as a conduit for motor, sensory, and autonomic signals between the brain and the body.

Clinical examination of spinal cord function involves assessing motor, sensory, and autonomic pathways.

Discrepancy exists between spinal segments and vertebral levels:

Afferent (or sensory) input to the nervous system arrives in the spinal cord via the dorsal root.

Efferents (or motor output) exit via the ventral root.

Descending tracts:

Ascending tracts:

Blood supply

A myotome refers to all the muscles or groups of muscles innervated by the motor horn cells within a segment of the cord.

Localising spinal cord lesions

During and after your examination you should seek answers to the following questions.

Small central lesion:

Large central cord lesion:

Brown-Squard syndrome (hemisection):

Complete transection:

Combined degeneration of the cord:

Tabes dorsalis:

Anterior spinal artery syndrome:

Posterior spinal artery syndrome:

Further reading

Publications

Robert Coni, DO, EdS, FAAN.Vascular neurologist and neurohospitalist and Neurology Subspecialty Coordinator at the Grand Strand Medical Center in South Carolina. Former neuroscience curriculum coordinator at St. Lukes / Temple Medical School and fellow of the American Academy of Neurology. Inmy spare time, I like to play guitar and go fly fishing. | Medmastery | Linkedin |

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Several factors can contribute to the narrowing of the spinal canal, leading to spinal stenosis. Normally, the vertebral canal provides enough room for the spinal cord, cauda equina, and the exiting nerves. However, aging and age-related changes in the spine, injury, other diseases, or inherited conditions can cause narrowing of the spaces.

Aging and age-related changes in the spine happen over a period of time and slowly cause loss of the normal structure of the spine. They are the most common causes of spinal stenosis. As people age, the ligaments that keep the vertebrae of the spine in place may thicken and calcify (harden from deposits of calcium salts). Bones and joints may also enlarge. When surfaces of the bone begin to project out from the body, these projections are called osteophytes (bone spurs). For example:

Arthritis is also a common cause of spinal stenosis. Two forms of arthritis that may affect the spine are osteoarthritis and rheumatoid arthritis.

The following conditions also may cause spinal stenosis:

Some people are born with a condition that can cause spinal stenosis. These conditions cause the spinal canal to narrow, leading to spinal stenosis. For example:

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IPS acquired Precision Electric Motor Works in February 2020. Precision, the only EASA-certified full-service repair facility in the tri-state area of New Jersey, New York, and Connecticut, now offers all IPS services and resources.

IPS expands Precisions capabilities in on-site and in-shop services for motors and rotating equipment. With the addition of IPS power management services and distribution resources, Precisions customers can also streamline their vendor management.

Single-source capabilities to respond, rethink, and resolveAs part of IPS, Precision is now aligned with the industrys leading provider of critical infrastructure services. The IPS network covers North America, the Caribbean, the United Kingdom, and Western Europe.

We serve over 30,000 customers competing in multiple markets. These include power generation, utilities, water and wastewater, petrochemicals, air separation, oil and gas, metals, mining, paper, aggregates, cement, hospitals, universities, commercial buildings, and data centers.

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The heart is a muscular organ found in humans and other animals. This organ pumps blood through the blood vessels.[1] The heart and blood vessels together make the circulatory system.[2] The pumped blood carries oxygen and nutrients to the tissue, while carrying metabolic waste such as carbon dioxide to the lungs. In humans, the heart is approximately the size of a closed fist and is located between the lungs, in the middle compartment of the chest, called the mediastinum.[4]

An illustration of the anterior view of the human heart

In humans, the heart is divided into four chambers: upper left and right atria and lower left and right ventricles.[5][6] Commonly, the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart. In a healthy heart, blood flows one way through the heart due to heart valves, which prevent backflow.[4] The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium.[8]

The heart pumps blood with a rhythm determined by a group of pacemaker cells in the sinoatrial node. These generate an electric current that causes the heart to contract, traveling through the atrioventricular node and along the conduction system of the heart. In humans, deoxygenated blood enters the heart through the right atrium from the superior and inferior venae cavae and passes to the right ventricle. From here, it is pumped into pulmonary circulation to the lungs, where it receives oxygen and gives off carbon dioxide. Oxygenated blood then returns to the left atrium, passes through the left ventricle and is pumped out through the aorta into systemic circulation, traveling through arteries, arterioles, and capillarieswhere nutrients and other substances are exchanged between blood vessels and cells, losing oxygen and gaining carbon dioxidebefore being returned to the heart through venules and veins. The adult heart beats at a resting rate close to 72 beats per minute. Exercise temporarily increases the rate, but lowers it in the long term, and is good for heart health.

Cardiovascular diseases were the most common cause of death globally as of 2008, accounting for 30% of all human deaths.[12][13] Of these more than three-quarters are a result of coronary artery disease and stroke.[12] Risk factors include: smoking, being overweight, little exercise, high cholesterol, high blood pressure, and poorly controlled diabetes, among others.[14] Cardiovascular diseases do not frequently have symptoms but may cause chest pain or shortness of breath. Diagnosis of heart disease is often done by the taking of a medical history, listening to the heart-sounds with a stethoscope, as well as with ECG, and echocardiogram which uses ultrasound.[4] Specialists who focus on diseases of the heart are called cardiologists, although many specialties of medicine may be involved in treatment.[13]

Structure

Location and shape

The human heart is situated in the mediastinum, at the level of thoracic vertebrae T5T8. A double-membraned sac called the pericardium surrounds the heart and attaches to the mediastinum.[16] The back surface of the heart lies near the vertebral column, and the front surface, known as the sternocostal surface, sits behind the sternum and rib cartilages.[8] The upper part of the heart is the attachment point for several large blood vesselsthe venae cavae, aorta and pulmonary trunk. The upper part of the heart is located at the level of the third costal cartilage.[8] The lower tip of the heart, the apex, lies to the left of the sternum (8 to 9cm from the midsternal line) between the junction of the fourth and fifth ribs near their articulation with the costal cartilages.[8]

The largest part of the heart is usually slightly offset to the left side of the chest (levocardia). In a rare congenital disorder (dextrocardia) the heart is offset to the right side and is felt to be on the left because the left heart is stronger and larger, since it pumps to all body parts. Because the heart is between the lungs, the left lung is smaller than the right lung and has a cardiac notch in its border to accommodate the heart.[8]The heart is cone-shaped, with its base positioned upwards and tapering down to the apex.[8] An adult heart has a mass of 250350 grams (912oz).[17] The heart is often described as the size of a fist: 12cm (5in) in length, 8cm (3.5in) wide, and 6cm (2.5in) in thickness,[8] although this description is disputed, as the heart is likely to be slightly larger.[18] Well-trained athletes can have much larger hearts due to the effects of exercise on the heart muscle, similar to the response of skeletal muscle.[8]

Chambers

The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The atria open into the ventricles via the atrioventricular valves, present in the atrioventricular septum. This distinction is visible also on the surface of the heart as the coronary sulcus. There is an ear-shaped structure in the upper right atrium called the right atrial appendage, or auricle, and another in the upper left atrium, the left atrial appendage. The right atrium and the right ventricle together are sometimes referred to as the right heart. Similarly, the left atrium and the left ventricle together are sometimes referred to as the left heart. The ventricles are separated from each other by the interventricular septum, visible on the surface of the heart as the anterior longitudinal sulcus and the posterior interventricular sulcus.

The fibrous cardiac skeleton gives structure to the heart. It forms the atrioventricular septum, which separates the atria from the ventricles, and the fibrous rings, which serve as bases for the four heart valves.[21] The cardiac skeleton also provides an important boundary in the heart's electrical conduction system since collagen cannot conduct electricity. The interatrial septum separates the atria, and the interventricular septum separates the ventricles.[8] The interventricular septum is much thicker than the interatrial septum since the ventricles need to generate greater pressure when they contract.[8]

Valves

The heart, showing valves, arteries and veins. The white arrows show the normal direction of blood flow.

The heart has four valves, which separate its chambers. One valve lies between each atrium and ventricle, and one valve rests at the exit of each ventricle.[8]

The valves between the atria and ventricles are called the atrioventricular valves. Between the right atrium and the right ventricle is the tricuspid valve. The tricuspid valve has three cusps, which connect to chordae tendinae and three papillary muscles named the anterior, posterior, and septal muscles, after their relative positions. The mitral valve lies between the left atrium and left ventricle. It is also known as the bicuspid valve due to its having two cusps, an anterior and a posterior cusp. These cusps are also attached via chordae tendinae to two papillary muscles projecting from the ventricular wall.

The papillary muscles extend from the walls of the heart to valves by cartilaginous connections called chordae tendinae. These muscles prevent the valves from falling too far back when they close.[24] During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight. As the heart chambers contract, so do the papillary muscles. This creates tension on the chordae tendineae, helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria.[8][g]

Two additional semilunar valves sit at the exit of each of the ventricles. The pulmonary valve is located at the base of the pulmonary artery. This has three cusps which are not attached to any papillary muscles. When the ventricle relaxes blood flows back into the ventricle from the artery and this flow of blood fills the pocket-like valve, pressing against the cusps which close to seal the valve. The semilunar aortic valve is at the base of the aorta and also is not attached to papillary muscles. This too has three cusps which close with the pressure of the blood flowing back from the aorta.[8]

Right heart

The right heart consists of two chambers, the right atrium and the right ventricle, separated by a valve, the tricuspid valve.[8]

The right atrium receives blood almost continuously from the body's two major veins, the superior and inferior venae cavae. A small amount of blood from the coronary circulation also drains into the right atrium via the coronary sinus, which is immediately above and to the middle of the opening of the inferior vena cava.[8] In the wall of the right atrium is an oval-shaped depression known as the fossa ovalis, which is a remnant of an opening in the fetal heart known as the foramen ovale.[8] Most of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface has prominent ridges of pectinate muscles, which are also present in the right atrial appendage.[8]

The right atrium is connected to the right ventricle by the tricuspid valve.[8] The walls of the right ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator band reinforces the thin walls of the right ventricle and plays a crucial role in cardiac conduction. It arises from the lower part of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle.[8] The right ventricle tapers into the pulmonary trunk, into which it ejects blood when contracting. The pulmonary trunk branches into the left and right pulmonary arteries that carry the blood to each lung. The pulmonary valve lies between the right heart and the pulmonary trunk.[8]

Left heart

The left heart has two chambers: the left atrium and the left ventricle, separated by the mitral valve.[8]

The left atrium receives oxygenated blood back from the lungs via one of the four pulmonary veins. The left atrium has an outpouching called the left atrial appendage. Like the right atrium, the left atrium is lined by pectinate muscles.[25] The left atrium is connected to the left ventricle by the mitral valve.[8]

The left ventricle is much thicker as compared with the right, due to the greater force needed to pump blood to the entire body. Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The left ventricle pumps blood to the body through the aortic valve and into the aorta. Two small openings above the aortic valve carry blood to the heart muscle; the left coronary artery is above the left cusp of the valve, and the right coronary artery is above the right cusp.[8]

Wall

The heart wall is made up of three layers: the inner endocardium, middle myocardium and outer epicardium. These are surrounded by a double-membraned sac called the pericardium.

The innermost layer of the heart is called the endocardium. It is made up of a lining of simple squamous epithelium and covers heart chambers and valves. It is continuous with the endothelium of the veins and arteries of the heart, and is joined to the myocardium with a thin layer of connective tissue.[8] The endocardium, by secreting endothelins, may also play a role in regulating the contraction of the myocardium.[8]

The middle layer of the heart wall is the myocardium, which is the cardiac musclea layer of involuntary striated muscle tissue surrounded by a framework of collagen. The cardiac muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart, with the outer muscles forming a figure 8 pattern around the atria and around the bases of the great vessels and the inner muscles, forming a figure 8 around the two ventricles and proceeding toward the apex. This complex swirling pattern allows the heart to pump blood more effectively.[8]

There are two types of cells in cardiac muscle: muscle cells which have the ability to contract easily, and pacemaker cells of the conducting system. The muscle cells make up the bulk (99%) of cells in the atria and ventricles. These contractile cells are connected by intercalated discs which allow a rapid response to impulses of action potential from the pacemaker cells. The intercalated discs allow the cells to act as a syncytium and enable the contractions that pump blood through the heart and into the major arteries.[8] The pacemaker cells make up 1% of cells and form the conduction system of the heart. They are generally much smaller than the contractile cells and have few myofibrils which gives them limited contractibility. Their function is similar in many respects to neurons.[8] Cardiac muscle tissue has autorhythmicity, the unique ability to initiate a cardiac action potential at a fixed ratespreading the impulse rapidly from cell to cell to trigger the contraction of the entire heart.[8]

There are specific proteins expressed in cardiac muscle cells.[26][27] These are mostly associated with muscle contraction, and bind with actin, myosin, tropomyosin, and troponin. They include MYH6, ACTC1, TNNI3, CDH2 and PKP2. Other proteins expressed are MYH7 and LDB3 that are also expressed in skeletal muscle.[28]

Pericardium

The pericardium is the sac that surrounds the heart. The tough outer surface of the pericardium is called the fibrous membrane. This is lined by a double inner membrane called the serous membrane that produces pericardial fluid to lubricate the surface of the heart. The part of the serous membrane attached to the fibrous membrane is called the parietal pericardium, while the part of the serous membrane attached to the heart is known as the visceral pericardium. The pericardium is present in order to lubricate its movement against other structures within the chest, to keep the heart's position stabilised within the chest, and to protect the heart from infection.[30]

Coronary circulation

Heart tissue, like all cells in the body, needs to be supplied with oxygen, nutrients and a way of removing metabolic wastes. This is achieved by the coronary circulation, which includes arteries, veins, and lymphatic vessels. Blood flow through the coronary vessels occurs in peaks and troughs relating to the heart muscle's relaxation or contraction.[8]

Heart tissue receives blood from two arteries which arise just above the aortic valve. These are the left main coronary artery and the right coronary artery. The left main coronary artery splits shortly after leaving the aorta into two vessels, the left anterior descending and the left circumflex artery. The left anterior descending artery supplies heart tissue and the front, outer side, and septum of the left ventricle. It does this by branching into smaller arteriesdiagonal and septal branches. The left circumflex supplies the back and underneath of the left ventricle. The right coronary artery supplies the right atrium, right ventricle, and lower posterior sections of the left ventricle. The right coronary artery also supplies blood to the atrioventricular node (in about 90% of people) and the sinoatrial node (in about 60% of people). The right coronary artery runs in a groove at the back of the heart and the left anterior descending artery runs in a groove at the front. There is significant variation between people in the anatomy of the arteries that supply the heart. The arteries divide at their furthest reaches into smaller branches that join at the edges of each arterial distribution.[8]

The coronary sinus is a large vein that drains into the right atrium, and receives most of the venous drainage of the heart. It receives blood from the great cardiac vein (receiving the left atrium and both ventricles), the posterior cardiac vein (draining the back of the left ventricle), the middle cardiac vein (draining the bottom of the left and right ventricles), and small cardiac veins. The anterior cardiac veins drain the front of the right ventricle and drain directly into the right atrium.[8]

Small lymphatic networks called plexuses exist beneath each of the three layers of the heart. These networks collect into a main left and a main right trunk, which travel up the groove between the ventricles that exists on the heart's surface, receiving smaller vessels as they travel up. These vessels then travel into the atrioventricular groove, and receive a third vessel which drains the section of the left ventricle sitting on the diaphragm. The left vessel joins with this third vessel, and travels along the pulmonary artery and left atrium, ending in the inferior tracheobronchial node. The right vessel travels along the right atrium and the part of the right ventricle sitting on the diaphragm. It usually then travels in front of the ascending aorta and then ends in a brachiocephalic node.

Nerve supply

The heart receives nerve signals from the vagus nerve and from nerves arising from the sympathetic trunk. These nerves act to influence, but not control, the heart rate. Sympathetic nerves also influence the force of heart contraction. Signals that travel along these nerves arise from two paired cardiovascular centres in the medulla oblongata. The vagus nerve of the parasympathetic nervous system acts to decrease the heart rate, and nerves from the sympathetic trunk act to increase the heart rate.[8] These nerves form a network of nerves that lies over the heart called the cardiac plexus.[8]

The vagus nerve is a long, wandering nerve that emerges from the brainstem and provides parasympathetic stimulation to a large number of organs in the thorax and abdomen, including the heart. The nerves from the sympathetic trunk emerge through the T1T4 thoracic ganglia and travel to both the sinoatrial and atrioventricular nodes, as well as to the atria and ventricles. The ventricles are more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation causes the release of the neurotransmitter norepinephrine (also known as noradrenaline) at the neuromuscular junction of the cardiac nerves[citation needed]. This shortens the repolarisation period, thus speeding the rate of depolarisation and contraction, which results in an increased heart rate. It opens chemical or ligand-gated sodium and calcium ion channels, allowing an influx of positively charged ions.[8] Norepinephrine binds to the beta1 receptor.[8]

Development

The heart is the first functional organ to develop and starts to beat and pump blood at about three weeks into embryogenesis. This early start is crucial for subsequent embryonic and prenatal development.

The heart derives from splanchnopleuric mesenchyme in the neural plate which forms the cardiogenic region. Two endocardial tubes form here that fuse to form a primitive heart tube known as the tubular heart.[36] Between the third and fourth week, the heart tube lengthens, and begins to fold to form an S-shape within the pericardium. This places the chambers and major vessels into the correct alignment for the developed heart. Further development will include the formation of the septa and the valves and the remodeling of the heart chambers. By the end of the fifth week, the septa are complete, and by the ninth week, the heart valves are complete.[8]

Before the fifth week, there is an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the lungs. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern. A depression in the surface of the right atrium remains where the foramen ovale was, called the fossa ovalis.[8]

The embryonic heart begins beating at around 22 days after conception (5 weeks after the last normal menstrual period, LMP). It starts to beat at a rate near to the mother's which is about 7580 beats per minute (bpm). The embryonic heart rate then accelerates and reaches a peak rate of 165185 bpm early in the early 7th week (early 9th week after the LMP).[37][38] After 9 weeks (start of the fetal stage) it starts to decelerate, slowing to around 145 (25) bpm at birth. There is no difference in female and male heart rates before birth.[39]

Physiology

Blood flow

The heart functions as a pump in the circulatory system to provide a continuous flow of blood throughout the body. This circulation consists of the systemic circulation to and from the body and the pulmonary circulation to and from the lungs. Blood in the pulmonary circulation exchanges carbon dioxide for oxygen in the lungs through the process of respiration. The systemic circulation then transports oxygen to the body and returns carbon dioxide and relatively deoxygenated blood to the heart for transfer to the lungs.[8]

The right heart collects deoxygenated blood from two large veins, the superior and inferior venae cavae. Blood collects in the right and left atrium continuously.[8] The superior vena cava drains blood from above the diaphragm and empties into the upper back part of the right atrium. The inferior vena cava drains the blood from below the diaphragm and empties into the back part of the atrium below the opening for the superior vena cava. Immediately above and to the middle of the opening of the inferior vena cava is the opening of the thin-walled coronary sinus.[8] Additionally, the coronary sinus returns deoxygenated blood from the myocardium to the right atrium. The blood collects in the right atrium. When the right atrium contracts, the blood is pumped through the tricuspid valve into the right ventricle. As the right ventricle contracts, the tricuspid valve closes and the blood is pumped into the pulmonary trunk through the pulmonary valve. The pulmonary trunk divides into pulmonary arteries and progressively smaller arteries throughout the lungs, until it reaches capillaries. As these pass by alveoli carbon dioxide is exchanged for oxygen. This happens through the passive process of diffusion.

In the left heart, oxygenated blood is returned to the left atrium via the pulmonary veins. It is then pumped into the left ventricle through the mitral valve and into the aorta through the aortic valve for systemic circulation. The aorta is a large artery that branches into many smaller arteries, arterioles, and ultimately capillaries. In the capillaries, oxygen and nutrients from blood are supplied to body cells for metabolism, and exchanged for carbon dioxide and waste products.[8] Capillary blood, now deoxygenated, travels into venules and veins that ultimately collect in the superior and inferior vena cavae, and into the right heart.

Cardiac cycle

The cardiac cycle is the sequence of events in which the heart contracts and relaxes with every heartbeat. The period of time during which the ventricles contract, forcing blood out into the aorta and main pulmonary artery, is known as systole, while the period during which the ventricles relax and refill with blood is known as diastole. The atria and ventricles work in concert, so in systole when the ventricles are contracting, the atria are relaxed and collecting blood. When the ventricles are relaxed in diastole, the atria contract to pump blood to the ventricles. This coordination ensures blood is pumped efficiently to the body.[8]

At the beginning of the cardiac cycle, the ventricles are relaxing. As they do so, they are filled by blood passing through the open mitral and tricuspid valves. After the ventricles have completed most of their filling, the atria contract, forcing further blood into the ventricles and priming the pump. Next, the ventricles start to contract. As the pressure rises within the cavities of the ventricles, the mitral and tricuspid valves are forced shut. As the pressure within the ventricles rises further, exceeding the pressure with the aorta and pulmonary arteries, the aortic and pulmonary valves open. Blood is ejected from the heart, causing the pressure within the ventricles to fall. Simultaneously, the atria refill as blood flows into the right atrium through the superior and inferior vena cavae, and into the left atrium through the pulmonary veins. Finally, when the pressure within the ventricles falls below the pressure within the aorta and pulmonary arteries, the aortic and pulmonary valves close. The ventricles start to relax, the mitral and tricuspid valves open, and the cycle begins again.

Cardiac output

Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle (stroke volume) in one minute. This is calculated by multiplying the stroke volume (SV) by the beats per minute of the heart rate (HR). So that: CO = SV x HR.[8]The cardiac output is normalized to body size through body surface area and is called the cardiac index.

The average cardiac output, using an average stroke volume of about 70mL, is 5.25 L/min, with a normal range of 4.08.0 L/min.[8] The stroke volume is normally measured using an echocardiogram and can be influenced by the size of the heart, physical and mental condition of the individual, sex, contractility, duration of contraction, preload and afterload.[8]

Preload refers to the filling pressure of the atria at the end of diastole, when the ventricles are at their fullest. A main factor is how long it takes the ventricles to fill: if the ventricles contract more frequently, then there is less time to fill and the preload will be less.[8] Preload can also be affected by a person's blood volume. The force of each contraction of the heart muscle is proportional to the preload, described as the Frank-Starling mechanism. This states that the force of contraction is directly proportional to the initial length of muscle fiber, meaning a ventricle will contract more forcefully, the more it is stretched.[8]

Afterload, or how much pressure the heart must generate to eject blood at systole, is influenced by vascular resistance. It can be influenced by narrowing of the heart valves (stenosis) or contraction or relaxation of the peripheral blood vessels.[8]

The strength of heart muscle contractions controls the stroke volume. This can be influenced positively or negatively by agents termed inotropes.[41] These agents can be a result of changes within the body, or be given as drugs as part of treatment for a medical disorder, or as a form of life support, particularly in intensive care units. Inotropes that increase the force of contraction are "positive" inotropes, and include sympathetic agents such as adrenaline, noradrenaline and dopamine.[42] "Negative" inotropes decrease the force of contraction and include calcium channel blockers.[41]

Electrical conduction

The normal rhythmical heart beat, called sinus rhythm, is established by the heart's own pacemaker, the sinoatrial node (also known as the sinus node or the SA node). Here an electrical signal is created that travels through the heart, causing the heart muscle to contract. The sinoatrial node is found in the upper part of the right atrium near to the junction with the superior vena cava.[43] The electrical signal generated by the sinoatrial node travels through the right atrium in a radial way that is not completely understood. It travels to the left atrium via Bachmann's bundle, such that the muscles of the left and right atria contract together.[44][45][46] The signal then travels to the atrioventricular node. This is found at the bottom of the right atrium in the atrioventricular septum, the boundary between the right atrium and the left ventricle. The septum is part of the cardiac skeleton, tissue within the heart that the electrical signal cannot pass through, which forces the signal to pass through the atrioventricular node only.[8] The signal then travels along the bundle of His to left and right bundle branches through to the ventricles of the heart. In the ventricles the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the heart muscle.[47]

Heart rate

Heart sounds of a 16 year old girl immediately after running, with a heart rate of 186 BPM.

The normal resting heart rate is called the sinus rhythm, created and sustained by the sinoatrial node, a group of pacemaking cells found in the wall of the right atrium. Cells in the sinoatrial node do this by creating an action potential. The cardiac action potential is created by the movement of specific electrolytes into and out of the pacemaker cells. The action potential then spreads to nearby cells.

When the sinoatrial cells are resting, they have a negative charge on their membranes. A rapid influx of sodium ions causes the membrane's charge to become positive; this is called depolarisation and occurs spontaneously.[8] Once the cell has a sufficiently high charge, the sodium channels close and calcium ions then begin to enter the cell, shortly after which potassium begins to leave it. All the ions travel through ion channels in the membrane of the sinoatrial cells. The potassium and calcium start to move out of and into the cell only once it has a sufficiently high charge, and so are called voltage-gated. Shortly after this, the calcium channels close and potassium channels open, allowing potassium to leave the cell. This causes the cell to have a negative resting charge and is called repolarisation. When the membrane potential reaches approximately 60 mV, the potassium channels close and the process may begin again.[8]

The ions move from areas where they are concentrated to where they are not. For this reason sodium moves into the cell from outside, and potassium moves from within the cell to outside the cell. Calcium also plays a critical role. Their influx through slow channels means that the sinoatrial cells have a prolonged "plateau" phase when they have a positive charge. A part of this is called the absolute refractory period. Calcium ions also combine with the regulatory protein troponin C in the troponin complex to enable contraction of the cardiac muscle, and separate from the protein to allow relaxation.[49]

The adult resting heart rate ranges from 60 to 100 bpm. The resting heart rate of a newborn can be 129 beats per minute (bpm) and this gradually decreases until maturity.[50] An athlete's heart rate can be lower than 60 bpm. During exercise the rate can be 150 bpm with maximum rates reaching from 200 to 220 bpm.[8]

Influences

The normal sinus rhythm of the heart, giving the resting heart rate, is influenced by a number of factors. The cardiovascular centres in the brainstem control the sympathetic and parasympathetic influences to the heart through the vagus nerve and sympathetic trunk.[51] These cardiovascular centres receive input from a series of receptors including baroreceptors, sensing the stretching of blood vessels and chemoreceptors, sensing the amount of oxygen and carbon dioxide in the blood and its pH. Through a series of reflexes these help regulate and sustain blood flow.[8]

Baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself. Baroreceptors fire at a rate determined by how much they are stretched, which is influenced by blood pressure, level of physical activity, and the relative distribution of blood. With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation. As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation.[8] There is a similar reflex, called the atrial reflex or Bainbridge reflex, associated with varying rates of blood flow to the atria. Increased venous return stretches the walls of the atria where specialized baroreceptors are located. However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase heart rate. The opposite is also true.[8] Chemoreceptors present in the carotid body or adjacent to the aorta in an aortic body respond to the blood's oxygen, carbon dioxide levels. Low oxygen or high carbon dioxide will stimulate firing of the receptors.

Exercise and fitness levels, age, body temperature, basal metabolic rate, and even a person's emotional state can all affect the heart rate. High levels of the hormones epinephrine, norepinephrine, and thyroid hormones can increase the heart rate. The levels of electrolytes including calcium, potassium, and sodium can also influence the speed and regularity of the heart rate; low blood oxygen, low blood pressure and dehydration may increase it.[8]

Clinical significance

Diseases

Cardiovascular diseases, which include diseases of the heart, are the leading cause of death worldwide.[54] The majority of cardiovascular disease is noncommunicable and related to lifestyle and other factors, becoming more prevalent with ageing.[54] Heart disease is a major cause of death, accounting for an average of 30% of all deaths in 2008, globally.[12] This rate varies from a lower 28% to a high 40% in high-income countries.[13] Doctors that specialise in the heart are called cardiologists. Many other medical professionals are involved in treating diseases of the heart, including doctors, cardiothoracic surgeons, intensivists, and allied health practitioners including physiotherapists and dieticians.[55]

Ischemic heart disease

Coronary artery disease, also known as ischemic heart disease, is caused by atherosclerosisa build-up of fatty material along the inner walls of the arteries. These fatty deposits known as atherosclerotic plaques narrow the coronary arteries, and if severe may reduce blood flow to the heart.[56] If a narrowing (or stenosis) is relatively minor then the patient may not experience any symptoms. Severe narrowings may cause chest pain (angina) or breathlessness during exercise or even at rest. The thin covering of an atherosclerotic plaque can rupture, exposing the fatty centre to the circulating blood. In this case a clot or thrombus can form, blocking the artery, and restricting blood flow to an area of heart muscle causing a myocardial infarction (a heart attack) or unstable angina. In the worst case this may cause cardiac arrest, a sudden and utter loss of output from the heart. Obesity, high blood pressure, uncontrolled diabetes, smoking and high cholesterol can all increase the risk of developing atherosclerosis and coronary artery disease.[54][56]

Heart failure

Heart failure is defined as a condition in which the heart is unable to pump enough blood to meet the demands of the body.[59] Patients with heart failure may experience breathlessness especially when lying flat, as well as ankle swelling, known as peripheral oedema. Heart failure is the result of many diseases affecting the heart, but is most commonly associated with ischemic heart disease, valvular heart disease, or high blood pressure. Less common causes include various cardiomyopathies. Heart failure is frequently associated with weakness of the heart muscle in the ventricles (systolic heart failure), but can also be seen in patients with heart muscle that is strong but stiff (diastolic heart failure). The condition may affect the left ventricle (causing predominantly breathlessness), the right ventricle (causing predominantly swelling of the legs and an elevated jugular venous pressure), or both ventricles. Patients with heart failure are at higher risk of developing dangerous heart rhythm disturbances or arrhythmias.[59]

Cardiomyopathies

Cardiomyopathies are diseases affecting the muscle of the heart. Some cause abnormal thickening of the heart muscle (hypertrophic cardiomyopathy), some cause the heart to abnormally expand and weaken (dilated cardiomyopathy), some cause the heart muscle to become stiff and unable to fully relax between contractions (restrictive cardiomyopathy) and some make the heart prone to abnormal heart rhythms (arrhythmogenic cardiomyopathy). These conditions are often genetic and can be inherited, but some such as dilated cardiomyopathy may be caused by damage from toxins such as alcohol. Some cardiomyopathies such as hypertrophic cardiomopathy are linked to a higher risk of sudden cardiac death, particularly in athletes.[8] Many cardiomyopathies can lead to heart failure in the later stages of the disease.[59]

Valvular heart disease

Heart sounds of a 16 year old girl diagnosed with mitral valve prolapse and mitral regurgitation. Auscultating her heart, a systolic murmur and click is heard. Recorded with the stethoscope over the mitral valve.

Healthy heart valves allow blood to flow easily in one direction, and prevent it from flowing in the other direction. A diseased heart valve may have a narrow opening (stenosis), that restricts the flow of blood in the forward direction. A valve may otherwise be leaky, allowing blood to leak in the reverse direction (regurgitation). Valvular heart disease may cause breathlessness, blackouts, or chest pain, but may be asymptomatic and only detected on a routine examination by hearing abnormal heart sounds or a heart murmur. In the developed world, valvular heart disease is most commonly caused by degeneration secondary to old age, but may also be caused by infection of the heart valves (endocarditis). In some parts of the world rheumatic heart disease is a major cause of valvular heart disease, typically leading to mitral or aortic stenosis and caused by the body's immune system reacting to a streptococcal throat infection.[60]

Cardiac arrhythmias

While in the healthy heart, waves of electrical impulses originate in the sinus node before spreading to the rest of the atria, the atrioventricular node, and finally the ventricles (referred to as a normal sinus rhythm), this normal rhythm can be disrupted. Abnormal heart rhythms or arrhythmias may be asymptomatic or may cause palpitations, blackouts, or breathlessness. Some types of arrhythmia such as atrial fibrillation increase the long term risk of stroke.[62]

Some arrhythmias cause the heart to beat abnormally slowly, referred to as a bradycardia or bradyarrhythmia. This may be caused by an abnormally slow sinus node or damage within the cardiac conduction system (heart block).[63] In other arrhythmias the heart may beat abnormally rapidly, referred to as a tachycardia or tachyarrhythmia. These arrhythmias can take many forms and can originate from different structures within the heartsome arise from the atria (e.g. atrial flutter), some from the atrioventricular node (e.g. AV nodal re-entrant tachycardia) whilst others arise from the ventricles (e.g. ventricular tachycardia). Some tachyarrhythmias are caused by scarring within the heart (e.g. some forms of ventricular tachycardia), others by an irritable focus (e.g. focal atrial tachycardia), while others are caused by additional abnormal conduction tissue that has been present since birth (e.g. Wolff-Parkinson-White syndrome). The most dangerous form of heart racing is ventricular fibrillation, in which the ventricles quiver rather than contract, and which if untreated is rapidly fatal.[64]

Pericardial disease

The sac which surrounds the heart, called the pericardium, can become inflamed in a condition known as pericarditis. This condition typically causes chest pain that may spread to the back, and is often caused by a viral infection (glandular fever, cytomegalovirus, or coxsackievirus). Fluid can build up within the pericardial sac, referred to as a pericardial effusion. Pericardial effusions often occur secondary to pericarditis, kidney failure, or tumours, and frequently do not cause any symptoms. However, large effusions or effusions which accumulate rapidly can compress the heart in a condition known as cardiac tamponade, causing breathlessness and potentially fatal low blood pressure. Fluid can be removed from the pericardial space for diagnosis or to relieve tamponade using a syringe in a procedure called pericardiocentesis.

Congenital heart disease

Some people are born with hearts that are abnormal and these abnormalities are known as congenital heart defects. They may range from the relatively minor (e.g. patent foramen ovale, arguably a variant of normal) to serious life-threatening abnormalities (e.g. hypoplastic left heart syndrome). Common abnormalities include those that affect the heart muscle that separates the two side of the heart (a "hole in the heart", e.g. ventricular septal defect). Other defects include those affecting the heart valves (e.g. congenital aortic stenosis), or the main blood vessels that lead from the heart (e.g. coarctation of the aorta). More complex syndromes are seen that affect more than one part of the heart (e.g. Tetralogy of Fallot).

Some congenital heart defects allow blood that is low in oxygen that would normally be returned to the lungs to instead be pumped back to the rest of the body. These are known as cyanotic congenital heart defects and are often more serious. Major congenital heart defects are often picked up in childhood, shortly after birth, or even before a child is born (e.g. transposition of the great arteries), causing breathlessness and a lower rate of growth. More minor forms of congenital heart disease may remain undetected for many years and only reveal themselves in adult life (e.g., atrial septal defect).[66]

Channelopathies

Channelopathies can be categorized based on the organ system they affect. In the cardiovascular system, the electrical impulse required for each heart beat is provided by the electrochemical gradient of each heart cell. Because the beating of the heart depends on the proper movement of ions across the surface membrane, cardiac ion channelopathies form a major group of heart diseases.[68][69] Cardiac ion channelopathies may explain some of the cases of sudden death syndrome and sudden arrhythmic death syndrome.[70] Long QT syndrome is the most common form of cardiac channelopathy.

Diagnosis

Heart disease is diagnosed by the taking of a medical history, a cardiac examination, and further investigations, including blood tests, echocardiograms, electrocardiograms, and imaging. Other invasive procedures such as cardiac catheterisation can also play a role.

Examination

The cardiac examination includes inspection, feeling the chest with the hands (palpation) and listening with a stethoscope (auscultation).[77] It involves assessment of signs that may be visible on a person's hands (such as splinter haemorrhages), joints and other areas. A person's pulse is taken, usually at the radial artery near the wrist, in order to assess for the rhythm and strength of the pulse. The blood pressure is taken, using either a manual or automatic sphygmomanometer or using a more invasive measurement from within the artery. Any elevation of the jugular venous pulse is noted. A person's chest is felt for any transmitted vibrations from the heart, and then listened to with a stethoscope.

Heart sounds

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: of, relating to, situated near, or acting on the heart

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: a person with heart disease

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Stephen Kopecky, M.D., Cardiovascular Disease, Mayo Clinic: I'm Dr. Stephen Kopecky, a cardiologist at Mayo Clinic. In this video, we'll cover the basics of coronary artery disease. What is it? Who gets it? The symptoms, diagnosis and treatment. Whether you're looking for answers for yourself or someone you love, we're here to give you the best information available.

Coronary artery disease, also called CAD, is a condition that affects your heart. It is the most common heart disease in the United States. CAD happens when coronary arteries struggle to supply the heart with enough blood, oxygen and nutrients. Cholesterol deposits, or plaques, are almost always to blame. These buildups narrow your arteries, decreasing blood flow to your heart. This can cause chest pain, shortness of breath or even a heart attack. CAD typically takes a long time to develop. So often, patients don't know that they have it until there's a problem. But there are ways to prevent coronary artery disease, and ways to know if you're at risk and ways to treat it.

Anyone can develop CAD. It begins when fats, cholesterols and other substances gather along the walls of your arteries. This process is called atherosclerosis. It's typically no cause for concern. However, too much buildup can lead to a blockage, obstructing blood flow. There are a number of risk factors, common red flags, that can contribute to this and ultimately lead to coronary artery disease. First, getting older can mean more damaged and narrowed arteries. Second, men are generally at a greater risk. But the risk for women increases after menopause. Existing health conditions matter, too. High blood pressure can thicken your arteries, narrowing your blood flow. High cholesterol levels can increase the rate of plaque buildup. Diabetes is also associated with higher risk, as is being overweight. Your lifestyle plays a large role as well. Physical inactivity, long periods of unrelieved stress in your life, an unhealthy diet and smoking can all increase your risk. And finally, family history. If a close relative was diagnosed at an early age with heart disease, you're at a greater risk. All these factors together can paint a picture of your risk for developing CAD.

When coronary arteries become narrow, the heart doesn't get enough oxygen-rich blood. Remember, unlike most pumps, the heart has to pump its own energy supply. It's working harder with less. And you may begin to notice these signs and symptoms of pressure or tightness in your chest. This pain is called angina. It may feel like somebody is standing on your chest. When your heart can't pump enough blood to meet your body's needs, you might develop shortness of breath or extreme fatigue during activities. And if an artery becomes totally blocked, it leads to a heart attack. Classic signs and symptoms of a heart attack include crushing, substernal chest pain, pain in your shoulders or arms, shortness of breath, and sweating. However, many heart attacks have minimal or no symptoms and are found later during routine testing.

Diagnosing CAD starts by talking to your doctor. They'll be able to look at your medical history, do a physical exam and order routine blood work. Depending on that, they may suggest one or more of the following tests: an electrocardiogram or ECG, an echocardiogram or soundwave test of the heart, stress test, cardiac catheterization and angiogram, or a cardiac CT scan.

Treating coronary artery disease usually means making changes to your lifestyle. This might be eating healthier foods, exercising regularly, losing excess weight, reducing stress or quitting smoking. The good news is these changes can do a lot to improve your outlook. Living a healthier life translates to having healthier arteries. When necessary, treatment could involve drugs like aspirin, cholesterol-modifying medications, beta-blockers, or certain medical procedures like angioplasty or coronary artery bypass surgery.

Discovering you have coronary artery disease can be overwhelming. But be encouraged. There are things you can do to manage and live with this condition. Reducing cholesterol, lowering blood pressure, quitting tobacco, eating healthier, exercising and managing your stress can make a world of difference. Better heart health starts by educating yourself. So don't be afraid to seek out information and ask your doctors about coronary artery disease. If you'd like to learn even more about this condition, watch our other related videos or visit Mayoclinic.org. We wish you well.

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