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A capillary is a small
blood vessel Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...
, from 5 to 10
micrometre The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...
s in diameter, and is part of the
microcirculation The microcirculation is the circulation of the blood in the smallest blood vessels, the microvessels of the microvasculature present within organ tissues. The microvessels include terminal arterioles, metarterioles, capillaries, and venules. ...
system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the
tunica intima The tunica intima (Neo-Latin "inner coat"), or intima for short, is the innermost tunica (biology), tunica (layer) of an artery or vein. It is made up of one layer of endothelium, endothelial cells (and macrophages in areas of disturbed blood flo ...
(the innermost layer of an artery or vein), consisting of a thin wall of simple squamous
endothelial cell The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and th ...
s. They are the site of the exchange of many substances from the surrounding
interstitial fluid In cell biology, extracellular fluid (ECF) denotes all body fluid outside the Cell (biology), cells of any multicellular organism. Body water, Total body water in healthy adults is about 50–60% (range 45 to 75%) of total body weight; women ...
, and they convey blood from the smallest branches of the arteries (
arteriole An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillary, capillaries. Arterioles have vascular smooth muscle, muscular walls (usually only one to two layers of smoo ...
s) to those of the veins (
venule A venule is a very small vein in the microcirculation that allows blood to return from the capillary beds to drain into the venous system via increasingly larger veins. Post-capillary venules are the smallest of the veins with a diameter of ...
s). Other substances which cross capillaries include water,
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
,
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
,
urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...
,
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
,
uric acid Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the Chemical formula, formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the meta ...
,
lactic acid Lactic acid is an organic acid. It has the molecular formula C3H6O3. It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as wel ...
and
creatinine Creatinine (; ) is a breakdown product of creatine phosphate from muscle and protein metabolism. It is released at a constant rate by the body (depending on muscle mass). Biological relevance Serum creatinine (a blood measurement) is an impor ...
.
Lymph capillaries Lymph capillaries or lymphatic capillaries are tiny, thin-walled microvessels located in the spaces between cells (except in the central nervous system and non-vascular tissues) which serve to drain and process extracellular fluid. Upon enterin ...
connect with larger
lymph vessel The lymphatic vessels (or lymph vessels or lymphatics) are thin-walled vessels (tubes), structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessel ...
s to drain
lymph Lymph () is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated. At the ori ...
atic fluid collected in microcirculation.


Etymology

''Capillary'' comes from the Latin word , meaning "of or resembling hair", with use in English beginning in the mid-17th century. The meaning stems from the tiny, hairlike diameter of a capillary. While capillary is usually used as a noun, the word also is used as an adjective, as in "
capillary action Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of external forces like Gravitation, gravity. The effe ...
", in which a liquid flows without influence of external forces, such as
gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...
.


Structure

Blood flows from the heart through
arteries An artery () is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions that carry deoxygenated blood are the pulmonary arteries in ...
, which branch and narrow into
arteriole An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillary, capillaries. Arterioles have vascular smooth muscle, muscular walls (usually only one to two layers of smoo ...
s, and then branch further into capillaries where nutrients and wastes are exchanged. The capillaries then join and widen to become
venule A venule is a very small vein in the microcirculation that allows blood to return from the capillary beds to drain into the venous system via increasingly larger veins. Post-capillary venules are the smallest of the veins with a diameter of ...
s, which in turn widen and converge to become
vein Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and feta ...
s, which then return blood back to the heart through the
venae cavae In anatomy, the ''venae cavae'' (; ''vena cava'' ; ) are two large veins (great vessels) that return deoxygenated blood from the body into the heart. In humans they are the superior vena cava and the inferior vena cava, and both empty into the ...
. In the
mesentery In human anatomy, the mesentery is an Organ (anatomy), organ that attaches the intestines to the posterior abdominal wall, consisting of a double fold of the peritoneum. It helps (among other functions) in storing Adipose tissue, fat and allowi ...
,
metarteriole A metarteriole is a short microvessel in the microcirculation that links arterioles and capillaries. Instead of a continuous tunica media The tunica media (Neo-Latin "middle coat"), or media for short, is the middle tunica (layer) of an arter ...
s form an additional stage between arterioles and capillaries. Individual capillaries are part of the capillary bed, an interweaving network of capillaries supplying tissues and
organs In a multicellular organism, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to a ...
. The more
metabolically Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the c ...
active a tissue is, the more capillaries are required to supply nutrients and carry away products of metabolism. There are two types of capillaries: true capillaries, which branch from arterioles and provide exchange between tissue and the capillary blood, and
sinusoids A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the inn ...
, a type of open-pore capillary found in the
liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...
,
bone marrow Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...
,
anterior pituitary gland The anterior pituitary (also called the adenohypophysis or pars anterior) is a major organ of the endocrine system. The anterior pituitary is the glandular, anterior lobe that together with the posterior pituitary (or neurohypophysis) makes up ...
, and brain
circumventricular organs Circumventricular organs (CVOs) (wiktionary:circum-, circum-: around ; ventricular: of Ventricular system, ventricle) are structures in the brain characterized by their extensive and highly vascular permeability, permeable capillary, capillarie ...
. Capillaries and sinusoids are short vessels that directly connect the arterioles and venules at opposite ends of the beds.
Metarteriole A metarteriole is a short microvessel in the microcirculation that links arterioles and capillaries. Instead of a continuous tunica media The tunica media (Neo-Latin "middle coat"), or media for short, is the middle tunica (layer) of an arter ...
s are found primarily in the
mesenteric In human anatomy, the mesentery is an organ that attaches the intestines to the posterior abdominal wall, consisting of a double fold of the peritoneum. It helps (among other functions) in storing fat and allowing blood vessels, lymphatics, and ...
microcirculation The microcirculation is the circulation of the blood in the smallest blood vessels, the microvessels of the microvasculature present within organ tissues. The microvessels include terminal arterioles, metarterioles, capillaries, and venules. ...
. Lymphatic capillaries are slightly larger in diameter than blood capillaries, and have closed ends (unlike the blood capillaries open at one end to the arterioles and open at the other end to the venules). This structure permits
interstitial fluid In cell biology, extracellular fluid (ECF) denotes all body fluid outside the Cell (biology), cells of any multicellular organism. Body water, Total body water in healthy adults is about 50–60% (range 45 to 75%) of total body weight; women ...
to flow into them but not out. Lymph capillaries have a greater internal
oncotic pressure Oncotic pressure, or colloid osmotic-pressure, is a type of osmotic pressure induced by the plasma proteins, notably albumin, in a blood vessel's plasma (or any other body fluid such as blood and lymph) that causes a pull on fluid back into ...
than blood capillaries, due to the greater concentration of
plasma proteins Plasma proteins, sometimes referred to as blood proteins, are proteins present in blood plasma. They perform many different functions, including transport of hormones, vitamins and minerals in activity and functioning of the immune system. Other b ...
in the
lymph Lymph () is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated. At the ori ...
.


Types

Blood capillaries are categorized into three types: continuous, fenestrated, and sinusoidal (also known as discontinuous).


Continuous

Continuous capillaries are continuous in the sense that the endothelial cells provide an uninterrupted lining, and they only allow smaller
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s, such as water and
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s, to pass through their
intercellular cleft An intercellular cleft is a channel between two cells through which molecules may travel and gap junctions and tight junctions may be present. Most notably, intercellular clefts are often found between epithelial cells and the endothelium of blood ...
s. Lipid-soluble molecules can passively diffuse through the endothelial cell membranes along concentration gradients. Continuous capillaries can be further divided into two subtypes: :# Those with numerous transport vesicles, which are found primarily in
skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...
s, fingers,
gonad A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...
s, and skin. :# Those with few vesicles, which are primarily found in the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...
. These capillaries are a constituent of the
blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable membrane, semipermeable border of endothelium, endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system ...
.


Fenestrated

Fenestrated capillaries have pores known as ''fenestrae'' (
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
for "windows") in the endothelial cells that are 60–80 
nanometre 330px, Different lengths as in respect to the Molecule">molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm), or nanometer (American spelling), is a unit of length ...
s (nm) in diameter. They are spanned by a diaphragm of radially oriented
fibril Fibrils () are structural biological materials found in nearly all living organisms. Not to be confused with fibers or protein filament, filaments, fibrils tend to have diameters ranging from 10 to 100 nanometers (whereas fibers are micro to ...
s that allows small molecules and limited amounts of protein to diffuse. In the
renal glomerulus The glomerulus (: glomeruli) is a network of small blood vessels (capillaries) known as a ''tuft'', located at the beginning of a nephron in the kidney. Each of the two kidneys contains about one million nephrons. The tuft is structurally suppor ...
the capillaries are wrapped in
podocyte foot processes Podocytes are cells in Bowman's capsule in the kidneys that wrap around capillaries of the glomerulus. Podocytes make up the epithelial lining of Bowman's capsule, the third layer through which filtration of blood takes place. Bowman's capsule ...
or pedicels, which have slit pores with a function analogous to the diaphragm of the capillaries. Both of these types of blood vessels have continuous
basal lamina The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often incorrectly referred to as the basement membrane, though it does constitute a portion of the basement membrane. The b ...
e and are primarily located in the
endocrine gland The endocrine system is a network of glands and organs located throughout the body. Along with the nervous system, it makes the neuroendocrine system, which controls and regulates many of the body's functions. Endocrine glands are ductless gland ...
s,
intestines The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. ...
,
pancreas The pancreas (plural pancreases, or pancreata) is an Organ (anatomy), organ of the Digestion, digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity, abdomen behind the stomach and functions as a ...
, and the glomeruli of the
kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...
.


Sinusoidal

Sinusoidal capillaries or discontinuous capillaries are a special type of open-pore capillary, also known as a ''sinusoid'', that have wider fenestrations that are 30–40 
micrometre The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...
s (μm) in diameter, with wider openings in the endothelium. Fenestrated capillaries have diaphragms that cover the pores whereas sinusoids lack a diaphragm and just have an open pore. These types of blood vessels allow
red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–750 nanometres. It is a primary color in the RGB color model and a seconda ...
and
white blood cell White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood cells are genera ...
s (7.5 μm – 25 μm diameter) and various serum proteins to pass, aided by a discontinuous basal lamina. These capillaries lack pinocytotic vesicles, and therefore use gaps present in cell junctions to permit transfer between endothelial cells, and hence across the membrane. Sinusoids are irregular spaces filled with blood and are mainly found in the
liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...
,
bone marrow Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...
,
spleen The spleen (, from Ancient Greek '' σπλήν'', splḗn) is an organ (biology), organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The spleen plays important roles in reg ...
, and brain
circumventricular organs Circumventricular organs (CVOs) (wiktionary:circum-, circum-: around ; ventricular: of Ventricular system, ventricle) are structures in the brain characterized by their extensive and highly vascular permeability, permeable capillary, capillarie ...
.


Development

During early
embryonic development In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...
, new capillaries are formed through
vasculogenesis Vasculogenesis is the process of blood vessel formation, occurring by a ''De novo synthesis, de novo'' production of endothelial cells. It is the first stage of the formation of the vascular network, closely followed by angiogenesis. Process ...
, the process of
blood vessel Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...
formation that occurs through a novel production of
endothelial cell The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and th ...
s that then form vascular tubes. The term ''
angiogenesis Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature mainly by processes of sprouting and ...
'' denotes the formation of new capillaries from pre-existing blood vessels and already-present endothelium which divides. The small capillaries lengthen and interconnect to establish a network of vessels, a primitive vascular network that vascularises the entire
yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...
,
connecting stalk The connecting stalk, or body stalk, is an embryonic structure that is formed by the third week of development and connects the embryo to its shell of trophoblasts. The connecting stalk is derived from the extraembryonic mesoderm. Initially it li ...
, and
chorionic villi Chorionic villi are Wiktionary:villus, villi that sprout from the chorion to provide maximal contact area with maternal blood. They are an essential element in pregnancy from a histology, histomorphologic perspective, and are, by definition, a pr ...
.


Function

The capillary wall performs an important function by allowing nutrients and waste substances to pass across it. Molecules larger than 3 nm such as
albumin Albumin is a family of globular proteins, the most common of which are the serum albumins. All of the proteins of the albumin family are water- soluble, moderately soluble in concentrated salt solutions, and experience heat denaturation. Alb ...
and other large proteins pass through
transcellular transport Transcellular transport involves the transportation of solutes by a cell ''through'' a cell. Transcellular transport can occur in three different ways active transport, passive transport, and transcytosis. Active Transport Active transport ...
carried inside
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle In a neuron, synaptic vesicles (or neurotransmitter vesicles) s ...
, a process which requires them to go through the cells that form the wall. Molecules smaller than 3 nm such as water and gases cross the capillary wall through the space between cells in a process known as
paracellular transport Paracellular transport refers to the transfer of substances across an epithelium by passing through the intercellular space ''between'' the cells. It is in contrast to transcellular transport, where the substances travel ''through'' the cell, pas ...
. These transport mechanisms allow bidirectional exchange of substances depending on
osmotic Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region o ...
gradients. Capillaries that form part of the
blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable membrane, semipermeable border of endothelium, endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system ...
only allow for transcellular transport as
tight junction Tight junctions, also known as occluding junctions or ''zonulae occludentes'' (singular, ''zonula occludens''), are multiprotein Cell junction, junctional complexes between epithelial cells, sealing and preventing leakage of solutes and water. Th ...
s between endothelial cells seal the paracellular space. Capillary beds may control their blood flow via
autoregulation Autoregulation is a process within many biological systems, resulting from an internal adaptive mechanism that works to adjust (or mitigate) that system's response to stimuli. While most systems of the body show some degree of autoregulation, i ...
. This allows an organ to maintain constant flow despite a change in central blood pressure. This is achieved by
myogenic response The myogenic mechanism is how arteries and arterioles react to an increase or decrease of blood pressure to keep the blood flow constant within the blood vessel. Myogenic response refers to a contraction initiated by the myocyte itself instead of ...
, and in the
kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...
by
tubuloglomerular feedback In the renal physiology, physiology of the kidney, tubuloglomerular feedback (TGF) is a feedback system inside the kidneys. Within each nephron, information from the nephron#Renal tubule, renal tubules (a downstream area of the tubular fluid) is si ...
. When blood pressure increases, arterioles are stretched and subsequently constrict (a phenomenon known as the Bayliss effect) to counteract the increased tendency for high pressure to increase blood flow. In the
lung The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...
s, special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise. When the
heart rate Heart rate is the frequency of the cardiac cycle, heartbeat measured by the number of contractions of the heart per minute (''beats per minute'', or bpm). The heart rate varies according to the body's Human body, physical needs, including the nee ...
increases and more blood must flow through the lungs, capillaries are recruited and are also distended to make room for increased blood flow. This allows blood flow to increase while resistance decreases. Extreme exercise can make capillaries vulnerable, with a breaking point similar to that of
collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...
. Capillary permeability can be increased by the release of certain
cytokine Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...
s,
anaphylatoxin Anaphylatoxins, or complement peptides, are fragments ( C3a, C4a and C5a) that are produced as part of the activation of the complement system. Complement components C3, C4 and C5 are large glycoproteins that have important functions in the immu ...
s, or other mediators (such as leukotrienes, prostaglandins, histamine, bradykinin, etc.) highly influenced by the
immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
.


Starling equation

The transport mechanisms can be further quantified by the
Starling equation The Starling principle holds that fluid movement across a semi-permeable blood vessel such as a capillary or small venule is determined by the hydrostatic pressures and colloid osmotic pressures ( oncotic pressure) on either side of a semiperm ...
. The Starling equation defines the forces across a semipermeable membrane and allows calculation of the net flux: :J_v = K_f P_c - P_i) - \sigma(\pi_c - \pi_i) where: : (P_c - P_i) - \sigma(\pi_c - \pi_i) is the net driving force, : K_f is the proportionality constant, and : J_v is the net fluid movement between compartments. By convention, outward force is defined as positive, and inward force is defined as negative. The solution to the equation is known as the net filtration or net fluid movement (''J''''v''). If positive, fluid will tend to ''leave'' the capillary (filtration). If negative, fluid will tend to ''enter'' the capillary (absorption). This equation has a number of important physiologic implications, especially when pathologic processes grossly alter one or more of the variables. According to Starling's equation, the movement of fluid depends on six variables: # Capillary
hydrostatic pressure Hydrostatics is the branch of fluid mechanics that studies fluids at hydrostatic equilibrium and "the pressure in a fluid or exerted by a fluid on an immersed body". The word "hydrostatics" is sometimes used to refer specifically to water and o ...
(''P''''c'') # Interstitial hydrostatic pressure (''P''''i'') # Capillary
oncotic pressure Oncotic pressure, or colloid osmotic-pressure, is a type of osmotic pressure induced by the plasma proteins, notably albumin, in a blood vessel's plasma (or any other body fluid such as blood and lymph) that causes a pull on fluid back into ...
(''c'') # Interstitial oncotic pressure (''i'') # Filtration coefficient (''K''''f'') # Reflection coefficient (''σ'')


Clinical significance

Disorders of capillary formation as a developmental defect or acquired disorder are a feature in many common and serious disorders. Within a wide range of cellular factors and cytokines, issues with normal genetic expression and bioactivity of the vascular growth and permeability factor
vascular endothelial growth factor Vascular endothelial growth factor (VEGF, ), originally known as vascular permeability factor (VPF), is a signal protein produced by many cells that stimulates the formation of blood vessels. To be specific, VEGF is a sub-family of growth factors ...
(VEGF) appear to play a major role in many of the disorders. Cellular factors include reduced number and function of bone-marrow derived
endothelial progenitor cell Endothelial progenitor cell (or EPC) is a term that has been applied to multiple different cell types that play roles in the regeneration of the endothelial lining of blood vessels. Outgrowth endothelial cells are an EPC subtype committed to end ...
s. and reduced ability of those cells to form blood vessels. * Formation of additional capillaries and larger blood vessels (angiogenesis) is a major mechanism by which a cancer may help to enhance its own growth. Disorders of retinal capillaries contribute to the pathogenesis of age-related
macular degeneration Macular degeneration, also known as age-related macular degeneration (AMD or ARMD), is a medical condition which may result in blurred vision, blurred or vision loss, no vision in the center of the visual field. Early on there are often no sym ...
. * Reduced capillary density (capillary rarefaction) occurs in association with cardiovascular
risk factor In epidemiology, a risk factor or determinant is a variable associated with an increased risk of disease or infection. Due to a lack of harmonization across disciplines, determinant, in its more widely accepted scientific meaning, is often use ...
s and in patients with
coronary heart disease Coronary artery disease (CAD), also called coronary heart disease (CHD), or ischemic heart disease (IHD), is a type of cardiovascular disease, heart disease involving Ischemia, the reduction of blood flow to the cardiac muscle due to a build-up ...
.


Therapeutics

Major diseases where altering capillary formation could be helpful include conditions where there is excessive or abnormal capillary formation such as cancer and disorders harming eyesight; and medical conditions in which there is reduced capillary formation either for familial or genetic reasons, or as an acquired problem. * In patients with the retinal disorder, neovascular age-related
macular degeneration Macular degeneration, also known as age-related macular degeneration (AMD or ARMD), is a medical condition which may result in blurred vision, blurred or vision loss, no vision in the center of the visual field. Early on there are often no sym ...
, local
anti-VEGF therapy Anti–vascular endothelial growth factor therapy, also known as anti-VEGF () therapy or medication, is the use of medications that block vascular endothelial growth factor. This is done in the treatment of certain cancers and in age-related macu ...
to limit the bio-activity of
vascular endothelial growth factor Vascular endothelial growth factor (VEGF, ), originally known as vascular permeability factor (VPF), is a signal protein produced by many cells that stimulates the formation of blood vessels. To be specific, VEGF is a sub-family of growth factors ...
has been shown to protect vision by limiting progression. In a wide range of cancers, treatment approaches have been studied, or are in development, aimed at decreasing tumour growth by reducing angiogenesis.


Blood sampling

Capillary blood sampling can be used to test for
blood glucose The blood sugar level, blood sugar concentration, blood glucose level, or glycemia is the measure of glucose concentrated in the blood. The body tightly blood sugar regulation, regulates blood glucose levels as a part of metabolic homeostasis ...
(such as in
blood glucose monitoring Blood glucose monitoring is the use of a glucose meter for testing the concentration of glucose in the blood (Blood sugar level, glycemia). Particularly important in diabetes management, a blood glucose test is typically performed by piercing ...
),
hemoglobin Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin ...
, pH and lactate. It is generally performed by creating a small cut using a
blood lancet A blood lancet, or simply lancet, is a small medical implement used for capillary blood sampling. A blood lancet, sometimes called a lance, is similar to a scalpel style lancet, but with a double-edged blade and a pointed end. It can even be a ...
, followed by sampling by
capillary action Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of external forces like Gravitation, gravity. The effe ...
on the cut with a test strip or small
pipette A pipette (sometimes spelled as pipet) is a type of laboratory tool commonly used in chemistry and biology to transport a measured volume of liquid, often as a media dispenser. Pipettes come in several designs for various purposes with differing ...
. It is also used to test for
sexually transmitted infections A sexually transmitted infection (STI), also referred to as a sexually transmitted disease (STD) and the older term venereal disease (VD), is an infection that is spread by sexual activity, especially vaginal intercourse, anal sex, or ...
that are present in the blood stream, such as
HIV The human immunodeficiency viruses (HIV) are two species of '' Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the im ...
,
syphilis Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms depend on the stage it presents: primary, secondary, latent syphilis, latent or tertiary. The prim ...
, and hepatitis B and C, where a finger is lanced and a small amount of blood is sampled into a
test tube A test tube, also known as a culture tube or sample tube, is a common piece of laboratory glassware consisting of a finger-like length of glass or clear plastic tubing, open at the top and closed at the bottom. Test tubes are usually placed in s ...
.


History

A 13th century manuscript by Ibn Nafis contains the earliest known description of capillaries. The manuscript records Ibn Nafis' prediction of the existence of the capillaries which he described as perceptible passages (''manafidh'') between pulmonary artery and pulmonary vein. These passages would later be identified by
Marcello Malpighi Marcello Malpighi (10 March 1628 – 30 November 1694) was an Italians, Italian biologist and physician, who is referred to as the "founder of microscopical anatomy, histology and father of physiology and embryology". Malpighi's name is borne by ...
as capillaries. He further states that the heart's two main chambers (right and left ventricles) are separate and that blood cannot pass through the (interventricular) septum.
William Harvey William Harvey (1 April 1578 – 3 June 1657) was an English physician who made influential contributions to anatomy and physiology. He was the first known physician to describe completely, and in detail, pulmonary and systemic circulation ...
did not explicitly predict the existence of capillaries, but he saw the need for some sort of connection between the arterial and venous systems. In 1653, he wrote, "...the blood doth enter into every member through the arteries, and does return by the veins, and that the veins are the vessels and ways by which the blood is returned to the heart itself; and that the blood in the members and extremities does pass from the arteries into the veins (either mediately by an anastomosis, or immediately through the porosities of the flesh, or both ways) as before it did in the heart and thorax out of the veins, into the arteries..."
Marcello Malpighi Marcello Malpighi (10 March 1628 – 30 November 1694) was an Italians, Italian biologist and physician, who is referred to as the "founder of microscopical anatomy, histology and father of physiology and embryology". Malpighi's name is borne by ...
was the first to observe directly and correctly describe capillaries, discovering them in a frog's lung 8 years later, in 1661.
August Krogh Schack August Steenberg Krogh (15 November 1874 – 13 September 1949) was a Danish professor at the department of zoophysiology at the University of Copenhagen from 1916 to 1945. He contributed a number of fundamental discoveries within seve ...
discovered how capillaries provide nutrients to animal tissue. For his work he was awarded the 1920
Nobel Prize in Physiology or Medicine The Nobel Prize in Physiology or Medicine () is awarded yearly by the Nobel Assembly at the Karolinska Institute for outstanding discoveries in physiology or medicine. The Nobel Prize is not a single prize, but five separate prizes that, acco ...
. His 1922 estimate that total length of capillaries in a human body is as long as 100,000 km, had been widely adopted by textbooks and other secondary sources. This estimate was based on figures he gathered from "an extraordinarily large person". More recent estimates give a number between 9,000 and 19,000 km.


See also

* * * *


References


External links

*
The Microcirculatory Society, Inc.The Histology Guide – Capillaries
{{Authority control Angiology Cardiovascular system anatomy Phlebotomy