A capillary is a small blood vessel from 5 to 10 micrometres (µm) in diameter, and having a wall one endothelial cell thick. They are the smallest blood vessels in the body: they convey blood between the arterioles and venules. These microvessels are the site of exchange of many substances with the interstitial fluid surrounding them. Substances which exit include water (proximal portion), oxygen, and glucose; substances which enter include water (distal portion), carbon dioxide, uric acid, lactic acid, urea and creatinine. Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in the microcirculation.
During early embryonic development new capillaries are formed through vasculogenesis, the process of blood vessel formation that occurs through a de novo production of endothelial cells which then form vascular tubes. The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels and already present endothelium which divides.
Transmission electron microscope image of a cross-section of a capillary occupied by a red blood cell.
|Pronunciation||US: /ˈkæpəlɛri/, UK: /kəˈpɪləri/|
Blood flows from the heart through arteries, which branch and narrow into arterioles, and then branch further into capillaries where nutrients and wastes are exchanged. The capillaries then join and widen to become venules, which in turn widen and converge to become veins, which then return blood back to the heart through the venae cavae.
Individual capillaries are part of the capillary bed, an interweaving network of capillaries supplying tissues and organs. The more metabolically active a tissue is, the more capillaries are required to supply nutrients and carry away waste products. There are two types of capillaries: true capillaries, which branch from arterioles and provide exchange between tissue and the capillary blood, and metarterioles, found only in the mesenteric circulation. They are short vessels that directly connect the arterioles and venules at opposite ends of the beds. Metarterioles are found primarily in the mesenteric microcirculation. The physiological mechanisms underlying precapillary resistance is no longer considered to be a result of precapillary sphincters outside of the mesentery organ.
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 to flow into them but not out. Lymph capillaries have a greater internal oncotic pressure than blood capillaries, due to the greater concentration of plasma proteins in the lymph.
There are three types of blood capillaries:
Continuous capillaries are continuous in the sense that the endothelial cells provide an uninterrupted lining, and they only allow smaller molecules, such as water and ions to pass through their intercellular clefts. Lipid-soluble molecules can passively diffuse through the endothelial cell membranes along concentration gradients. Continuous capillaries can be further divided into two subtypes:
Fenestrated (derived from fenestra, Latin for "window") capillaries have pores in the endothelial cells (60–80 nm in diameter) that are spanned by a diaphragm of radially oriented fibrils and allow small molecules and limited amounts of protein to diffuse. In the renal glomerulus there are cells with no diaphragms, called podocyte foot processes 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 laminae and are primarily located in the endocrine glands, intestines, pancreas, and the glomeruli of the kidney.
Sinusoid capillaries (also known as a discontinuous) are a special type of open-pore capillary, that have larger openings (30–40 µm in diameter) in the endothelium. These types of blood vessels allow red and white blood cells (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 utilize gaps present in cell junctions to permit transfer between endothelial cells, and hence across the membrane. Sinusoid blood vessels are primarily located in the bone marrow, lymph nodes, and adrenal glands. Some sinusoids are distinctive in that they do not have the tight junctions between cells. They are called discontinuous sinusoidal capillaries, and are present in the liver and spleen, where greater movement of cells and materials is necessary. A capillary wall is only 1 cell thick and is simple squamous epithelium.
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 and other large proteins pass through transcellular transport carried inside vesicles, a process which requires them to go through the cells that form the wall. Molecules smaller than 3 nm such as water, ions and gases cross the capillary wall through the space between cells in a process known as paracellular transport. These transport mechanisms allow bidirectional exchange of substances depending on osmotic gradients and can be further quantified by the Starling equation. Capillaries that form part of the blood–brain barrier however only allow for transcellular transport as tight junctions between endothelial cells seal the paracellular space.
Capillary beds may control their blood flow via autoregulation. This allows an organ to maintain constant flow despite a change in central blood pressure. This is achieved by myogenic response, and in the kidney by tubuloglomerular feedback. 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 lungs special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise. When the heart rate 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.
Capillary permeability can be increased by the release of certain cytokines, anaphylatoxins, or other mediators (such as leukotrienes, prostaglandins, histamine, bradykinin, etc.) highly influenced by the immune system.
The Starling equation defines the forces across a semipermeable membrane and allows calculation of the net flux:
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 (Jv). 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:
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 (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 cells. and reduced ability of those cells to form blood vessels.
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.
Contrary to a popular misconception, William Harvey did not explicitly predict the existence of capillaries, but he clearly saw the need for some sort of connection between the arterial and venous systems. 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…"
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 by processes of sprouting and splitting. Vasculogenesis is the embryonic formation of endothelial cells from mesoderm cell precursors, and from neovascularization, although discussions are not always precise (especially in older texts). The first vessels in the developing embryo form through vasculogenesis, after which angiogenesis is responsible for most, if not all, blood vessel growth during development and in disease.Angiogenesis is a normal and vital process in growth and development, as well as in wound healing and in the formation of granulation tissue. However, it is also a fundamental step in the transition of tumors from a benign state to a malignant one, leading to the use of angiogenesis inhibitors in the treatment of cancer. The essential role of angiogenesis in tumor growth was first proposed in 1971 by Judah Folkman, who described tumors as "hot and bloody," illustrating that, at least for many tumor types, flush perfusion and even hyperemia are characteristic.Blood–air barrier
The blood–air barrier (alveolar–capillary barrier or membrane) exists in the gas exchanging region of the lungs. It exists to prevent air bubbles from forming in the blood, and from blood entering the alveoli. It is formed by the type 1 pneumocytes of the alveolar wall, the endothelial cells of the capillaries and the basement membrane between the two cells. The barrier is permeable to molecular oxygen, carbon dioxide, carbon monoxide and many other gases.Blood–brain barrier
The blood–brain barrier (BBB) is a highly selective semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). The blood–brain barrier is formed by endothelial cells of the capillary wall, astrocyte end-feet ensheathing the capillary, and pericytes embedded in the capillary basement membrane. This system allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function. Specialized structures participating in sensory and secretory integration within neural circuits – the circumventricular organs and choroid plexus – do not have a blood–brain barrier.
The blood–brain barrier restricts the diffusion of solutes in the blood (e.g., bacteria) and large or hydrophilic molecules into the cerebrospinal fluid (CSF), while allowing the diffusion of hydrophobic molecules (O2, CO2, hormones) and small polar molecules. Cells of the barrier actively transport metabolic products such as glucose across the barrier using specific transport proteins.Capillary action
Capillary action (sometimes capillarity, capillary motion, capillary effect, or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied carbon fiber, or in a cell. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container wall act to propel the liquid.Capillary electrophoresis
Capillary electrophoresis (CE) is a family of electrokinetic separation methods performed in submillimeter diameter capillaries and in micro- and nanofluidic channels. Very often, CE refers to capillary zone electrophoresis (CZE), but other electrophoretic techniques including capillary gel electrophoresis (CGE), capillary isoelectric focusing (CIEF), capillary isotachophoresis and micellar electrokinetic chromatography (MEKC) belong also to this class of methods. In CE methods, analytes migrate through electrolyte solutions under the influence of an electric field. Analytes can be separated according to ionic mobility and/or partitioning into an alternate phase via non-covalent interactions. Additionally, analytes may be concentrated or "focused" by means of gradients in conductivity and pH.Capillary electrophoresis–mass spectrometry
Capillary electrophoresis–mass spectrometry (CE-MS) is an analytical chemistry technique formed by the combination of the liquid separation process of capillary electrophoresis with mass spectrometry. CE-MS combines advantages of both CE and MS to provide high separation efficiency and molecular mass information in a single analysis. It has high resolving power and sensitivity, requires minimal volume and can analyze at high speed. Ions are typically formed by electrospray ionization, but they can also be formed by matrix-assisted laser desorption/ionization or other ionization techniques. It has applications in basic research in proteomics and quantitative analysis of biomolecules as well as in clinical medicine.
Since its introduction in 1987, new developments and application has made CE-MS powerful separation and identification technique. Use of CE-MS has increased for protein and peptides analysis and other biomolecules. However, the development of online CE-MS is not without challenges. Understanding of CE, the interface setup, ionization technique and mass detection system is important to tackle problems while coupling capillary electrophoresis to mass spectrometry.Capillary hemangioma
A capillary hemangioma (also known as an Infantile hemangioma, Strawberry hemangioma, and Strawberry nevus) is the most common variant of hemangioma which appears as a raised, red, lumpy area of flesh anywhere on the body, though 83% occur on the head or neck area. These marks occur in about 10% of all births, and usually appear between one and four weeks after birth. It may grow rapidly, before stopping and slowly fading. Some are gone by the age of 2, about 60% by 5 years, and 90–95% by 9 years. Capillary hemangioma is a vascular anomaly.
Capillary hemangiomas occur 5 times more often in female infants than in males, and mostly in Caucasian populations. Additionally, low birthweight infants have a 26% chance of developing a hemangioma.It is the most common tumor of orbit and periorbital areas in childhood. It may occur in the skin, subcutaneous tissues and mucous membranes of oral cavities and lips as well as in the liver, spleen and kidneys. While this birthmark may be alarming in appearance, physicians generally counsel that it be left to disappear on its own, unless it is in the way of vision or blocking the nostrils.Capillary lamina of choroid
The capillary lamina of choroid or choriocapillaris is a layer of capillaries that is immediately adjacent to Bruch's membrane in the choroid.Capillary leak syndrome
Capillary leak syndrome is characterized by the escape of blood plasma through capillary walls, from the blood circulatory system to surrounding tissues, muscle compartments, organs or body cavities. It is a phenomenon most commonly witnessed in sepsis, and less frequently in autoimmune diseases, differentiation syndrome, engraftment syndrome, hemophagocytic lymphohistiocytosis, the ovarian hyperstimulation syndrome, viral hemorrhagic fevers, and snakebite and ricin poisoning. Pharmaceuticals, including the chemotherapy medications gemcitabine and tagraxofusp, as well as certain interleukins and monoclonal antibodies, can also cause capillary leaks. These conditions and factors are sources of secondary capillary leak syndrome.
Systemic capillary leak syndrome (SCLS, or Clarkson's disease), or primary capillary leak syndrome, is a rare, grave and episodic medical condition observed largely in otherwise healthy individuals mostly in middle age. It is characterized by self-reversing episodes during which the endothelial cells which line the capillaries, usually of the extremities, separate for one to three days, causing a leakage of plasma mainly into the muscle compartments of the arms and legs. The abdomen, the central nervous system, and the organs (including the lungs) are typically spared, but the extravasation in the extremities is sufficiently massive to cause circulatory shock and compartment syndromes, with a dangerous hypotension (low blood pressure), hemoconcentration (thickening of the blood) and hypoalbuminemia (drop in albumin, a major protein) in the absence of other causes for such abnormalities. SCLS is thus a limb- and life-threatening illness, because each episode has the potential to cause damage to limb muscles and nerves, as well as to vital organs due to limited perfusion. It is often misdiagnosed as polycythemia, polycythemia vera, hyperviscosity syndrome, or sepsis.Capillary number
In fluid dynamics, the capillary number (Ca) represents the relative effect of viscous drag forces versus surface tension forces acting across an interface between a liquid and a gas, or between two immiscible liquids. For example, an air bubble in a liquid flow tends to be deformed by the friction of the liquid flow due to viscosity effects, but the surface tension forces tend to minimize the surface. The capillary number is defined as:
where µ is the dynamic viscosity of the liquid, V is a characteristic velocity and is the surface tension or interfacial tension between the two fluid phases.
The capillary number is a dimensionless quantity, hence its value does not depend on the system of units. In the petroleum industry, capillary number is denoted instead of .
For low capillary numbers (a rule of thumb says less than 10−5), flow in porous media is dominated by capillary forces whereas for high capillary number the capillary forces are negligible compared to the viscous forces. Flow through the pores in an oil field reservoir have capillary number on the order of 10−6, whereas flow of oil through an oil well drill pipe has a capillary number on the order of 1.
The capillary number plays a role in the dynamics of capillary flow, in particular it governs the dynamic contact angle of a flowing droplet at an interface.Capillary wave
A capillary wave is a wave traveling along the phase boundary of a fluid, whose dynamics and phase velocity are dominated by the effects of surface tension.
Capillary waves are common in nature, and are often referred to as ripples. The wavelength of capillary waves on water is typically less than a few centimeters, with a phase speed in excess of 0.2–0.3 meter/second.
A longer wavelength on a fluid interface will result in gravity–capillary waves which are influenced by both the effects of surface tension and gravity, as well as by fluid inertia. Ordinary gravity waves have a still longer wavelength.
When generated by light wind in open water, a nautical name for them is cat's paw waves, since they may resemble paw prints. Light breezes which stir up such small ripples are also sometimes referred to as cat's paws. On the open ocean, much larger ocean surface waves (seas and swells) may result from coalescence of smaller wind-caused ripple-waves.Extracellular fluid
Extracellular fluid (ECF) denotes all body fluid outside the cells. Total body water in humans makes up between 45 to 75% of total body weight. About two thirds of this is intracellular fluid within cells, and one third is the extracellular fluid. The main component of the extracellular fluid is the interstitial fluid that bathes cells.
Extracellular fluid is the internal environment of all multicellular animals, and in those animals with a blood circulatory system a proportion of this fluid is blood plasma. Plasma and interstitial fluid are the two components that make up at least 97% of the ECF. Lymph makes up a small percentage of the interstitial fluid. The remaining small portion of the ECF includes the transcellular fluid (about 2.5%). The ECF can also be seen as having two components – plasma and lymph as a delivery system, and interstitial fluid for water and solute exchange with the cells.The extracellular fluid, in particular the interstitial fluid, constitutes the body's internal environment that bathes all of the cells in the body. The ECF composition is therefore crucial for their normal functions, and is maintained by a number of homeostatic mechanisms involving negative feedback. Homeostasis regulates, among others, the pH, sodium, potassium, and calcium concentrations in the ECF. The volume of body fluid, blood glucose, oxygen, and carbon dioxide levels are also tightly homeostatically maintained.
The volume of extracellular fluid in a young adult male of 70 kg (154 lbs) is 20% of body weight – about fourteen litres. Eleven litres is interstitial fluid and the remaining three litres is plasma.Glomerulus (kidney)
The glomerulus (), plural glomeruli, is a network of capillaries known as a tuft, located at the beginning of a nephron in the kidney. The tuft is structurally supported by intraglomerular mesangial cells. The blood is filtered across the capillary walls of this tuft through the glomerular filtration barrier, which yields its filtrate of water and soluble substances to a cup-like sac known as Bowman's capsule. The filtrate then enters the renal tubule, of the nephron.The glomerulus receives its blood supply from an afferent arteriole of the renal arterial circulation. Unlike most capillary beds, the glomerular capillaries exit into efferent arterioles rather than venules. The resistance of the efferent arterioles causes sufficient hydrostatic pressure within the glomerulus to provide the force for ultrafiltration.
The glomerulus and its surrounding Bowman's capsule constitute a renal corpuscle, the basic filtration unit of the kidney. The rate at which blood is filtered through all of the glomeruli, and thus the measure of the overall renal function, is the glomerular filtration rate (GFR).Liver sinusoid
A liver sinusoid is a type of capillary known as a sinusoid capillary, or discontinuous capillary that is similar to a fenestrated capillary, having discontinuous endothelium that serves as a location for mixing of the oxygen-rich blood from the hepatic artery and the nutrient-rich blood from the portal vein.The liver sinusoid has a larger caliber than other types of capillaries and has a lining of specialised endothelial cells known as the liver sinusoidal endothelial cells (LSECs), and Kupffer cells. The cells are porous and have a scavenging function. The LSECs make up around half of the non-parenchymal cells in the liver and are flattened and fenestrated. LSECs have many fenestrae that gives easy communication between the sinusoidal lumen and the space of Disse. They play a part in filtration, endocytosis, and in the regulation of blood flow in the sinusoids.The Kuppfer cells can take up and destroy foreign material such as bacteria. Hepatocytes are separated from the sinusoids by the space of Disse. Hepatic stellate cells are present in the space of Disse and are involved in scar formation in response to liver damage.
Defenestration also known as capillarisation happens when LSECs are lost rendering the sinusoid as an ordinary capillary. This process precedes fibrosis.Millerite
Millerite is a nickel sulfide mineral, NiS. It is brassy in colour and has an acicular habit, often forming radiating masses and furry aggregates. It can be distinguished from pentlandite by crystal habit, its duller colour, and general lack of association with pyrite or pyrrhotite.Port-wine stain
A port-wine stain (nevus flammeus), also commonly called a firemark, is a discoloration of the human skin caused by a vascular anomaly (a capillary malformation in the skin). They are so named for their coloration, which is similar in color to port wine, a fortified red wine from Portugal.
A port-wine stain is vascular malformation, seen at birth. Port-wine stains always persist throughout life. The area of skin affected grows in proportion to general growth.
Port-wine stains occur most often on the face but can appear anywhere on the body, particularly on the neck, upper trunk, arms and legs. Early stains are usually flat and pink in appearance. As the child matures, the color may deepen to a dark red or purplish color. In adulthood, thickening of the lesion or the development of small lumps may occur.Port-wine stains may be part of a syndrome such as Sturge–Weber syndrome or Klippel–Trénaunay–Weber syndrome.Pulmonary wedge pressure
The pulmonary wedge pressure or PWP, or cross-sectional pressure (also called the pulmonary arterial wedge pressure or PAWP, pulmonary capillary wedge pressure or PCWP, or pulmonary artery occlusion pressure or PAOP), is the pressure measured by wedging a pulmonary catheter with an inflated balloon into a small pulmonary arterial branch. It estimates the left atrial pressure.
Pulmonary venous wedge pressure (PVWP) is not synonymous with the above; PVWP has been shown to correlate with pulmonary artery pressures in studies, albeit unreliably.
Physiologically, distinctions can be drawn among pulmonary artery pressure, pulmonary capillary wedge pressure, pulmonary venous pressure and left atrial pressure, but not all of these can be measured in a clinical context.Noninvasive estimation techniques have been proposed.Pyogenic granuloma
Pyogenic granuloma (also known as a "eruptive hemangioma", "granulation tissue-type hemangioma", "granuloma gravidarum", "lobular capillary hemangioma", "pregnancy tumor", and "tumor of pregnancy") is a vascular lesion that occurs on both mucosa and skin, and appears as an overgrowth of tissue due to irritation, physical trauma, or hormonal factors. It is often found to involve the gums, the skin and nasal septum, and has also been found far from the head such as in the thigh.Pyogenic granulomas may be seen at any age, and are more common in females than males. In pregnant women, lesions may occur in the first trimester with an increasing incidence up until the seventh month, and are often seen on the gums.Vascular permeability
Vascular permeability, often in the form of capillary permeability or microvascular permeability, characterizes the capacity of a blood vessel wall to allow for the flow of small molecules (drugs, nutrients, water, ions) or even whole cells (lymphocytes on their way to the site of inflammation) in and out of the vessel. Blood vessel walls are lined by a single layer of endothelial cells. The gaps between endothelial cells (cell junctions) are strictly regulated depending on the type and physiological state of the tissue.
There are several techniques to measure vascular permeability to certain molecules. For instance, the cannulation of a single microvessel with a micropipette, the microvessel is perfused with a certain pressure, occluded downstream and then the velocity of some cells will be related to the permeability. Another technique uses multiphoton fluorescence intravital microscopy through which the flow is related to fluorescence intensity and the permeability is estimated from the Patlak transformation of the intensity data In cancer research, the study of permeability of the microvasculature that surrounds tumours is of great interest as the vascular wall is a barrier of large molecules into the tumours, the vessels control the microenvironment which affect tumour progression and changes to the permeability may indicate vascular damage with drugs.An example of increased vascular permeability is in the initial lesion of periodontal disease, in which the gingival plexus becomes engorged and dilated, allowing large numbers of neutrophils to extravasate and appear within the junctional epithelium and underlying connective tissue.