Capillary
Overview
Capillaries are the smallest of a body's blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s and are parts of the microcirculation
Microcirculation
The microcirculation is a term used to describe the small vessels in the vasculature which are embedded within organs and are responsible for the distribution of blood within tissues; as opposed to larger vessels in the macrocirculation which transport blood to and from the organs...

. They are only 1 cell thick. These microvessels, measuring 5-10 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 in diameter, connect arteriole
Arteriole
An arteriole is a small diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries.Arterioles have muscular walls and are the primary site of vascular resistance...

s and venule
Venule
A venule is a very small blood vessel in the microcirculation that allows deoxygenated blood to return from the capillary beds to the larger blood vessels called veins. Venules range from 8 to 100μm in diameter and are formed when capillaries unite .Venules are blood vessels that drain blood...

s, and enable the exchange of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, and many other nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...

 and waste
Waste
Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...

 chemical substances between blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 and surrounding tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

s. During embryological development, new capillaries are formed by vasculogenesis
Vasculogenesis
Vasculogenesis is the process of blood vessel formation occurring by a de novo production of endothelial cells.-Process:Though similar to angiogenesis, the two are different in one aspect: The term angiogenesis denotes the formation of new blood vessels from pre-existing ones, whereas...

, the process of blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

 formation occurring by a de novo
De novo
In general usage, de novo is a Latin expression meaning "from the beginning," "afresh," "anew," "beginning again." It is used in:* De novo transcriptome assembly, the method of creating a transcriptome without a reference genome...

production of endothelial cells and their formation into vascular tubes.
Encyclopedia
Capillaries are the smallest of a body's blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s and are parts of the microcirculation
Microcirculation
The microcirculation is a term used to describe the small vessels in the vasculature which are embedded within organs and are responsible for the distribution of blood within tissues; as opposed to larger vessels in the macrocirculation which transport blood to and from the organs...

. They are only 1 cell thick. These microvessels, measuring 5-10 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 in diameter, connect arteriole
Arteriole
An arteriole is a small diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries.Arterioles have muscular walls and are the primary site of vascular resistance...

s and venule
Venule
A venule is a very small blood vessel in the microcirculation that allows deoxygenated blood to return from the capillary beds to the larger blood vessels called veins. Venules range from 8 to 100μm in diameter and are formed when capillaries unite .Venules are blood vessels that drain blood...

s, and enable the exchange of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, and many other nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...

 and waste
Waste
Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...

 chemical substances between blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 and surrounding tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

s. During embryological development, new capillaries are formed by vasculogenesis
Vasculogenesis
Vasculogenesis is the process of blood vessel formation occurring by a de novo production of endothelial cells.-Process:Though similar to angiogenesis, the two are different in one aspect: The term angiogenesis denotes the formation of new blood vessels from pre-existing ones, whereas...

, the process of blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

 formation occurring by a de novo
De novo
In general usage, de novo is a Latin expression meaning "from the beginning," "afresh," "anew," "beginning again." It is used in:* De novo transcriptome assembly, the method of creating a transcriptome without a reference genome...

production of endothelial cells and their formation into vascular tubes. The term angiogenesis
Angiogenesis
Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...

 denotes the formation of new capillaries from pre-existing blood vessels.

Anatomy

Blood flows away from the heart via arteries, which branch and narrow into the arterioles, and then branch further still into the capillaries. After the tissue has been perfused, capillaries join and widen to become venules and then widen more to become veins, which return blood to the heart.

Capillaries do not function on their own. The "capillary bed" is an interweaving network of capillaries supplying an organ. The more metabolically
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 active the cells, the more capillaries they will require to supply nutrients and carry away waste products.

A capillary bed can consist of two types of vessels: true capillaries which branch mainly from metarterioles and provide exchange between cells and the circulation. Secondly, capillary beds also consist of a vascular shunt which is a short vessel that directly connects the arteriole and venule at opposite ends of the bed.

Metarteriole
Metarteriole
A Metarteriole is a short vessel that links arterioles and venules. Instead of a continuous tunica media, they have individual smooth muscle cells placed a short distance apart, each forming a precapillary sphincter that encircles the entrance to that capillary bed...

s provide direct communication between arterioles and venules and are important in bypassing the bloodflow through the capillaries. The internal diameter of 8 μm forces the red blood cell
Blood cell
A blood cell, also called a hematocyte, is a cell normally found in blood. In mammals, these fall into three general categories:* red blood cells — Erythrocytes* white blood cells — Leukocytes* platelets — Thrombocytes...

s to partially fold into bullet-like shapes and to go into single file in order for them to pass through.

Precapillary sphincter
Precapillary sphincter
The precapillary sphincter is a band of smooth muscle that adjusts the blood flow into each capillary. At the point where each true capillary originates from a metarteriole, a smooth muscle fiber usually encircles the capillary. This is called the precapillary sphincter. This sphincter can open and...

s are rings of smooth muscle
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

s at the origin of true capillaries that regulate blood flow into true capillaries and thus control blood flow through a tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

.

Types

There are two main types of capillaries:
  • Continuous - They are continuous in the sense that the endothelial cells provide an uninterrupted lining, and only allow small molecules, like water and ions to diffuse through tight junctions which leave gaps of unjoined membrane which are called intercellular clefts. Tight junctions can be further divided into two subtypes:
  1. Those with numerous transport vesicles that are primarily found in skeletal muscles, finger, gonads, and skin.
  2. Those with few vesicles that are primarily found in the central nervous system
    Central nervous system
    The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

    . These capillaries are a constituent of the blood-brain-barrier.
    • Fenestrated - Fenestrated capillaries (derived from "fenestra," Latin
      Latin
      Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

       for "window") 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 an analogous function to the diaphragm of the capillaries. Both of these types of blood vessels have continuous basal lamina and are primarily located in the endocrine glands
      Endocrine glands
      Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into the blood rather than through a duct. The main endocrine glands include the pituitary gland, pancreas, ovaries, testes, thyroid gland, and adrenal glands. The hypothalamus is a neuroendocrine...

      , intestines, pancreas
      Pancreas
      The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...

      , and glomeruli of kidney
      Kidney
      The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

      .
    • Sinusoidal
      Sinusoid (blood vessel)
      A sinusoid is a small blood vessel similar to a capillary but with a fenestrated endothelium. Fenestrations are pores in the endothelial cells that greatly increase their permeability. In addition, permeability is increased by large inter-cellular clefts and fewer tight junctions...

      - Sinusoidal capillaries are a special type of fenestrated capillaries that have larger openings (30-40 μm in diameter) in the endothelium. These types of blood vessels allow red and white blood cells (7.5μm - 25μm diameter) and various serum
      Blood plasma
      Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

       proteins to pass using a process that is aided by a discontinuous basal lamina. These capillaries lack pinocytotic vesicles
      Pinocytosis
      In cellular biology, pinocytosis is a form of endocytosis in which small particles are brought into the cell—forming an invagination, and then suspended within small vesicles that subsequently fuse with lysosomes to hydrolyze, or to break down, the particles...

      , 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 gland. Some sinusoids are special, 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.

The membrane in the capillary is only 1 cell thick and is squamous epithelium.

Physiology

The capillary wall is a one-layer endothelium that allows gas and lipophilic molecules to pass through without the need for special transport mechanisms. This transport mechanism allows bidirectional diffusion depending on osmotic gradients and is further explained by the Starling equation
Starling equation
The Starling equation is an equation that illustrates the role of hydrostatic and oncotic forces in the movement of fluid across capillary membranes.Capillary fluid movement may occur as a result of three processes:...

.

Capillary beds may control their blood flow via autoregulation
Autoregulation
Autoregulation is a process within many biological systems, resulting from some internal adaptive mechanism that works to adjust the systems response to stimuli. While most systems of the body show some degree of autoregulation, it is most clearly observed in the kidney, the heart, and the brain...

. 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 the arterioles that lead to the capillaries bed are stretched and subsequently constrict 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
Vascular permeability
Vascular permeability, often in the form of capillary permeability, characterizes the capacity of a blood vessel wall to allow for the flow of small molecules or even whole cells in and out of the vessel. Blood vessel walls are lined by a single layer of endothelial cells...

 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
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

.

The Starling equation defines the forces across a semipermeable membrane and allows calculation of the net flux:


where:
  • is the net driving force,
  • is the proportionality constant, and
  • 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 (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.

The variables

According to Starling's equation, the movement of fluid depends on six variables:
  1. Capillary hydrostatic pressure ( Pc )
  2. Interstitial hydrostatic pressure ( Pi )
  3. Capillary oncotic pressure ( πz )
  4. Interstitial oncotic pressure ( πi )
  5. Filtration coefficient ( Kf )
  6. Reflection coefficient ( σ )


  • Note that oncotic pressure is not illustrated in the image.

Pathophysiology

Disorders of capillary formation as a developmental problem or acquired disorder are a feature in many common and serious disorders. Within a wide range of cellular factors and cytokines, problems with normal genetic expression and bioactivity of the vascular growth and permeability factor vascular endothelial growth factor
Vascular endothelial growth factor
Vascular endothelial growth factor is a signal protein produced by cells that stimulates vasculogenesis and angiogenesis. It is part of the system that restores the oxygen supply to tissues when blood circulation is inadequate....

 (VEGF) appear to play a major role in many of these disorders. Cellular factors include reduced numbers and function of bone-marrow derived endothelial progenitor cell
Endothelial progenitor cell
Endothelial progenitor cells are a population of rare cells that circulate in the blood with the ability to differentiate into endothelial cells, the cells that make up the lining of blood vessels. The process by which blood vessels are born de novo from endothelial progenitor cells is known as...

s. and reduced ability of those cells to form blood vessels.
  • Formation of additional capillaries and larger blood vessels (angiogenesis
    Angiogenesis
    Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...

    ) 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
    Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...

    .
  • Reduced capillary density (capillary rarefaction) occurs in association with cardiovascular risk factors
    Risk factors
    A risk factor is a concept in finance theory such as the CAPM, APT and other theories that use pricing kernels. In these models, the rate of return of an asset is a random variable whose realization in any time period is a linear combination of other random variables plus a disturbance term or...

     and in patients with coronary heart disease

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
    Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...

    , local anti-VEGF treatment to limit the bio-activity of vascular endothelial growth factor
    Vascular endothelial growth factor
    Vascular endothelial growth factor is a signal protein produced by cells that stimulates vasculogenesis and angiogenesis. It is part of the system that restores the oxygen supply to tissues when blood circulation is inadequate....

     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
    Angiogenesis
    Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...

    .

History

Ibn al-Nafis theorized a "premonition of the capillary circulation in his assertion that the pulmonary vein
Pulmonary vein
The pulmonary veins are large blood vessels that carry blood from the lungs to the left atrium of the heart. In humans there are four pulmonary veins, two from each lung...

 receives what comes out of the pulmonary artery
Pulmonary artery
The pulmonary arteries carry deoxygenated blood from the heart to the lungs. They are the only arteries that carry deoxygenated blood....

, this being the reason for the existence of perceptible passages between the two."

Park ji-sung
Park Ji-Sung
Park Ji-Sung is a South Korean footballer who plays as a midfielder for English club Manchester United. He was also the captain of the South Korean national team until his retirement from international football...

 was the first to observe and correctly describe capillaries when he discovered them in a frog's lung in 1661.

See also

  • Angiogenesis
    Angiogenesis
    Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...

  • Alveolar-capillary barrier
    Alveolar-capillary barrier
    The alveolar-capillary barrier 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...

  • Blood brain barrier
  • Capillary action
    Capillary action
    Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...

  • Hagen-Poiseuille equation
    Hagen-Poiseuille equation
    In fluid dynamics, the Hagen–Poiseuille equation is a physical law that gives the pressure drop in a fluid flowing through a long cylindrical pipe. The assumptions of the equation are that the flow is laminar viscous and incompressible and the flow is through a constant circular cross-section that...

  • Vasculogenesis
    Vasculogenesis
    Vasculogenesis is the process of blood vessel formation occurring by a de novo production of endothelial cells.-Process:Though similar to angiogenesis, the two are different in one aspect: The term angiogenesis denotes the formation of new blood vessels from pre-existing ones, whereas...


External links

  • {http://microcirc.org Microcirculatory Society, Inc}
  • {http://www.bishoujyunkan.co.jp/bisyoujyunkan1.htm Microcirculation Research Institute Ltd.(Japan)}
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK