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A red blood cell in a capillary, pancreatic tissue - TEM.jpg
Transmission electron microscope image of a capillary with a red blood cell within the pancreas. The capillary lining consists of long, thin endothelial cells, connected by tight junctions.
Capillary system CERT.jpg
A simplified illustration of a capillary network (not showing precapillary sphincters (which are only present in the mesenteric circulation).
CodeTH H3.
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A red blood cell in a capillary, pancreatic tissue - TEM.jpg
Transmission electron microscope image of a capillary with a red blood cell within the pancreas. The capillary lining consists of long, thin endothelial cells, connected by tight junctions.
Capillary system CERT.jpg
A simplified illustration of a capillary network (not showing precapillary sphincters (which are only present in the mesenteric circulation).
CodeTH H3.

Capillaries /ˈkæpɨlɛriz/ are the smallest of a body's blood vessels and are parts of its microcirculation. Their endothelial linings are only one cell layer thick. These microvessels, measuring around 5 to 10 micrometre in diameter, connect arterioles and venules, and they help to enable the exchange of water, oxygen, carbon dioxide, and many other nutrients and waste chemical substances between blood and the tissues[1] surrounding them. During embryological development,[2] new capillaries are formed through vasculogenesis, the process of blood vessel formation that occurs through a de novo production of endothelial cells followed by their forming into vascular tubes.[3] The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels and already present endothelium which divides.[4]


Simplified image showing flood-flow through the body, passing through capillary networks in its path.

Blood flows away from a body's heart via arteries, which branch and narrow into arterioles, and then branch further still into capillaries. After their tissues have been perfused 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 body's heart through the different great veins.

Capillaries do not function on their own, but instead in a capillary bed, an interweaving network of capillaries supplying organs and tissues. The more metabolically active a cell or environment is, the more capillaries are required to supply nutrients; and to carry away waste products. Capillary beds can consist of two types of vessels: true capillaries which branch from arterioles and provide exchange between cells and the blood, and vascular shunts or metarterioles (found mostly in the mesenteric microcirculation[5]), short vessels that directly connect the arterioles and venules at opposite ends of the bed. Metarterioles provide direct communication between arterioles and venules and are important in bypassing the bloodflow through the capillaries.[citation needed]


See also Sinusoid (blood vessel), Fenestra (histology) for individual articles.

There are three main types of capillaries:

Depiction of the major types of capillaries, showing fenestrations as well as intercellular gaps.


  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. These capillaries are a constituent of the blood brain barrier.



A capillary wall is only 1 cell thick and is simple squamous epithelium.[citation needed]


Diagram of a capillary

The capillary wall is a one-layer endothelium that allows gas and lipophilic molecules such as water and ions 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.

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.

Depiction of the filtration and reabsorption present in capillaries.

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] )


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:

  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 )


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.[8] and reduced ability of those cells to form blood vessels.[9]


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.

Blood sampling[edit]

Capillary blood sampling can be used to test for, for example, blood glucose (such as in blood glucose monitoring), hemoglobin, pH and lactate (the two latter can be quantified in fetal scalp blood testing to check the acid base status of a fetus during childbirth).

Capillary blood sampling is generally performed by creating a small cut using a blood lancet, followed by sampling by capillary action on the cut with a test strip or small pipe.


Ibn al-Nafis theorized a "premonition of the capillary circulation in his assertion that the pulmonary vein receives what exits the pulmonary artery, explaining the existence of perceptible passages between the two."[13][verification needed]

Marcello Malpighi was the first to observe and correctly describe capillaries, discovering them in a frog's lung in 1661.[14]

See also[edit]


  1. ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey: Prentice Hall. ISBN 0-13-981176-1.  [page needed]
  2. ^
  3. ^ John S. Penn (11 March 2008). Retinal and Choroidal Angiogenesis. Springer. pp. 119–. ISBN 978-1-4020-6779-2. Retrieved 26 June 2010. 
  4. ^ "Endoderm -- Developmental Biology -- NCBI Bookshelf". Retrieved 2010-04-07. 
  5. ^ Sakai et. al (2013). "Are the precapillary sphincters and metarterioles universal components of the microcirculation? An historical review". J Physiol Sci. 2013; 63: 319–331. PMC 3751330. 
  6. ^ BU Histology Learning System: 22401lba
  7. ^ Pavelka, Margit; Jürgen Roth (2005). Functional Ultrastructure: An Atlas of Tissue Biology and Pathology. Springer. p. 232. 
  8. ^ Gittenberger-De Groot, Adriana C.; Winter, Elizabeth M.; Poelmann, Robert E (2010). "Epicardium derived cells (EPDCs) in development, cardiac disease and repair of ischemia". Journal of Cellular and Molecular Medicine 14 (5): 1056–60. doi:10.1111/j.1582-4934.2010.01077.x. PMID 20646126. 
  9. ^ a b Lambiase, P. D.; Edwards, RJ; Anthopoulos, P; Rahman, S; Meng, YG; Bucknall, CA; Redwood, SR; Pearson, JD; Marber, MS (2004). "Circulating Humoral Factors and Endothelial Progenitor Cells in Patients with Differing Coronary Collateral Support". Circulation 109 (24): 2986–92. doi:10.1161/01.CIR.0000130639.97284.EC. PMID 15184289. 
  10. ^ Noon, J P; Walker, B R; Webb, D J; Shore, A C; Holton, D W; Edwards, H V; Watt, G C (1997). "Impaired microvascular dilatation and capillary rarefaction in young adults with a predisposition to high blood pressure". Journal of Clinical Investigation 99 (8): 1873–9. doi:10.1172/JCI119354. PMC 508011. PMID 9109431. 
  11. ^ Bird, Alan C. (2010). "Therapeutic targets in age-related macular disease". Journal of Clinical Investigation 120 (9): 3033–41. doi:10.1172/JCI42437. PMC 2929720. PMID 20811159. 
  12. ^ Cao, Yihai (2009). "Tumor angiogenesis and molecular targets for therapy". Frontiers in Bioscience 14 (14): 3962–73. doi:10.2741/3504. PMID 19273326. 
  13. ^ Dr. Paul Ghalioungui (1982), "The West denies Ibn Al Nafis's contribution to the discovery of the circulation", Symposium on Ibn al-Nafis, Second International Conference on Islamic Medicine: Islamic Medical Organization, Kuwait (cf. The West denies Ibn Al Nafis's contribution to the discovery of the circulation, Encyclopedia of Islamic World)
  14. ^ John Cliff, Walter (1976). Blood Vessels. CUP Archives. p. 14. 

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