Capillaries are the smallest of a body's blood vessels, measuring 5-10 μm in diameter, which connect arterioles and venules, and enable the interchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between blood and surrounding tissues. [cite book
last = Maton
first = Anthea
authorlink =
coauthors = Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright
title = Human Biology and Health
publisher = Prentice Hall
date = 1993
location = Englewood Cliffs, New Jersey
pages =
url =
doi =
id =
isbn = 0-13-981176-1


Blood flows from the heart to arteries, which branch and narrow into arterioles, and then branch further still into 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.

The "capillary bed" is the network of capillaries supplying an organ. The more metabolically active the cells, the more capillaries it will require to supply nutrients and carry away waste products.

Metarterioles provide direct communication between arterioles and venules and are important in bypassing the bloodflow through the capillaries. True capillaries branch mainly from metarterioles and provide exchange between cells and the circulation. The internal diameter of 8 μm forces the red blood cells to partially fold into bullet-like shapes in order to by pass them in single file.

Precapillary sphincters are rings of smooth muscles at the origin of true capillaries that regulate blood flow into true capillaries and thus control blood flow through a tissue.


Capillaries come in three types:
*Continuous - Continuous capillaries have a sealed endothelium and only allow small molecules, like water and ions to diffuse. Continuous capillaries can be further divided into two subtypes: :# Those with numerous transport vesicles and tight junctions that are primarily found in skeletal muscles, lungs, gonads, and skin. :# Those with few vesicles and tight junctions that are primarily found in the central nervous system.
*Fenestrated - Fenestrated capillaries (derived from "fenestra," the Latin word for "window") have pores in the the endothelial cells (60-80 nm in diameter) that are spanned by a diaphragm of radially oriented fibrils and allow small molecules [BUHistology|22401lba] [cite book | title = Functional Ultrastructure: An Atlas of Tissue Biology and Pathology| author = Pavelka, Margit; Jürgen Roth| publisher = Springer | year = 2005 | page = 232] and limited amounts of protein to diffuse. In the renal glomerulus there are larger fenestrae which have no diaphragms. Both types of fenestrated blood vessels have continuous basal lamina and are primarily located in the endocrine glands, intestines, pancreas and glomeruli of kidney.
*Sinusoidal - Sinusoidal or discontinuous capillaries are special fenestrated capillaries that have larger openings (30-40 μm in diameter) in the epithelium to allow red blood cells and serum proteins to enter, a process that is aided by a discontinuous basal lamina. These capillaries lack pinocytotic vesicles and gaps may be present in cell junctions permitting leakage between endothelial cells. Sinusoid blood vessels are primarily located in the liver, spleen, bone marrow, lymph nodes, and adrenal cortex.


The capillary wall is a one-layer endothelium so thin that gas and molecules such as oxygen, water, proteins and lipids can pass through them driven by osmotic and hydrostatic gradients. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body. The physics of this exhange is explained by the Starling equation.

The capillary bed usually carries no more than 25% of the amount of blood it could contain, although this amount can be increased through auto regulation by inducing relaxation of smooth muscle in the arterioles that lead to the capillary bed as well as constriction of the metarterioles.

The capillaries do not possess this smooth muscle in their own wall, and so any change in their diameter is passive. Any signaling molecules they release (such as endothelin for constriction and nitric oxide for dilation) act on the smooth muscle cells in the walls of nearby, larger vessels, e.g. arterioles.

Capillary permeability can be increased by the release of certain cytokines, such as in an immune response.

Immune response

In an immune response, the endothelial cells of the capillary will upregulate receptor molecules, thus it signals the need for an immune response by the site of infection and aids extravasion of these cells into the tissue.


Ibn al-Nafis theorized a "premonition of the capillary circulation in his assertion that the pulmonary vein receives what comes out of the pulmonary artery, this being the reason for the existence of perceptible passages between the two."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")]

Marcello Malpighi was the first to observe and correctly describe capillaries when he discovered them in a frog's lung in 1661. [cite book | title = Blood Vessels | author = John Cliff, Walter | year = 1976 | publisher = CUP Archives | page = 14]

ee also

*Alveolar-capillary barrier
*Blood brain barrier
*Capillary action
*Hagen-Poiseuille equation


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  • Capillary — Cap il*la*ry (k[a^]p [i^]l*l[asl]*r[y^] or k[.a]*p[i^]l l[.a]*r[y^]; 277), a. [L. capillaris, fr. capillus hair. Cf. {Capillaire}.] 1. Resembling a hair; fine; minute; very slender; having minute tubes or interspaces; having very small bore; as,… …   The Collaborative International Dictionary of English

  • capillary — 1650s (adj.), 1660s (n.), from L. capillaris of hair, from capillus hair (of the head); probably related to caput head. Borrowed earlier as capillar (14c.). Capillary attraction attested from 1813. Capillarity is recorded from 1830, from Fr.… …   Etymology dictionary

  • capillary — ► NOUN 1) Anatomy any of the fine branching blood vessels that form a network between the arterioles and venules. 2) (also capillary tube) a tube with an internal diameter of hair like thinness. ► ADJECTIVE ▪ relating to capillaries or… …   English terms dictionary

  • capillary — [kap′ə ler΄ē; ] Brit [ kə pil′ə ri] adj. [L capillaris < capillus, hair] 1. of or like a hair, esp. in being very slender 2. having a very small bore 3. in or of capillaries n. pl. capillaries 1. a tube with a very small bo …   English World dictionary

  • Capillary — Cap il*la*ry, n.; pl. {Capillaries}. 1. A tube or vessel, extremely fine or minute. [1913 Webster] 2. (Anat.) A minute, thin walled vessel; particularly one of the smallest blood vessels connecting arteries and veins, but used also for the… …   The Collaborative International Dictionary of English

  • Capillary — One of the tiny blood vessels that connect the arterioles (the smallest divisions of the arteries) and the venules (the smallest divisions of the veins). The capillaries form a fine network in many parts of the body. Although minute, the… …   Medical dictionary

  • capillary — I. adjective Etymology: French or Latin; French capillaire, from Latin capillaris, from capillus hair Date: 14th century 1. a. resembling a hair especially in slender elongated form < capillary leaves > b. having a very small bore < a capillary… …   New Collegiate Dictionary

  • capillary — /kap euh ler ee/, adj., n., pl. capillaries. adj. 1. pertaining to or occurring in or as if in a tube of fine bore. 2. resembling a strand of hair; hairlike. 3. Physics. a. pertaining to capillarity. b. of or pertaining to the apparent attraction …   Universalium

  • capillary — /kəˈpɪləri / (say kuh piluhree) noun (plural capillaries) 1. Anatomy one of the minute blood vessels between the terminations of the arteries and the beginnings of the veins. 2. Also, capillary tube. a tube with a small bore. –adjective 3.… …  

  • capillary — adj. & n. adj. 1 of or like a hair. 2 (of a tube) of hairlike internal diameter. 3 of one of the delicate ramified blood vessels intervening between arteries and veins. n. (pl. ies) 1 a capillary tube. 2 a capillary blood vessel. Phrases and… …   Useful english dictionary

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