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[[File:Blood_vessels-en.svg|Blood vessels-en|thumb]] [[File:Different_Types_of_Capillaries.jpg|Different Types of Capillaries|thumb|left]] [[File:Capillary.svg|Capillary|thumb|left]] [[File:Sinusoid.jpeg|Sinusoid|thumb]] [[File:The_exchange_between_capillary_and_body_tissue_diagram.svg|The exchange between capillary and body tissue diagram|thumb]] '''Capillaries''' are the smallest of a body's [[blood vessel]]s, playing a critical role in the [[circulatory system]] by enabling the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between [[blood]] and surrounding [[tissues]]. This exchange is vital for the maintenance of homeostasis within the body and supports the overall function of [[organ systems]].

==Structure and Function==
Capillaries are microscopically thin blood vessels, with a diameter so small that [[red blood cells]] can only pass through them in single file. The walls of capillaries are composed of a single layer of [[endothelial cells]] surrounded by a [[basement membrane]], making them exceptionally thin and allowing for efficient exchange of substances between the blood and tissues.

There are three main types of capillaries:
# '''Continuous capillaries''', which have uninterrupted endothelial cell walls except for small gaps called [[intercellular clefts]]. These are found in most tissues.
# '''Fenestrated capillaries''', which have pores in their endothelial lining to allow for faster exchange of materials. These are commonly found in the [[kidneys]], [[intestines]], and [[endocrine glands]].
# '''Sinusoidal capillaries''' or '''discontinuous capillaries''', which have larger openings in the endothelium to permit the passage of [[red blood cells]] and serum proteins. These are typically found in the [[liver]], [[spleen]], and [[bone marrow]].

The primary function of capillaries is the exchange of materials such as oxygen, carbon dioxide, nutrients, and waste products between the blood and tissues. This exchange occurs through several mechanisms, including [[diffusion]], [[filtration]], and [[osmosis]], facilitated by the thinness of the capillary walls.

==Capillary Networks==
Capillaries are organized into networks called [[capillary beds]], which are supplied with blood by [[arterioles]] and drained by [[venules]]. The flow of blood through capillary beds is regulated by [[precapillary sphincters]], which are bands of smooth muscle at the entrance of capillary beds that can open and close, directing blood flow according to the body's needs.

==Clinical Significance==
Capillaries play a key role in many physiological and pathological processes. For example, in [[inflammation]], capillaries become more permeable, allowing [[immune cells]] to more easily reach the site of injury or infection. In [[diabetes mellitus]], high blood sugar levels can damage capillaries, leading to complications such as [[retinopathy]], [[nephropathy]], and [[neuropathy]]. Understanding capillary function is also crucial in the treatment of [[edema]], where excessive fluid leaks out of capillaries into the tissues, and in [[tumor]] growth, where the formation of new capillaries ([[angiogenesis]]) supports the growth and spread of cancer cells.

==See Also==
* [[Blood vessel]]
* [[Circulatory system]]
* [[Endothelial cell]]
* [[Homeostasis]]

[[Category:Cardiovascular system]]
[[Category:Angiology]]

{{medicine-stub}}

Capillary - Revision history

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