A precapillary sphincter is a band of contractile mural cells either classified as

smooth muscle Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...

pericytes Pericytes (formerly called Rouget cells) are multi-functional mural cells of the microcirculation that wrap around the endothelial cells that line the capillaries throughout the body. Pericytes are embedded in the basement membrane of blood capil ...

that adjusts

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...

flow into

capillaries A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the in ...

. They were originally described in the mesenteric

microcirculation The microcirculation is the circulation of the blood in the smallest blood vessels, the microvessels of the microvasculature present within organ tissues. The microvessels include terminal arterioles, metarterioles, capillaries, and venules. ...

, and were thought to only reside there. At the point where each of the capillaries originates from an

arteriole An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillary, capillaries. Arterioles have vascular smooth muscle, muscular walls (usually only one to two layers of smoo ...

, contractile mural cells encircle the capillary. This is called the precapillary sphincter. The precapillary sphincter has now also been found in the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

, where it regulates blood flow to the capillary bed. The sphincter can open and close the entrance to the capillary, by which contraction causes blood flow in a capillary to change as vasomotion occurs. In some tissues, the entire

capillary bed A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the inn ...

may be bypassed by blood flow through arteriovenous anastomoses or through preferential flow through

metarteriole A metarteriole is a short microvessel in the microcirculation that links arterioles and capillaries. Instead of a continuous tunica media The tunica media (Neo-Latin "middle coat"), or media for short, is the middle tunica (layer) of an arter ...

s. If the sphincter is damaged or cannot contract, blood can flow into the capillary bed at high pressures. When capillary pressures are high (as per gravity, etc.), fluid passes out of the capillaries into the

interstitial space An interstitial space is an intermediate space located between regular-use floors, commonly located in Hospital, hospitals and laboratory-type buildings to allow space for the mechanical systems of the building. By providing this space, laborator ...

, and

edema Edema (American English), also spelled oedema (British English), and also known as fluid retention, swelling, dropsy and hydropsy, is the build-up of fluid in the body's tissue (biology), tissue. Most commonly, the legs or arms are affected. S ...

or fluid swelling is the result.

Dispute over concept

Precapillary sphincters and metarterioles were discovered in the mesenteric circulation in the 1950s. Medical and physiological textbooks, such as '' Guyton'', ''

Boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

'' and ''Fulton'', etc. were quick to claim the existence of said sphincters and metarterioles all over the body, despite lack of evidence. At least since 1976 there has been considerable debate about the existence of precapillary sphincters and metarterioles. In 2020, precapillary sphincters were identified as a mechanism for controlling cerebral blood flow.

Dispute over concept

References

Further reading