The precapillary sphincter is a band 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...

 that adjusts the 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....

 flow into each capillary

Capillary

Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...

. At the point where each true capillary originates from a 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...

, a smooth muscle fiber usually encircles the capillary. This is called the precapillary sphincter. This sphincter can open and close the entrance to the capillary. Blood flow in a capillary changes as vasomotion

Vasomotion

Vasomotion is the spontaneous oscillation in tone of blood vessels, independent of heart beat, innervation or respiration. While vasomotion was first observed by Jones in 1852, the complete mechanisms responsible for its generation and its physiological importance remain to be elucidated, however...

 occurs. The entire capillary bed may be bypassed by blood flow through arteriovenous anastomoses. A precapillary sphincter encircles each capillary branch at the point where it branches from the arteriole. Contraction of the precapillary sphincter can close the branches off to blood flow. If the sphincter is damaged or can not contract, blood can flow into the capillary bed at high pressures. When capillary pressures are high (and this can be the result of gravity), fluid passes out of the capillaries into the interstitial space

Fluid compartments

Fluid compartments in the mammalian body broadly comprise two compartments, each with several subdivisions:intracellular fluid, which makes up approximately 60-65% of body water, and...

, and edema

Edema

Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...

 or fluid swelling is the result.

Exteranal links

• Central neural influence on precapillary microvessels and sphincter. PubMed
  PubMed
  PubMed is a free database accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine at the National Institutes of Health maintains the database as part of the Entrez information retrieval system...
  
   Abstract, American Journal of Physiology
  American Journal of Physiology
  The American Journal of Physiology is a peer-reviewed scientific journal on physiology published by the American Physiological Society.Vols...
  
  , 1977 Jul;233(1):H141-7. Baez S, Feldman SM, Gootman PM.