platelet

(redirected from Georges Hayem)
Also found in: Dictionary, Thesaurus, Medical.
Related to Georges Hayem: l'chaim

blood clotting

blood clotting, process by which the blood coagulates to form solid masses, or clots. In minor injuries, small oval bodies called platelets, or thrombocytes, tend to collect and form plugs in blood vessel openings. To control bleeding from vessels larger than capillaries a clot must form at the point of injury. The coagulation of the blood is also initiated by the blood platelets. The platelets produce a substance that combines with calcium ions in the blood to form thromboplastin, which in turn converts the protein prothrombin into thrombin in a complex series of reactions. Thrombin, a proteolytic enzyme, converts fibrinogen, a protein substance, into fibrin, an insoluble protein that forms an intricate network of minute threadlike structures called fibrils and causes the blood plasma to gel. The blood cells and plasma are enmeshed in the network of fibrils to form the clot. Blood clotting can be initiated by the extrinsic mechanism, in which substances from damaged tissues are mixed with the blood, or by the intrinsic mechanism, in which the blood itself is traumatized. More than 30 substances in blood have been found to affect clotting; whether or not blood will coagulate depends on a balance between those substances that promote coagulation (procoagulants) and those that inhibit it (anticoagulants). Prothrombin, a substance essential to the clotting mechanism, is produced by the liver in the presence of vitamin K. When the body is deficient in this vitamin, bleeding is more difficult to control. In hemophiliacs, or “bleeders,” the blood's coagulation time is greatly prolonged (see hemophilia). The coagulation of blood within blood vessels in the absence of injury can cause serious illness or death, especially when a clot forms in the coronary arteries (thrombosis) or cerebral arteries (stroke or apoplexy). To prevent coagulation of the blood in persons with known tendency to clot formation, and also as prophylaxis before performing surgery or blood transfusion, the blood's natural anticlotting substance, heparin, is reinforced by an additional amount of an anticoagulant such as Dicumarol injected into the body.
The Columbia Electronic Encyclopedia™ Copyright © 2022, Columbia University Press. Licensed from Columbia University Press. All rights reserved.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Platelet

 

(also thrombocyte), a formed element found in the blood of vertebrate animals and man that plays an important role in blood coagulation. The platelets of vertebrate animals (except mammals) are small ovoid cells with a solid nucleus and weakly basophilic cytoplasm. In mammals, including man, platelets are anuclear bodies 2–5 µ in diameter. The platelets normally number about 250,000–350,000 per cubic mm of blood. They have granules containing serotonin and substances involved in blood coagulation, as well as mitochondria, microtubules, glycogen granules, and sometimes ribosomes. In mammals, platelets originate in the blood-forming organs when fragments of the cytoplasm of megakaryocytes separate from the megakaryocytes. The life-span of the platelets of mammals, including man, is about five to nine days.

REFERENCE

Markosian, A. A. Fiziologiia trombotsitov. Leningrad, 1970.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

platelet

[′plāt·lət]
(histology)
(hydrology)
A small ice crystal which, when united with other such crystals, forms a layer of floating ice, especially sea ice, and serves as seed crystals for further thickening of the ice cover.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

platelet

a minute cell occurring in the blood of vertebrates and involved in clotting of the blood
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005