E V E N T I O N S S

crease interstitial pressure enough to eliminate the pressure gradient required for continued blood loss. Accumulation of blood in the tissues can occur as a result of bleeding from any vessel type and is termed a hematoma.

When a blood vessel is severed or otherwise injured, its immediate inherent response is to constrict (the mechanism is unclear). This short-lived response slows the flow of blood in the affected area. In addition, this constriction presses the opposed endothelial surfaces of the vessel together, and this contact induces a stickiness capable of keeping them "glued" together.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

III. Coordinated Body Functions

14. Circulation

© The McGraw-Hill Companies, 2001

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

Permanent closure of the vessel by constriction and contact stickiness occurs only in the very smallest vessels of the microcirculation, however, and the staunching of bleeding ultimately is dependent upon two other processes that are interdependent and occur in rapid succession: (1) formation of a platelet plug; and (2) blood coagulation (clotting). The blood platelets—cell fragments circulating in blood that are derived from bone-marrow-cells known as megakaryocytes—are involved in both processes.

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Essentials of Human Physiology

Essentials of Human Physiology

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