Porth's Essentials of Pathophysiology, 4e

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Hematopoietic Function

U N I T 3

Hemostasis, which refers to the stoppage of blood flow, is divided into five stages: (1) vessel spasm, (2) formation of the platelet plug, (3) development of a blood clot (coagulation cascade), (4) clot retraction, and (5) clot dissolution.This multistep process involves the interaction of substrates, enzymes, protein cofactors, and calcium ions that circulate in the blood or are released from platelets and cells in the vessel wall. U N D E R S T A N D I N G Hemostasis

Vessel Spasm. Injury to a blood vessel causes vascular smooth mus- cle in the vessel wall to contract and thus instantaneously reduce blood flow. Both local neural reflexes and local humoral factors such as thromboxane A 2 (TXA 2 ), which is released from platelets, contribute to the vasoconstriction. 1

Injury

Vessel spasm

Platelets

Injury to endothelium

Smooth muscle

Subendothelium

Formation of the Platelet Plug. Seconds after vessel injury, von Willebrand factor, released from the endothelium, binds to platelet receptors, causing adhesion of the platelets to the exposed collagen fibers ( inset ). As the platelets adhere to the collagen fibers on the damaged vessel wall, they become activated and release adenosine diphosphate (ADP) and TXA 2 . The ADP and TXA 2 attract additional platelets, leading to platelet aggregation. 2

Endothelium

von Willebrand factor (vWf)

Factor VIII

Platelet

ADP TXA 2

Collagen

Collagen

Platelet aggregation

Smooth muscle