Hemostasis page 563

Hemostasis refers to the stoppage of bleeding. Hemostatic mechanisms are most effective in controlling blood loss from small vessels.

1. Blood vessel spasm (vasospasm)

a. Smooth muscles in walls of smaller blood vessels reflexly contract following injury.

b. Platelets release serotonin that stimulates vasoconstriction and helps maintain vessel spasm.

2. Platelet plug formation a. Platelets adhere to rough surfaces and exposed collagen.

b. Platelets stick together at the sites of injuries and form platelet plugs in broken vessels.

3. Blood coagulation a. Blood clotting, the most effective means of hemostasis, involves a series of reactions wherein each reaction stimulates the next reaction (cascade), which may be initiated by extrinsic or intrinsic mechanisms.

b. The extrinsic clotting mechanism is triggered when blood contacts damaged tissue.

c. The intrinsic clotting mechanism is triggered when blood contacts a foreign surface.

d. Clot formation depends on the balance between clotting factors that promote clotting and those that inhibit clotting.

e. The basic event of coagulation is the conversion of soluble fibrinogen into insoluble fibrin.

f. After forming, the clot retracts and pulls the edges of a broken vessel closer together.

g. A thrombus is an abnormal blood clot in a vessel; an embolus is a clot or fragment of a clot that moves in a vessel.

h. Fibroblasts invade a clot, forming connective tissue throughout.

i. Protein-splitting enzymes may eventually destroy a clot.

4. Prevention of coagulation a. The smooth lining of blood vessels discourages the accumulation of platelets.

b. As a clot forms, fibrin adsorbs thrombin and prevents the reaction from spreading.

c. Antithrombin interferes with the action of excess thrombin.

d. Some cells secrete heparin, an anticoagulant.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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