Blood Coagulation

Coagulation (ko-ag"u-la'shun), the most effective hemostatic mechanism, causes formation of a blood clot by a series of reactions, each one activating the next in a chain reaction, or cascade. Coagulation may occur by an extrinsic or an intrinsic clotting mechanism. Release of biochemicals from broken blood vessels or damaged tissues triggers the extrinsic clotting mechanism. Blood contact with foreign surfaces in the absence of tissue damage stimulates the intrinsic clotting mechanism. These clotting mechanisms are described in the sections titled "Extrinsic Clotting Mechanism" and "Intrinsic Clotting Mechanism."

Blood coagulation is very complex and utilizes many biochemicals called clotting factors. They are designated by Roman numerals indicating the order of their discovery. Vitamin K is necessary for some clotting factors to function. Whether or not the blood coagulates depends on the balance between factors that promote coagulation (procoagulants) and others that inhibit it (anticoagulants). Normally, the anticoagulants prevail, and the blood does not clot. However, as a result of injury (trauma), biochemicals that favor coagulation may increase in concentration, and the blood may coagulate.

The major event in blood clot formation is conversion of the soluble plasma protein fibrinogen (factor I) into insoluble threads of the protein fibrin. Activation of certain plasma proteins by still other protein factors triggers conversion of fibrinogen to fibrin. Table 14.9 summarizes the three primary hemostatic mechanisms.

Extrinsic Clotting Mechanism

The extrinsic clotting mechanism is triggered when blood contacts damaged blood vessel walls or tissues outside blood vessels. Such damaged tissues release a complex of substances called tissue thromboplastin (factor III) that is associated with disrupted cell membranes. The presence of tissue thromboplastin activates factor VII, which combines with and activates factor X. Further, factor X combines with and activates factor V. These reactions, which also depend on the presence of calcium ions (factor IV), lead to production and release of prothrombin activator by the platelets.

Prothrombin (factor II) is an alpha globulin that the liver continually produces and is thus a normal constituent of plasma. In the presence of calcium ions, pro-thrombin activator converts prothrombin into thrombin (factor IIa). Thrombin, in turn, catalyzes a reaction that fragments fibrinogen (factor I). The fibrinogen fragments join, forming long threads of fibrin. Fibrinogen is a solu-

H Hemostatic

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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