Initiation of Coagulation Formation of Prothrombin Activator

Now that we have discussed the clotting process itself, we must turn to the more complex mechanisms that initiate clotting in the first place. These mechanisms are set into play by (1) trauma to the vascular wall and adjacent tissues, (2) trauma to the blood, or (3) contact of the blood with damaged endothelial cells or with collagen and other tissue elements outside the blood vessel. In each instance, this leads to the formation of prothrombin activator, which then causes prothrombin conversion to thrombin and all the subsequent clotting steps.

Prothrombin activator is generally considered to be formed in two ways, although, in reality, the two ways interact constantly with each other: (l) by the extrinsic pathway that begins with trauma to the vascular wall and surrounding tissues and (2) by the intrinsic pathway that begins in the blood itself.

In both the extrinsic and the intrinsic pathways, a series of different plasma proteins called blood-clotting factors play major roles. Most of these are inactive forms of proteolytic enzymes. When converted to the active forms, their enzymatic actions cause the successive, cascading reactions of the clotting process.

Most of the clotting factors, which are listed in Table 36-1, are designated by Roman numerals. To indicate the activated form of the factor, a small letter "a" is added after the Roman numeral, such as Factor VIIIa to indicate the activated state of Factor VIII.

Extrinsic Pathway for Initiating Clotting

The extrinsic pathway for initiating the formation of prothrombin activator begins with a traumatized vascular wall or traumatized extravascular tissues that come in contact with the blood. This leads to the following steps, as shown in Figure 36-3: 1. Release of tissue factor. Traumatized tissue releases a complex of several factors called tissue factor or tissue thromboplastin. This factor is composed especially of phospholipids from the membranes of the tissue plus a lipoprotein complex that functions mainly as a proteolytic enzyme.

Tissue trauma

Tissue factor t

Platelet phospholipids

Prothrombin Activator

Prothrombin s> \

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Essentials of Human Physiology

Essentials of Human Physiology

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  • Veronica
    What initiates the formation of prothrombin?
    8 years ago
  • Belba
    How prothrombin is formed?
    2 years ago

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