Platelet Plug Formation
Platelet Plug Formation
A platelet plug is an accumulation of platelets that can seal small breaks in blood vessels. Platelet plug formation is very important in maintaining the integrity of the circulatory system because small tears occur in the smaller vessels and capillaries many times each day, and platelet plug formation quickly closes them. People who lack the normal number of platelets tend to develop numerous small hemorrhages in their skin and internal organs.
The formation of a platelet plug can be described as a series of steps, but in actuality many of the steps take place simultaneously ( fi g u re 1 9 . 9 ) :
1. Platelet adhesion occurs when platelets bind to collagen ex- posed by blood vessel damage. Most platelet adhesion is medi- ated through von Willebrand factor (vWF), which is a protein produced and secreted by blood vessel endothelial cells. Von Willebrand factor forms a bridge between exposed collagen of the blood vessel wall and platelets by binding to platelet sur- face receptors and collagen. In addition, other platelet surface receptors can bind directly to collagen.
2. After platelets adhere to collagen, they become activated; in the platelet release reaction, adenosine diphosphate (ADP), thromboxanes, and other chemicals are extruded from the platelets by exocytosis. The ADP and thromboxane bind to their respective receptors on the surfaces of other platelets, activating them. These activated platelets release additional chemicals, thereby producing a cascade of chemical release by the platelets. Thus, more and more platelets become activated. This is an example of positive feedback.
3. As platelets become activated, they change shape and express fibrinogen receptors that can bind to fibrinogen, a plasma pro- tein. In platelet aggregation, fibrinogen forms a bridge be- tween the fibrinogen receptors of different platelets, resulting in a platelet plug.
In addition to forming a platelet plug, activated platelets also release
A platelet plug is an accumulation of platelets that can seal small breaks in blood vessels. Platelet plug formation is very important in maintaining the integrity of the circulatory system because small tears occur in the smaller vessels and capillaries many times each day, and platelet plug formation quickly closes them. People who lack the normal number of platelets tend to develop numerous small hemorrhages in their skin and internal organs.
The formation of a platelet plug can be described as a series of steps, but in actuality many of the steps take place simultaneously ( fi g u re 1 9 . 9 ) :
1. Platelet adhesion occurs when platelets bind to collagen ex- posed by blood vessel damage. Most platelet adhesion is medi- ated through von Willebrand factor (vWF), which is a protein produced and secreted by blood vessel endothelial cells. Von Willebrand factor forms a bridge between exposed collagen of the blood vessel wall and platelets by binding to platelet sur- face receptors and collagen. In addition, other platelet surface receptors can bind directly to collagen.
2. After platelets adhere to collagen, they become activated; in the platelet release reaction, adenosine diphosphate (ADP), thromboxanes, and other chemicals are extruded from the platelets by exocytosis. The ADP and thromboxane bind to their respective receptors on the surfaces of other platelets, activating them. These activated platelets release additional chemicals, thereby producing a cascade of chemical release by the platelets. Thus, more and more platelets become activated. This is an example of positive feedback.
3. As platelets become activated, they change shape and express fibrinogen receptors that can bind to fibrinogen, a plasma pro- tein. In platelet aggregation, fibrinogen forms a bridge be- tween the fibrinogen receptors of different platelets, resulting in a platelet plug.
In addition to forming a platelet plug, activated platelets also release