Overview of Hemostasis - Blood physiology Animated video lecture for MBBS

Hemostasis: Understanding the Process

Overview of Hemostasis

• Hemostasis is defined as the physiological process that prevents or stops bleeding from damaged blood vessels. It involves three main steps: vasoconstriction, formation of a temporary hemostatic plug, and formation of a definitive hemostatic clot.

Vasoconstriction

• The initial phase of hemostasis begins with vasoconstriction, which is triggered by direct injury to vascular smooth muscles. This response is transient but can be sustained for several minutes to hours through humoral facilitation involving substances like 5-hydroxytryptophan and other vasoconstrictors.

Formation of Temporary Hemostatic Plug

Platelet Adhesion

• Upon injury, platelets adhere to exposed collagen fibers and endothelial cells in the vessel wall, changing their shape by swelling and forming pseudopodia. This adhesion is crucial for initiating the hemostatic process.

Platelet Activation

• Activated platelets release ADP and thromboxane A2, which further activate nearby platelets, enhancing their stickiness and promoting aggregation at the site of injury. This creates a cycle that amplifies platelet activation and adherence.

Platelet Aggregation

• The aggregation process involves large numbers of activated platelets sticking together, facilitated by platelet activating factor released from neutrophils, monocytes, and platelets themselves. This aggregation leads to further reactions resulting in thromboxane A2 production from membrane phospholipids. Notably, aspirin inhibits this aggregation by blocking thromboxane A2 formation, making it useful in preventing myocardial infarction.

Temporary Hemostatic Plug Formation

• The collective actions of platelet adherence and aggregation lead to the creation of a temporary hemostatic plug that effectively blocks blood loss if the vascular opening is small enough. However, its formation can be inhibited by factors such as icland derived from membrane phospholipids that curtail thromboxane production and localize plug formation.

Formation of Definitive Hemostatic Plug

• The transition from a temporary platelet plug to a definitive hemostatic clot involves complex processes where platelets contribute significantly to forming intrinsic prothrombin activators necessary for clotting initiation. This step solidifies the hemostatic response ensuring effective cessation of bleeding following vascular injury.