FUNCTION OF PLATELETS IN HAEMOSTASIS AND COAGULATION

FUNCTION OF PLATELETS IN HAEMOSTASIS AND COAGULATION

Platelets are disc shaped particles produced by cytoplasmic fragmentation of megakaryocytes in the bone marrow. They are non-nucleated and are surrounded by a layer of proteins and carbohydrates which participate in coagulation and fibrinolysis. The platelets also contain granules which are rich in adenosine triphosphate (ATP) and other substances.

Platelets perform three important functions:

1. Form a platelet plug which offers a physical obstruction to blood loss: Adhesion of platelets to the exposed collagen fibres of the blood vessels starts activation of platelets. Activated platelets change their form and become spherical with long projections. In this way, the surface area of platelets is increased and enhances interaction with other platelets and coagulation factors. The changes on their outer coats also enhances the tendency to aggregate to form a plug.

2. Help in activation of coagulation factors and participate in the process of coagulation: The platelets serve as a barrier in extensive blood loss by participating in blood coagulation in various ways. Platelets produce a phospholipoprotein on their plasma membrane called platelet factor 3 (PF3) which is necessary for the activation of certain coagulation factors. Many coagulation proteins such as factors II, V, VIII, IX, XII and XIII are found in or on the platelets.

3. Maintain the endothelial lining of the blood vessels: Platelets also secrete certain products which help to maintain and repair the endothelium of the blood vessels. Platelets plug the gaps in the endothelial lining and prevent leakage of red cells in the surrounding tissues.

Role of Blood Vessels

The vascular system is a closed system of tubules which serves to regulate the flow of blood to and from the heart and the tissues. The entire system is internally lined by a single layer of endothelium. In larger vessels, the endothelium is surrounded by smooth muscle which helps in regulation of blood flow. 

The thickness of this muscle varies with the size of the vessels. The capillaries are much smaller and are lined by a single layer of endothelial cells without any muscular tissue. This allows the passage of molecules across the endothelium. The endothelium is supported by a basement membrane and collagen.

The vascular endothelium monitors the integrity of the vessel wall in two ways.

(i) It synthesises and releases substances such as prostaglandins which inhibit aggregation and adhesion of platelets to the vessel wall. 

(ii) It stores substances such as plasminogen activator. When stimulated, this activator activates plasminogen which gets converted to plasmin. The plasmin, in turn, dissolves fibrin clots within the vessel, thus allowing a free flow of blood .