PLATELETS AND HEMOSTASIS Dr Joan Kumar Asst Prof. Dept of Physiology

Platelets Platelets (thromobocytes) are 1-4 microns in diameter. Smallest of the blood cells. They are formed in the bone marrow from Megakaryocytes. After entering the circulation 1/3 is sequestered in the spleen and the rest circulate in the blood for 7 days. Platelet production is controlled by colony stimulating factors and thrombopoietin from liver and kidneys The normal platelet count in peripheral blood is 150,000 to 300,000/microliter.

Platelets are non-nucleated,round or oval discs with diameter of 2.5µm. The cell membrane has a coating of glycoproteins which prevents adhesion to the normal smooth vascular endothelium and phosholipids which play a role in the clotting process. The cytoplasm has microfilaments and contractile proteins myosin,actin and thrombosthenin The contractile proteins are necessary for clot retraction.

Two types of granules are present – dense granules containing ATP and ADP, calcium and serotonin α granules which secrete fibrinogen, albumin, von willebrand factor, PAF,Fibrin stabilizing factor,and enzymes that synthesize thromboxane It also contain growth factors (PDGF) which helps in regrowth of vascular smooth muscle and fibroblasts.

FUNCTIONS OF PLATELETS 1.Play a role in hemostasis – platelet plug formation. This is important for closing minute vascular holes formed during everyday life Adhesion – platelets adhere to the exposed collagen on the injured endothelium and to von willebrand factor Activation - they undergo a shape change and become activated and release the contents of the granules. Aggregation – the released ADP,PAF,and thromboxane act to produce even more aggregation of platelets leading to formation of platelet plug.

2. They have an important role in coagulation 2. They have an important role in coagulation. The platelet factors and phospholipids have a role in the formation of prothrombin activator in the intrinsic pathway of coagulation. 3. Platelets are essential for clot retraction.

VARIATION IN PLATELET COUNT Thrombocytopenia – decrease in platelet count. Maybe due to decreased production , increased destruction or increased sequestration in the spleen. Thrombocytosis- increase in count ; Due to malignant proliferation of megakaryocytes or increased production in response to chronic bleeding or inflammation.

Thrombocytopenic purpura There are areas of bleeding in the skin and mucosa and spontaneous bleeding from nose, mouth, GIT etc. This is due to failure to produce platelet plugs. ITP- Idiopathic Thrombocytopenic purpura The coagulation time is normal, but the bleeding time is prolonged. Clot retraction is delayed. The platelet count is less than 10000 per cu.mm. Sometimes the count is normal but the platelet function is defective. (thrombasthenic purpurae) Splenectomy helps in certain cases.