Physiology - Platelets PDF

# Physiology - Platelets ## Platelets Platelets, or thrombocytes, are small, colorless cell fragments in our blood that form clots and stop or prevent bleeding. Platelets are made in our bone marrow. Life span is 8 days. ## 1. Constriction of blood vessels * Constriction of blood vessels occurs...

# Physiology - Platelets ## Platelets Platelets, or thrombocytes, are small, colorless cell fragments in our blood that form clots and stop or prevent bleeding. Platelets are made in our bone marrow. Life span is 8 days. ## 1. Constriction of blood vessels * Constriction of blood vessels occurs immediately after injury of vessels to decrease blood loss from the injured site. * **Causes of vasoconstriction:** * **Local myogenic contraction** due to direct trauma. * **Nervous reflexes** start due to pain from the injured vessel. * **Local humoral factors** liberated from platelets such as: * ADP * Serotonin * Thromboxane A2 ## 2. Formation of a temporary hemostatic plug * **Hemostatic plug** is an aggregation of platelets formed during early stages of hemostasis. * Injury of the vessel stimulates the platelets to form the mechanical plug to seal the vascular injury. ## Platelet reactions in hemostasis 1. **Platelet adhesion:** After blood vessel injury platelets adhere to the exposed subendothelial collagen. 2. **Platelet activation:** After adhesion, platelets swell, change their shape, and discharge the contents of granules. 3. **Platelet release reaction:** Platelet release content of its granules which have a role in clot formation. * ADP * Serotonin * Thromboxane A2 4. **Platelet aggregation:** Platelet aggregation, the process by which platelets adhere to each other at sites of vascular injury, has long been recognized as critical for hemostatic plug formation and thrombosis. 5. **Platelet fusion:** Irreversible slow process, in which the aggregated platelets fuse at the site of injury. * Platelet fusion is caused by: * High concentration of ADP * Thrombin ## Blood coagulation * The clotting mechanism is the process of conversion of temporary platelets plug into definitive clots by fibrin. * **Coagulation factors** are plasma proteins that are synthesized by the liver. * The **coagulation cascade** involves several steps of activation of clotting factors. * The final step of coagulation is the formation of **fibrin mesh**. ## Blood coagulation - Diagram The diagram illustrates how blood clots formed by the intrinsic and extrinsic pathways. * **Intrinsic Pathway:** This pathway is triggered by a change in the blood itself, such as the presence of collagen. * **Extrinsic Pathway:** This pathway is triggered by an external factor, such as trauma to the blood vessel. ### Intrinsic Pathway 1. HMWK and PK activate Factor XII 2. Factor XIIa activates Factor XI 3. Factor XIa activates Factor IX 4. Factor IXa, along with Factor VIIIa activate Factor X 5. Factor Xa, along with Factor Va, convert prothrombin (Factor II) into thrombin (Factor IIa) 6. Thrombin (Factor IIa) converts fibrinogen into fibrin, which forms the clot. 7. Factor XIIla cross-links fibrin, strengthening the clot. ### Extrinsic Pathway 1. Trauma leads to the release of tissue factor (TF) from damaged cells 2. TF activates Factor VII 3. Factor VIIa, along with Factor Xa, activates Factor X 4. Factor Xa, along with Factor Va, convert prothrombin (Factor II) into thrombin (Factor IIa) 5. Thrombin (Factor IIa) converts fibrinogen into fibrin, which forms the clot. 6. Factor XIIla cross-links fibrin, strengthening the clot. The diagram shows the various factors and enzymes involved in both pathways, along with the final formation of the fibrin mesh. # Thanks