Hemostasis .pdf

Hemostasis Enas Sabah MSc. Medical Physiology Hemostasis Means prevention of blood loss. Whenever a vessel is severed or ruptured, hemostasis is achieved by several mechanisms: 1. Vascular constriction. 2. Formation of a platelet plug. 3. Formation of a blood clot as a result of blood coagulation....

Hemostasis Enas Sabah MSc. Medical Physiology Hemostasis Means prevention of blood loss. Whenever a vessel is severed or ruptured, hemostasis is achieved by several mechanisms: 1. Vascular constriction. 2. Formation of a platelet plug. 3. Formation of a blood clot as a result of blood coagulation. 4. Eventual growth of fibrous tissue into the blood clot to close the hole in the vessel permanently. Vascular constriction • Results from: • Local myogenic spasm • Factors released from damaged tissue & platelets • Nerve reflex Formation of a platelet plug • To understand this process, its important to discuss the nature of platelets first. Characteristics of the platelets • Platelets (also called thrombocytes) are minute discs formed in the bone marrow from megakaryocytes • The normal concentration of platelets in the blood is between 150,000 and 300,000 per microliter. • They don’t have nucleus • Have a life span of 8 to 12 days,then removed by the tissue macrophage system in spleen Formation of a platelet plug • When platelets come in contact with a damaged vascular surface, especially with collagen fibers, they immediately change their own characteristics • They swell, pseudopods form on their surfaces and become sticky, so that they adhere to collagen in the vessel wall and to each other forming the plug. • If the cut in the blood vessel is very small, the cut is often sealed by a platelet plug, rather than by a blood clot. Blood coagulation and formation of blood clot • Factors from damaged tissue, platelets & from blood proteins initiate clotting process. • This process catalyzed by clotting factors & results in the formation of fibrin fibers running in all direction producing a meshwork that entrap platelets & blood cells to form the clot. • This followed by formation of more fibrous tissue into the blood clot to close the hole in the vessel permanently. Fibrinolysis • Extra clot and old clot cleaned up by factors that breakdown clot. • This allows the repair of the wall and normalize blood flow through the vessel. Activation of Plasminogen to Form Plasmin, Then Lysis of Clots The injured tissues and vascular endothelium very slowly release a powerful activator called tissue plasminogen activator(t-PA) that a few days later, after the clot has stopped the bleeding, eventually converts plasminogen to plasmin, which in turn removes the remaining unnecessary blood clot. •Patients with bleeding disorders and tendencies require the following investigations: 1. Hess test (capillary resistance test). 2. Platelet count. 3. Bleeding time. 4. Clotting time (whole blood coagulation time). 5. Other tests like prothrombin time, partial thromboplastin time and factor assay (testing clotting factor activity). •Hess test: ∙ With a pencil, mark a circle of 6 cm diameter in the antecubital fossa. ∙ Apply the cuff of the sphygmomanometer 1-2 inches above the circle and raise the pressure of the cuff to 50 mmHg and maintain the pressure for five minutes. ∙ Release the cuff and count the number of petichial hemorrhages (blue dots) on the circle. Counts of 0 – 8 are regarded as normal. ∙ If the test is positive (above 8 dots), this means that the capillary wall is abnormal and weakened by a disease. •Bleeding time: is the time required to arrest bleeding from a ruptured capillary. It includes two tests: A. Duke’s method ∙ After cleaning the finger, do a puncture of about 4 mm deep in the thumb. Touch the bleeding site with a filter paper every 30 seconds. ∙ When bleeding is stopped count the number of spots formed on the paper and then divide it by two. ∙ This will give the bleeding time in minutes. By this method, the normal BT is 2 – 6 minutes. B. Ivy’s method ∙ Wrap the cuff of the sphygmomanometer around the upper arm and inflate it to a pressure of 40 mmHg and maintain the pressure. ∙ Do a puncture of about 3 mm deep on the forearm (avoid puncturing nearby veins) and touch the bleeding site by the filter paper every 30 seconds until the bleeding stops. ∙ Then count the number of spots formed on the paper and divide it by two. ∙ By this method, the normal BT is 2 – 8 minutes. ∙ This method gives information about the function of the platelets and the capillary wall. •Clotting time: This is the time taken for the blood to clot out side the vascular system. •Capillary tube method : ∙ Do a puncture on the thumb and fill a none-heparinized capillary tube with the blood and wait for two minutes. ∙ Break off a piece of the capillary every 30 seconds until you see the thread of the clot sticks to each end of the broken parts. ∙ Then the CT is equal to 2 + (number of pieces÷2), By this method, the CT is 4 – 9 minutes. Conditions That Cause Excessive Bleeding in Humans • • • • (1) vitamin K deficiency (2) hemophilia (3) thrombocytopenia (platelet deficiency) (4) side effect of certain drugs (anticoagulants) Thromboembolic Conditions • An abnormal clot that develops in a blood vessel is called a thrombus. • This can occlude small blood vessels and prevents blood from flowing normally. • Emboli is a clot that freely circulating (travels) through the bloodstream.