What is Hemostasis?
The process which stops the bleeding from a wound or injury is called Hemostasis. This is the first stage of wound healing.
1. When there is an injury to tissues, blood vessels may rupture and cause bleeding.
2. To prevent or minimise the loss of blood, the body reacts by:
(a) Reflex spasm of the wall of the blood vessel which temporarily closes the blood vessel and prevents loss of blood.
(b) Clotting of blood at the site of injury to stop the bleeding permanently. Later the clot shrinks and pulls together the wall of the blood vessel making a permanent seal.
Coagulation (Clotting of blood)
1. The first step is that the platelets stick to the damaged and rough surface of the injured blood vessel and form a plug.
2. The platelet piüg then releases chemical substances and also itself participates in the process of clotting.
3. In the blood there are 10 known and important clotting factors which participitate.in the process of clotting.
4. Platelets and some of the clotting factors form thrombin which together with fibrinogen in the biood forms an interconnected web called fibrin.
5. RBCs are caught,.in the web giving rise to the blood clot.
6. Thrombin is formed from prothrombin which is its precursor. Prothrombin is converted to thrombin by the action of platelets and other clotting factors.
7. Blood clots can form only when blood does not flow. Therefore the clot formed in an injured blood vessel does not spread to uninjured blood vessels where blood is flowing.
Abnormal Blood Clotting
1. Normal clotting takes place when a blood vessel is injured or damaged. Clotting should not take place in a normal blood vessel.
2. However in some diseases, blood clotting may take place inside an uninjured blood vessel. Such abnormal clotting called intravascular clotting will block the flow of blood in the blood vessel. The part of the body or tissue supplied by the blocked blood vessel will die because of lack of blood supply.
An example of this is coronary thrombosis with myocardial infraction. In this condition, clotting of blood inside the coronary arteries (blood vessel supplying the heart muscle) causes stoppage of blood flow to a part of the heart. The heart muscle deprived of blood supply dies, causing what is called a heart attack or myocardial infarction.
Blood Coagulation Tests
There are many very sophisticated tests to assess blood coagulation. The 3 simple tests which are used in clinical practice are:
1. Bleeding time. It is the time taken for bleeding to stop when a finger tip or lobe of the ear is pierced with a needle or a sharp knife. Bleeding time may be prolonged because of deficiency of platelets, excessive fragility of the blood vessel, and also when some clotting factors are deficient.
2. Clotting time. It is the time taken for blood (collected in a test tube) to clot. Clotting time is prolonged in deficiency of one or more of the clotting factors.
3. Prothrombin time. This test estimates the time taken for conversion of prothrombin to thrombin—a very essential step in the clotting of Anticoagulant therapy which reduces the quantity of prothrombin in the blood, is used to reduce the clotting tendency in patients who have had a heart attack. To prevent overdosage with anticoagulants, which could cause fatal bleeding, the dosage of anticoagulant drugs is controlled by estimating the prothrombin time.