Blood Coagulation (clotting) and Dissolution of Clot (Clot Lysis)

Human Physiology

Lesson 30 : Blood Coagulation & Blood Groups

Blood Coagulation (clotting) and Dissolution of Clot (Clot Lysis)

One might have observed that within few minutes after a minor injury, the blood flow stops from the injury site and a dark coloured solid mass appear after few hours that permanently seal the site of injury. This process is a wonderful property of blood by which excessive blood loss from injuries is prevented. Blood coagulation is a process of formation of clot with the help of clotting factors. It is a biochemical phenomenon involving 12 clotting factors present in blood. All clotting factors are present in blood, yet blood does not clot inside blood vessels, because the factors are in inactive form. These factors are activated once the blood comes in contact with foreign surface (other than blood vessels) or mixed with tissue fluid. All clotting factors are protein in nature and act as enzyme (inactive form is proenzyme) except factor IV which is calcium. During clotting fibrinogen is converted into fibrin which later gets polymerized to become insoluble fibrin clot that seal the site of injury. Fluid that oozes out after clot formation is called serum.
Summarized reaction of clotting mechanism is as below:

  1. Proteolysis of fibrinogen by thrombin enzyme to yield fibrin monomer.
  2. Polymerization of fibrin monomer to form fibrin polymer (soluble).
  3. Stabilization of soluble fibrin polymer to insoluble clot with the help of factor XIII.

Once the clot is formed it remains at the place of injuries few days until gets dissolved or broken down. Fibrinolysis is process of dissolution of clot which initiates after clot formation with the help of separate set of enzymes. During this process first plasminogen is converted to plasmin by plasminogen activators (clot itself & other factors); subsequently plasmin converts fibrin into small peptides and which are removed by phagocytic cells. Fibrinolysis is important process for maintaining patency of blood vessels by removing undesirable clots.

  • Immune functions of blood.

One might have observed that children are vaccinated against several diseases like diphtheria, tuberculosis, small pox, hepatitis, polio and so on in order to protect them against these diseases in future. The protection against these micro organisms (bacteria and viruses) is provided through immune functions of blood. Immunity is defined as protection of body against disease with the help of immune cells. Various types of white blood cells play an active role in immune function. Neutrophils and monocyte engulf bacteria and destroy them with the help of their enzymes. Lymphocytes and plasma cells provides protection through secretion of antibodies (specialized immunoglobulins), that can kill or neutralize small viruses and antigens present in them. Once the lymphocytes are activated by antigen, they start producing antibodies which can provide immunity for years or even life long in certain cases.

  • BLOOD Groups And Their Significance.

Blood grouping is also an immunological property of blood. Two blood group systems are very important in human beings, one is ABO system and other is Rh system. The blood groups are classified based on type of antigen present on their RBCs. In ABO system, the RBC either have ‘A’ antigen (A group), or ‘B’ antigen (B group) or both A & B antigens (AB group) or no antigen (O group). Rh antigen was first identified in rhesus monkey is either present on RBC (Rh +ve group) or absent on RBC (Rh –ve group). Apart from antigen, specific blood group also possesses reciprocal antibody in their serum as well e.g. A group has anti B antibody, B group has anti A antibody, AB group has no antibody and O group has both anti A and anti B antibodies in their serum.

  • Blood transfusion and matching

The presence of specific antigen on RBC and reciprocal antibody in the serum makes it difficult to transfuse blood from any healthy person to a patient. Healthy person donating the blood is called donor and patient receiving the blood is called recipient. Matching of donor and recipient blood is prerequisite for blood transfusion because mismatching results in transfusion reaction (a type of immunological reaction in which antigen and antibody of donor and recipients reacts with each other) and life of patient becomes difficult to save. Transfusion reaction involves mismatching of ABO system, whereas, Rh system is involved in immunological reaction in newborn at the time of child birth.

Last modified: Tuesday, 10 April 2012, 11:20 AM