How clotting of Blood occurs in humans?

Bleeding occurs usually when any cut occurs on our body. Within 3 to 5 minutes time a clot occurs at the place of cut to stop further loss of blood. In this resource I have explained the simple mechanism of clotting and the various steps involved in this process. I also gave some insight into various clotting facts in human body.


Usually when we work with sharp instruments like blade, knife, razors, a pair of scissors, chisel, peelers etc. accidentally you may get a cut over the surface of your body and from the cut place blood may start oozing out. You may find within 3 to 5 minutes a clot may be formed at the site of cut and the bleeding will be stopped. The clot formation will protect our body from any further loss of blood as well as any entry of germs through the cut portion of the blood vessels. It is a kind of defense mechanism to stop the entry of any germs through our cut blood vessels.

Mechanism of Blood clotting

Blood platelets (Thrombocytes), one of the formed elements or blood corpuscles present in blood plasma play an important role in blood clotting process. Clotting is a very complex process which require not less than 12 different important factors which have to work in harmony with each other. Clotting process doesn't occur if any of the 12 clotting factors are not present in required level in the blood plasma. The formation of clotting factors and the series of steps in clotting process occur in a chain like fashion in our body. The clotting mechanism of blood occurs in a series of steps as follows.

  1. Conversion of Prothrombin into Thrombin
  2. Prothrombin is a protein present in blood plasma which is synthesized in liver. This protein present in blood plasma is in inactive form and should be converted into an active enzyme form thrombin in order to initiate blood clotting process.
    At the site of cut, the damaged cells and the platelets present in the oozing out blood secrete thromboplastin or thrombokinase enzyme, clotting factors VII and X (Stuart factor), calcium ions, Vitamin K. These substances together convert the inactive prothrombin present in blood plasma into thrombin. Calcium ions present in blood plasma are very essential for this conversion. Vitamin K is essential for the synthesis of prothrombin and fibrinogen proteins by the liver cells as these are essential for clotting of blood.

  3. Conversion of Fibrinogen into Fibrin
  4. Fibrinogen is a soluble protein present in blood plasma. This soluble protein is now converted into insoluble fibrin with the help of the enzyme thrombin in the above step. To occur this step also requires the presence of calcium ions. Fibrin is simply long microscopic silky thread like structures which become interwoven to form a mesh like structure in front of the opening of cut blood vessel.

  5. Formation of clot
  6. Blood cells got trapped in these meshes of network of fibrin. This network then shrinks and squeezes out the blood plasma in the form of a clear pale yellow color liquid called serum. The solid mass left behind is known as clot or thrombus.

Why clotting doesn't occur in our blood vessels

The lining of the undamaged blood vessels is very smooth and doesn't promote cell or platelets rupture. A substance heparin present in low concentration in blood also doesn't allow the blood to clot. Heparin is synthesized by mast cells of connective tissue and also by liver. Heparin prevent the conversion of prothrombin into thrombin and fibrinogen into fibrin. These two substances are essential for clotting of blood. Sometimes blood blisters are formed on our skin due to injuries like sharp pinch if the skin doesn't break.

How coronary thrombosis occur

Coronary artery is the artery which supply oxygenated blood to heart muscles. If endothelium of this blood vessel got damaged or the inner surface become rough due to the formation of a rough layer of cholesterol which promotes breakdown of platelets resulting in clot formation. The clot formation results in stoppage of transport of blood to heart muscles resulting in heart attack or myocardial infarction. The clinical condition of forming thrombus in coronary artery is known as coronary thrombosis. When the thrombus is transported by blood stream is called em-bolus. It is known as embolism when it is lodged in blood vessel and cuts of blood circulation.
Here is Human Excretory system and its structure

Why hemophilia is found in some individuals

Hemophilia is a genetic defect caused through one of the two defective genes present on X chromosome of these individuals. One of the defective gene present in an individual cause hemophilia A type of defect in 85% of the people. The reason for this is the defective gene controls the production of a clotting factor VIII. Similarly the other defective gene controls the production of another clotting factor IX which causes hemophilia B type of defect and can be found in about 15% of the people.


The substances which prevent the process of clotting of blood are known as anticoagulants. Coagulation means clotting of blood. Sodium oxalate and sodium citrate are used as anticoagulants in blood banks to preserve blood. Heparin present in low concentration in our blood and also the salivary glands of Leeches are also anticoagulants.
Here is the video of 'How does blood clot':-



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