Blood clotting time is used to detect many diseases.

Clotting is also called the coagulation of blood. It is the time required by the blood to clot. In order for blood to clot, the enzyme thrombin must be generated from the plasma precursor prothrombin. Thrombin then converts soluble fibrinogen into insoluble fibrin

The generation of thrombin involves the sequential activation of a number of other plasma clotting factors, this process is also being assisted by Ca++ and by factors released by platelets and damaged tissues. The time taken for blood to clot mainly reflects the time required for the generation of thrombin in this manner. 

If the plasma concentration of prothrombin or of some other factors is low (or if the factor is absent, or functionally inactive), clotting time will be prolonged. The expected range for clotting time is 4-10 min.

There are 2 methods used to measure blood time.

1-Capillary blood methods:

  • Slide method                       2 - 6 min,
  • Capillary tube method        2  - 6 min.

2- Venous blood methods:

  • Howell methods                      0  to  30 min,
  • Lee and White method           5  to  15 min.

Benefit:

There are 2 methods used to measure blood time. Clotting time is used to detect many diseases, such as

  • Clotting time is more in hemophilia,
  • vitamin k deficiency,
  • Obstructive jaundice, 
  • heparin therapy, 
  • pneumonia, and 
  • leukemia.

Blood clotting mechanism:

All Vertebrates include Human beings, have developed a mechanism for preventing the accidental loss of blood. Whenever a blood vessel is ruptured, one of the soluble plasma proteins, fibrinogen, is enzymatically converted into an insoluble protein, fibrin, which forms a semi-solid clot.



Some people think that blood clots when it becomes exposed to the air or when it stops flowing. However, if one were to carefully remove blood from a vessel- without allowing it to contact the damaged part of the vessel, and then place this blood on a smooth plastic dish or one lined with paraffin, it would not clot. However, if this blood were allowed to touch any damaged tissues or were placed on glass or some other relatively rough surface, the blood would clot.



Blood clotting factor:

Either the damaged tissues or the blood itself must release some chemical that initiates the clotting Mechanism. 

Actually, it is both damaged tissues and disintegrated platelets in the plasma that release substances responsible for the clotting reaction. Platelets are very small disc-shaped bodies found in mammalian blood and formed in the bone marrow from large cells called megakaryocytes. Platelets seem to disintegrate more readily upon contacting glass surfaces than on plastic surfaces. Platelets are also called thrombocytes.



When a blood vessel is cut, the damaged tissues release a lipoprotein, called thromboplastin, which initiates the clotting mechanism. Calcium ions and certain protein factors in the plasma must be present in order for thrombo­plastin to be effective. Thromboplastin interacts with Ca+2 and these proteins to produce prothrombinase, the enzyme that catalyzes the second step in the clotting mechanism. Prothrombinase can also be made by the interaction of a substance released from the disintegrated platelets (platelet factor # 3) and other factors in the plasma, including Ca2+ and proteins. The prothrombinase made from either the tissues or the platelets catalyzes the conversion of prothrombin, a plasma globulin into thrombin. Finally, thrombin enzymatically converts fibrinogen into fibrin, an insoluble protein. Fibrin forms long fibers that mesh and trap red cells, white cells, and platelets, forming the clot. Usually the clot forms within 5 minutes of the rupturing of the vessel. The clot then begins to contract and squeeze out most of the plasma from itself within an hour. This process, called clot retraction, serves to increase the strength of the clot and also pulls the vessel walls adhering to the clot closer together.

The extruded plasma is now called serum since all the fibrinogen and most other clotting factors have been removed. Because it lacks these constituents, the serum cannot clot.







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