The McGill Physiology Virtual Lab

Blood Laboratory

Hemostasis > Clotting and bleeding time tests
  Hemostasis (literally - blood halting) depends on three interrelated and overlapping sets of events:
  • Constriction of the blood vessels and formation of a platelet "plug"
  • Blood clotting
  • Clot retraction
Review the topics in your textbook.
Clotting time test

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. Generation of thrombin involves the sequential activation of a number of other plasma clotting factor, 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 of the 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 mins.


Observe and listen to the procedure for determining clotting time
Bleeding time test
This test measures the time taken for blood vessel constriction and platelet plug formation to occur. No clot is allowed to form, so that the arrest of bleeding depends exclusively on blood vessel constriction and platelet action.

Observe and listen to the procedure for determining bleeding time.

To continue with the next section, Prothrombin Time and Partial Thromboplastin Time, click here