The haemostatic plug or blood clot is formed initially by aggregation of blood platelets; later fibrin deposition occurs and the fully formed blood clot contains also red cells and leucocytes within the fibrin meshwork.
The haemostatic plug is relatively stable except in haemophilia in which fibrin formation fails to occur.
The rapidity with which the process takes place and the degree of consistency of jelly produced vary greatly in different species of animals and sometimes in the animals of the same species but at different times.
The exact mechanism of blood clotting is not known. Many theories have been put forward to explain this process. All these theories differ only in the exact details but agree on the few main steps.
The first theory of blood clotting, which later became classic, was put forward by Alexander Schmidt.
Originally (in 1861) it was formulated as a chemical theory but subsequently (in 1872) as an enzyme one.
In contrast to the classic theory of blood coagulation as a biphasic process (phase I—thrombin formation ; phase II—fibrin formation) the current theory recognizes three main phases in the process (phase I—thromboplastin formation ; phase II—thrombin formation ; and phase III—fibrin formation).