The ability of the body to control the flow of blood following any vascular injury is of paramount importance to continued survival. The process of blood clotting and then the subsequent dissolution of the clot, following repair of the injured tissue, is termed hemostasis.
There is a total of 4 major events that occur during hemostasis in a set order following the loss of vascular integrity.
During the initial phase called vascular constriction, the affected vascular tissues limits the flow of blood to the area of injury.
During the second phase, platelets become activated by thrombin and aggregate at the site of injury, forming a temporary, loose platelet plug. The protein fibrinogen is primarily responsible for stimulating platelet clumping. Platelets clump by binding to collagen that becomes exposed following rupture of the endothelial lining of vessels. Upon activation, platelets release ADP and TXA2 (which activate additional platelets), serotonin, phospholipids, lipoproteins, and other proteins important for the coagulation cascade. In addition to induced secretion, activated platelets change their shape to accommodate the formation of the plug.
During the third phase, to insure stability of the initially loose platelet plug, a fibrin mesh (also called the clot) forms and entraps the plug. If the plug contains only platelets it is termed a white thrombus; if red blood cells are present it is called a red thrombus.
During the last phase, the clot must be dissolved in order for normal blood flow to resume following tissue repair. The dissolution of the clot occurs through the action of plasmin.
Two pathways lead to the formation of a fibrin clot: the intrinsic and extrinsic pathway. Although they are initiated by distinct mechanisms, the two converge on a common pathway that leads to clot formation. The formation of a red thrombus or a clot in response to an abnormal vessel wall in the absence of tissue injury is the result of the intrinsic pathway. Fibrin clot formation in response to tissue injury is the result of the extrinsic pathway. Both pathways are complex and involve numerous different proteins termed clotting factors.
N.B. For purposes of laboratory tests, plasma is obtained from whole blood. To prevent clotting, an anticoagulant such as citrate or heparin is added to the blood specimen immediately after it is obtained. (Usually the anticoagulant is already in the evacuated blood collection tube (e.g. Vacutainer or Vacuette when the patient is bled.) The specimen is then centrifuged to separate plasma from blood cells. Plasma can be frozen below negative 80 degree celcius nearly indefinitely for subsequent analysis.
For many biochemical laboratory tests, plasma and blood serum can be used interchangeably. Serum resembles plasma in composition but lacks the coagulation factors. It is obtained by letting a blood specimen clot prior to centrifugation. For this purpose, a serum-separating tube can be used which contains an inert catalyst (such as glass beads or powder) to facilitate clotting as well as a portion of gel with a density designed to sit between the liquid and cellular layers in the tube after centrifugation, making separation more convenient.
Barring the use of the inert catalyst and the centrifugation, if the blood cells and the blood serum were to be allowed to separate naturally, it would take at least 20 minutes and above to occur. As such, if blood that were found on evidence taken from the crime scene were confirmed forensically to have the blood cells to be separated from the blood serum, then it would be extremely likely that the blood were actually planted onto the evidence after the crime was committed rather than transferred onto the evidence during the occurance of the crime.
On a separate note, it is interesting to note that the approximate distribution of blood types in the United States of America population is as follows. The distribution may be differ for specific racial and ethnic groups.
O Rh-positive: 38 percent
O Rh-negative: 7 percent
A Rh-positive: 34 percent
A Rh-negative: 6 percent
B Rh-positive: 9 percent
B Rh-negative: 2 percent
AB Rh-positive: 3 percent
AB Rh-negative: 1 percent