Blood Clotting

The clotting process is initiated by a protein called tissue factor, which is bound to the membranes of most cells outside the bloodstream. Blood coming into contact with these membranes sets off a sequence of reactions involving various blood protein called factors. Thus, a molecule of factor VII coming into contact with tissue factor converts to an active form that in turn converts ten molecules of factor X into thromboplastin, with the help of calcium ions. Thromboplastin in turn binds to factor Va, which then binds to the protein prothrombin to form two million molecules of the clotting enzyme thrombin. Thrombin acts as a catalyst to convert fibrinogen to billions of molecules of the insoluble protein fibrin, the basis of the final clot. The clot is made firm by factor XIII, reinforced by blood platelets.

Because minor injuries often occur, the clotting process is almost constantly taking place somewhere in the body. It must be terminated quickly, however, or blood would clot everywhere and death would ensue. To prevent this form happening, plasma proteins known as fibrinol-ysins, or plasmin, dissolve old clots in the bloodstream.

Blood Transfusions
Transfusions involve the intravenous administration of blood from a donor to a patient. The procedure's safety and effectiveness relates, in large measure, to careful donor selection and to pre-transfusion testing. Blood is now mainly transfused as components rather than as whole blood; a donated unit is separated into its cellular and plasma constituents and transfused according to the patient's clinical needs. Thus, blood from single donor can provide or the needs of several patients.

There is no true artificial substitute for blood. Some experimental solutions can transport oxygen to the tissues temporarily, and synthetic plasma substitutes are used to raise blood fluid levels.