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Blood Physiology


                 When the lining of a blood vessel breaks and endothelial cells are damaged, revealing collagen
            proteins in the vessel wall, platelets swell, grow spikey extensions, and start clumping together. They
            start to stick to each other and the walls of the vessel. This continues as more platelets congregate and
            undergo these same transformations. This process results in a platelet plug that seals the injured area. If
            the injury is small, a platelet plug may be able to form and close it within several seconds. If the
            damage is more serious, the next step of blood clotting will take place. Platelets contain secretory
            granules. When they stick to the proteins in the vessel walls, they  degranulate, thus releasing their
            products, which include ADP (adenosine diphosphate), serotonin, and thromboxane A2.

                 A Blood Clot Forms: If the platelet plug is not enough to stop the bleeding, the third stage of
            hemostasis begins: the formation of a blood clot. First, blood changes from a liquid to a gel. At least 12
            substances called clotting factors take part in a series of chemical reactions that eventually create a
            mesh of protein fibers within the blood. Each of the clotting factors has a very specific function. We
            will discuss just three of the substances here: prothrombin, thrombin, and fibrinogen. Prothrombin and
            fibrinogin are proteins that are produced and deposited in the blood by the liver.

                     • Prothrombin: When blood vessels are damaged, vessels and nearby platelets are stimulated
                   to release a substance called prothrombin activator, which in turn activates the conversion of
                   prothrombin, a plasma protein, into an enzyme called thrombin. This reaction requires calcium
                   ions.

                     • Thrombin:  Thrombin  facilitates   the   conversion   of   a   soluble   plasma   protein   called
                   fibrinogen into long insoluble fibers or threads of the protein fibrin.

                     • Fibrin: Fibrin threads wind around the platelet plug at the damaged area of the blood
                   vessel, forming an interlocking network of fibers and a framework for the clot. This net of fibers
                   traps and helps hold platelets, blood cells and other molecules tight to the site of injury,
                   functioning as the initial clot. This temporary fibrin clot can form in less than a minute, and
                   usually does a good job of reducing the blood flow. Next, platelets in the clot begin to shrink,
                   tightening the clot and drawing together the vessel walls. Usually, this whole process of clot
                   formation and tightening takes less than a half hour.

                 The use of adsorbent chemicals, such as zeolites, and other hemostatic agents, are also being
            explored for use in sealing severe injuries quickly.



            ABO Group System


                 The ABO blood group is represented by substances on the surface of red blood cells (RBCs).
            These   substances   are   important   because   they   contain   specific   sequences   of   amino   acid   and
            carbohydrates which are antigenic. As well as being on the surface of RBCs, some of these antigens are
            also present on the cells of other tissues. A complete blood type describes the set of 29 substances on
            the surface of RBCs, and an individual's blood type is one of the many possible combinations of blood
            group antigens. Usually only the ABO blood group system and the presence or absence of the Rhesus
            D antigen (also known as the Rhesus factor or RH factor) are determined and used to describe the
            blood type. Over 400 different blood group antigens have been found, many of these being very rare. If
            an individual is exposed to a blood group antigen that is not recognized as self, the individual can
            become   sensitized   to   that   antigen;   the   immune   system   makes   specific   antibodies   which   binds
            specifically to a particular blood group antigen and an immunological memory against that particular



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