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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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