Chapter III Blood
Blood is composed of cells (erythrocytes, leukocytes, and platelets) and plasma, the liquid in which the cells are suspended. The total blood volume of an average person is approximately 5.5L.
Components and Osmotic Pressure of Plasma
Plasma contains water, proteins (albumin, globulin, and fibrinogen), nutrients, metabolic end products, hormones, and mineral electrolytes. Albumin and globulin substantially determine the colliodal osmotic pressure of plasma. Globulin and fibrinogen decide the suspension stability of the red blood cells in plasma. Fibrunogen is essential for coagulation. The globulins are important in antibody formation. The plasma proteins also act as substrates for the release of peptides such as angiotensin and bradykinin and as binding vehicles for the transport of substances such as metal ions, fatty acids, steriods, hormones and drugs.
The osmotic pressure of plasma is about 300 mOsm/L. Since water is in diffusion equilibrium across cell membranes, the intracellular fluid also has an osmolarity of 300 mOsm. Na+ and CL ions are the major effectively nonpenetrating solutes in the extracellular fluids, while K+ ions and the organic solutes are the major nonpenetrating solutes in the intracellular fluid. Cells placed in hypotonic solutions swell, while cells placed in hypertonic solutions shrink.
Blood Cells
Erythrocytes, which make up more than 99 percent of blood cells, contain hemoglobin, an oxygen-binding protein consisting of heme and globin. Oxygen binds to the iron in heme. Erythrocytes are produced in the bone marrow and destroyed in the spleen. Iron, folic acid, and vitamin B12 are essential for erythrocyte formation. Control of erythrocyte production is exerted by the hormone erythropoietin, which is produced by the kidneys in response to low oxygen supply and which stimulates the bone marrow. Anemia can be caused not only by deficiency of iron, vitamin B12, and folic acid but by bone-marrow failure, excessive blood loss, excessive destruction of erythrocytes, and by erythropoietin deficiency.
The leukocytes include three types of polymorphonuclear granulocytes (neutrophils, eosinophils, and basophils), monocytes, and lymphocytes .All the leukocyte types are produced in the bone marrow. In addition, many lymphocytes undergo further development and mitosis in tissue outside the bone marrow. The peripheral lymphoid orgains are the lymph node, spleen, tonsils, and the lymphocyte accumulations in the linings of the interstial, respiratory, genital, and urinary tract. It is in the periperal lymphoid orgains that lymphocytes are stimulted to participate in specific immune responses (to see immulogy).
The platelets are cell fragments essential for the hemostasis.
Hemostasis
The rupture of a blood vessel results in blood loss that, if unchecked, would eventually lead to hemorrhagic shock and death. During normal activity, minor accidents frequently occur in which minute blood vessels are ruptured. Yet blood loss is minimal .The initial response to blood-vessel damage is vasoconstriction as well as sticking together of the opposed endothelial surfaces. The next events are formation of a platelet plug followed by blood coagulation (clotting). Platelets adhere to exposed collagen in a damaged vessel and release the contents of their secretory vesicles. These substances enhance further platelet aggregation. This process is also enhanced by thromboxane A2, produced by the platelets. Contractile elements in the platelets compress and strengthen the plug. The platelet plug does not spread along normal endothelium because the latter secretes prostacyclin, which inhibits platelet aggregation.
Blood Coagulation
Blood is transformed into a solid gel when, at the site of vessel damage, plasma fibrinogen is converted into fibrin molecules, which bind to each other to form a mesh. This reaction is catalyzed by the enzyme thrombin, which also activates factor XIII, a plasma protein that stabilizes the fibrin meshwork. . The formation of thrombin from the plasma protein prothrombin is the end result of a cascade of reactions in which an inactive plasma protein is activated and then enzymatically activates the next protein in the series. In the intrinsic clotting pathway, the cascade begins with the activation of factor XII by contact with collagen underlying a damaged vessel. In the extrinsic clotting pathway, the cascade begins when tissue factor forms an active complex with plasma factor VII, and this complex activates factors IX and X, plugging into the intrinsic pathway at this point. Both calcium and a platelet phospholipid, PF3, are required at several steps in the cascade. Two of the factors involved in clotting, factors VIII and V are activated in a positive-feedback manner by thrombin. Vitamin K is required by the liver for normal production of clotting factors
Anticlotting Systems
Clotting is opposed by antithrombin III and protein C. Protein C is activated by thrombin and inactivates factors VIII and V. Antithrombin III binds to heparin on endothelial cells and inactivates thrombin. Clots are dissolved by the fibrinolytic system, in which the plasma proenzyme, plasminogen, is activated by plasminogen activators to plasmin, which digests fibrin. Tissue plasminogen activator is secreted by endothelial cells and is activated by fibrin in a clot. Thrombin inactivates a plasma inhibitor of tissue plasminogen activator.
A-B-O blood types
Bloods are divided into different groups and types in accordance with the types of antigens present in the cells. Two different but related antigens, type A and type B, are present in the red cells in A-B-O grouping. Because of the way these antigens are inherited, a person may have neither of them in the cells, or may have one or both simultaneously. Therefore, there are four major A-B-O blood groups, namely A, B, O and AB groups.
