Revision:Composition of Blood

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There are four major types of blood cells: red blood cells, platelets, lymphocytes, and phagocyte cells. Collectively, the lymphocytes and phagocyte cells constitute the white blood cells. Each type of blood cell has a specialized function: red cells take up oxygen from the lungs and deliver it to the tissues; platelets participate in forming blood clots; lymphocytes are involved with immunity; and phagocyte cells occur in two varieties-granulocytes and monocytes- and ingest and break down microorganisms and foreign particles. The circulating blood functions as a carrier, bringing the various kinds of cells to the regions of the body in which they are needed: red cells to tissues requiring oxygen, platelets to seal over points of injury, lymphocytes to areas of infection, and phagocyte cells to sites of microbial invasion and inflammation.

White Blood Cell

White cells, unlike red cells, are nucleated and independently mobile. Highly differentiated for their specialized functions, they do not undergo mitosis (ordinary cell division) in the bloodstream, but some retain the capability of cell division. As a group they are involved in the body's defense mechanisms and reparative activity. The number of leukocytes in normal blood ranges between 4,500 and 11,000 per cubic millimetre. Fluctuations occur during the day; lower values are obtained during rest and higher values during exercise. Violent physical exertion may cause the count to exceed 20,000 per cubic millimetre. Most of the leukocytes are outside the circulation, and the few in the bloodstream are in transit from one site to another. As living cells, their survival depends on their continuous production of energy. The chemical pathways utilized are more complex than those of the red cells and are similar to those of other tissue cells. Leukocytes, containing a nucleus and able to produce RNA, can synthesize protein.

Platelets

Platelets are formed when cytoplasmic fragments of megakaryocytes, which are very large cells in the bone marrow, pinch off into the circulation as they age. The platelet is metabolically more active than the red blood cell and has a variety of functions. Platelets play an important and not fully understood role in the formation of the blood clot by coagulating to occlude a cut blood vessel and provide a surface on which strands of fibrin form an organized clot, by contracting to pull the fibrin strands together to make the clot firm and permanent, and, perhaps most important, by providing or mediating a series of coagulation factors necessary to the formation of the clot. Platelets also store and transport several chemicals, including serotonin, epinephrine, and histamine (the importance of which in this capacity is unknown), and they phagocytose (absorb) foreign bodies, including viruses, as well.

Plasma

The liquid portion of the blood, the plasma, is a complex solution containing more than 90 percent water. The water of the plasma is freely exchangeable with that of body cells and other extra cellular fluids and is available to maintain the normal state of hydration of all tissues. Water, the single largest constituent of the body, is essential to the existence of every living cell. The major solute of plasma is a heterogeneous group of proteins constituting about 7 percent of the plasma by weight. The principle difference between the plasma and the extra cellular fluid of the tissues is the high protein content of the plasma. Plasma protein exerts an osmotic effect by which water tends to move from other extra cellular fluid to the plasma. Fatty substances (lipids) are present in plasma in suspension and in solution. Other plasma constituents include salts, glucose, amino acids, vitamins, hormones, and waste products of metabolism.

Red Blood Cell

The red cell is enclosed in a thin membrane that is composed of chemically complex lipids, proteins, and carbohydrates in a highly organized structure. Extraordinary distortion of the red cell occurs in its passage through minute blood vessels, many of which have a diameter less than that of the red cell. When the deforming stress is removed, the cell springs back to its original shape. The red cell readily tolerates bending and folding, but, if appreciable stretching of the membrane occurs, the cell is damaged or destroyed. The membrane is freely permeable to water, oxygen, carbon dioxide, glucose, urea, and certain other substances, but it is impermeable to hemoglobin. Within the cell the major cation is potassium; in contrast, in plasma and extra cellular fluids the major cation is predominantly sodium. A pumping mechanism, driven by enzymes within the red cell, maintains its sodium and potassium concentrations.

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