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Blood is composed of cells and plasma. Plasma is largely water in which are dissolved minerals, proteins, gases, and other chemicals that assist in the work of the blood. Whether all are in the right proportions or not depends on the state of nutrition and metabolism of the body and on the activity of the blood-forming organs. Most commonly plasma abnormalities occur when something goes wrong with the respiratory, digestive, or excretory organs.

Blood cells are produced in special tissues or organs. Bone marrow, the largest blood-forming organ, supplies red cells, certain white cells called granulocytes, and platelets. Other white cells (the lymphocytes and plasma cells) are formed in the spleen, lymph nodes, and other lymphoid tissues. Granulocytes once were considered the most important of the white cells because their activity could be easily observed in the laboratory. Recently, however, it has been discovered that lymphocytes and plasma cells-both classified as white cells-are very important in overcoming infections caused by viruses and in maintaining immunity against certain diseases; e.g., measles, whooping cough, and smallpox.

Diseases of the blood may involve red cells white cells, platelets, or plasma constituents. The effects of disease may result in too few or too many of the item concerned. When there are too many red cells, the condition is called polycythemia; when there are too few red cells, we speak of anemia.

An increased number of white cells occurs in response to an infection or in a case of leukemia and is called leukocytosis. A decrease in white cells is called leukopenia. A condition in which there are too few platelets is known as thrombocytopenia. Conditions in which there is an excess of plasma protein are rare, but too little of the right kind of protein in the plasma may cause abnormal bleeding problems, as in hemophilia. Too little of the gamma globulin component of the plasma's protein causes susceptibility to infection. Diagnosis of these conditions requires the facilities of modern hospitals and laboratories, and treatment demands the services of a physician.

The primary function of the red cells is to carry oxygen from the lungs to the tissues. Each red cell (erythrocyte) is a small elastic package of hemoglobin’s red, iron-containing substance capable of transporting oxygen.

All living tissues require a continuous supply of oxygen. The more active the tissue, the greater its need for oxygen. Three factors help to determine the amount of oxygen the blood can bring to a tissue:
(1) the hemoglobin in each red blood cell
(2) the relative number of red blood cells in the bloodstream, and
(3) the speed with which the red blood cells arrive in the tissue.

In anemia there is either a reduced amount of hemoglobin in each of the red blood cells or else the total number of red cells is less than usual. In either case the oxygen-carrying capacity of the blood is reduced and the tissues suffer. Strangely, when there is an excess of red blood cells, the tissues may also suffer for lack of oxygen. In this case the diffi culty is that with a high population of red cells the blood becomes syrupy and moves so slowly that it cannot deliver its oxygen quickly.

Of all the tissues, brain tissue has about the greatest and most continuous need for oxygen. Headache is one symptom that may herald the brain's need for more oxygen. (This is only one of the many possible causes of headache.) The symptoms of blood diseases depend on what effects these diseases produce on the body's tissues rather than on what changes can be observed in the blood itself.