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
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, eg. measles, whopping
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, a 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:
The hemoglobin in each red blood cell
The relative number of red blood cells
in the bloodstream
The speed with which the red blood cells
arrive in the tissue
Of all the tissues, brain tissue has
about the greatest and most continuous need for oxygen. 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.