BLOOD

Blood is fluid tissue composed of formed elements (cells) suspended in plasma. It is pumped by the heart through miles of arteries, capillaries, and veins to all parts of the body. Total blood volume of the average adult is 5 to 6 liters.

PLASMA is the liquid part of blood; the whole blood minus cells. Plasma constitutes 50 to 60 percent of whole blood. It is a clear, slightly alkaline, straw-colored liquid consisting of about 92 percent water. The remainder is made up mainly of proteins. One of these, fibrinogen, contributes to coagulation.

BLOOD SERUM is a clear, pale yellow liquid. It is the liquid portion of blood after coagulation. Plasma and serum differ in that plasma is whole blood minus the cells, and serum is plasma minus the clotting elements.

Red blood cells (RBCs), or ERYTHROCYTES, are small, biconcave, nonnucleated disks, formed in the red bone marrow. Blood of the average man contains 5 million red cells per cubic millimeter. Women have fewer red cells, 4.5 million per cubic millimeter. Emotional stress, strenuous exercise, high altitudes, and some diseases may cause an increase in the number of RBCs.

During the development of the red blood cell, a substance called hemoglobin is combined with it. HEMOGLOBIN is the key of the red cell’s ability to carry oxygen and carbon dioxide. Thus the main function of erythrocytes is the transportation of respiratory gases. The red cells deliver oxygen to the body tissues, holding some oxygen in reserve for an emergency. Carbon dioxide is picked up by the same cells and discharged via the lungs.

The color of the red blood cell is determined by the hemoglobin content. Bright red, or arterial, blood is due to the combination of oxygen and hemoglobin. Dark red, or venous, blood is the result of hemoglobin combining with carbon dioxide.

A red blood cell will live only about 100 to 120 days in the body. There are several reasons for its short life span. This delicate cell has to withstand constant knocking around as it is pumped into the arteries by the heart. It travels through blood vessels at high speed, bumps into other cells, bounces off the walls of arteries and veins, and squeezes through narrow passages. It must adjust to continual pressure changes. Fragments of red blood cells are found in the spleen and other body tissues. The spleen is the “graveyard” where old, worn out cells are removed from the blood stream.

White blood cells (WBCs), or LEUKOCYTES, are almost colorless, nucleated cells originating in the bone marrow and in certain lyrnphoid tissues of the body. There is only one white cell to every 600 red cells. Normal WBC count is 6,000 to 8,000 per cubic millimeter, although the number of white cells maybe 15,000 to 20,000 or higher during infection.

Leukocytes are important for the protection of the body against disease. Their AMEBOID movement permits them to leave the blood stream through the capillary wall and to attack pathogenic bacteria. They can travel anywhere in the body and are often named “the wandering cells.” They protect the body tissues by engulfing disease-bearing bacteria and foreign matter, a process called phagocytosis. When white cells are undermanned, more are produced, causing an increase in their number and a condition known as leukocytosis. Another way they protect the body from disease is by the production of bacteriolysins that dissolve the foreign bacteria. The secondary function of WBCS is to aid in blood clotting.

Blood platelets, or THROMBOCYTES, are round bodies in the blood that contain no nucleus, only cytoplasm. They are smaller than red blood cells and average about 250,000 per cubic millimeter of blood. They play an important role in the process of blood coagulation, clumping together in the presence of jagged, torn tissue.