red (venous) blood is the result of hemoglobin combining with carbon dioxide. Red blood cells live only about 100 to 120 days in the body. There are several reasons for their short life span. These delicate cells have to withstand constant knocking around as they are pumped into the arteries by the heart. These cells travel through blood vessels at high speed, bumping into other cells, bouncing off the walls of arteries and veins, and squeezing through narrow passages. They must adjust to continual pressure changes. The spleen is the “graveyard” where old, worn out cells are removed from the blood stream. Fragments of red blood cells are found in the spleen and other body tissues. WHITE BLOOD CELLS.—White blood cells, or leukocytes, are almost colorless, nucleated cells originating in the bone marrow and in certain lymphoid tissues of the body (fig. 1-32). 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 may be 15,000 to 20,000 or higher during infection. Leukocytes are important for the protection of the body against disease. Leukocytes can squeeze between the cells that form blood cell walls. This movement, called diapedesis, permits them to leave the blood stream through the capillary wall and 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 WBC's protect the body from disease is by producing bacteriolysins that dissolve the foreign bacteria. The secondary function of WBCs is to aid in blood clotting. BLOOD PLATELETS.—Blood platelets, or thrombocytes, are irregular- or oval-shaped discs in the blood that contain no nucleus, only cytoplasm (fig. 1-32). They are smaller than red blood cells and average about 250,000 per cubic millimeter of blood. Blood platelets play an important role in the process of blood coagulation, clumping together in the presence of jagged, torn tissue. Blood Coagulation To protect the body from excessive blood loss, blood has its own power to coagulate, or clot. If blood components and linings of vessels are normal, circulating blood will not clot. Once blood escapes from its vessels, however, a chemical reaction begins that causes it to become solid. Initially a blood clot is a fluid, but soon it becomes thick and then sets into a soft jelly that quickly becomes firm enough to act as a plug. This plug is the result of a swift, sure mechanism that changes one of the soluble blood proteins, fibrinogen, into an insoluble protein, fibrin, whenever injury occurs. Other necessary elements for blood clotting are calcium salts; a substance called prothrombin, which is formed in the liver; blood platelets; and various factors necessary for the completion of the successive steps in the coagulation process. Once the fibrin plug is formed, it quickly enmeshes red and white blood cells and draws them tightly together. Blood serum, a yellowish clear liquid, is squeezed out of the clot as the mass shrinks. Formation of the clot closes the wound, preventing blood loss. A clot also serves as a network for the growth of new tissues in the process of healing. Normal clotting time is 3 to 5 minutes, but if any of the substances necessary for clotting are absent, severe bleeding will occur. Hemophilia is an inherited disease characterized by delayed clotting of the blood and consequent difficulty in controlling hemorrhage. Hemophiliacs can bleed to death as a result of minor wounds. THE HEART The heart is a hollow, muscular organ, somewhat larger than the closed fist, located anteriorly in the chest and to the left of the midline. It is shaped like a cone, its base directed upward and to the right, the apex down and to the left. Lying obliquely in the chest, much of the base of the heart is immediately posterior to the sternum. Heart Composition The heart is enclosed in a membranous sac, the pericardium. The smooth surfaces of the heart and pericardium are lubricated by a serous secretion called pericardial fluid. The inner surface of the heart is lined with a delicate serous membrane, the endocardium, similar to and continuous with that of the inner lining of blood vessels. The interior of the heart (fig. 1-33) is divided into two parts by a wall called the interventricular septum. In each half is an upper chamber, the atrium, which receives blood from the veins, and a lower chamber, the ventricle, which receives blood from the 1-25