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 WBCsis 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 atrium and pumps it out into the arteries. The openings between the chambers on each side of the heart are separated by flaps of tissue that act as valves to prevent backward flow of blood. The valve on the right has three flaps, or cusps, and is called the tricuspid valve. The valve on the left has two flaps and is called the mitral, orbicuspid, valve. The outlets of the ventricles are supplied with similar valves. In the right ventricle, the pulmonary valve is at the origin of the pulmonary artery. In the left ventricle, the aortic valve is at the origin of the aorta. See figure 1-33 for valve locations.

The heart muscle, the myocardium, is striated like the skeletal muscles of the body, but involuntary in action, like the smooth muscles. The walls of the atria are thin with relatively little muscle fiber because the blood flows from the atria to the ventricles under low pressure. However, the walls of the ventricles, which comprise the bulk of the heart, are thick and muscular. The wall of the left ventricle is considerably thicker than that of the right, because more force is required to pump the blood into distant or outlying locations of the circulatory system than into the lungs located only a short distance from the heart.