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