evaporation does not remove all the sweat that has been excreted, the sweat collects in beads on the surface of the skin. CERUMINOUS   GLANDS.—Ceruminous glands  are  modified  sweat  glands  found  only  in  the auditory canal. They secrete a yellow, waxy substance called  cerumen  that protects the eardrum. THE CIRCULATORY SYSTEM LEARNING OBJECTIVE: Identify the parts of  the  circulatory  system,  and  recognize  their major components and functions. The  circulatory  system,  also  called  the  vascular system,  consists  of  blood,  heart,  and  blood  vessels. The circulatory system is close circuited (i.e., there is no opening to external environment of the body). The function of this system is to move blood between the cells  and  the  organs  of  the  integumentary,  digestive, respiratory, and urinary system that communicate with the external environment of the body. This function is facilitated by the heart pumping blood through blood vessels.   The   blood   travels   throughout   the   body transporting nutrients and wastes, and permitting the exchange of gases (carbon dioxide and oxygen). BLOOD Blood is fluid tissue composed of formed elements (i.e., cells) suspended in plasma. It is pumped by the heart through arteries, capillaries, and veins to all parts of the body. Total blood volume of the average adult is 5 to 6 liters. Plasma Plasma   is   the   liquid   part   of   blood   (fig.   1-31). Plasma constitutes 55 percent of whole blood (plasma and cells). 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 proteins,  fibrinogen, contributes to coagulation. Blood Cells The blood cells suspended in the plasma constitute 45 percent of whole blood. Its cells, which are formed mostly  in  red  bone  marrow,  include  red  blood  cells (RBCs) and white blood cells (WBCs). The blood also contains   cellular   fragments   called   blood   platelets. When blood components are separated, the WBCs and platelets   form   a   thin   layer,   called   the   buffy  coat, between the layers of plasma and RBCs. These layers are illustrated in figure 1-31. RED   BLOOD   CELLS.—Red   blood   cells,   or erythrocytes,   are   small,   biconcave,   nonnucleated disks,   formed   in   the   red   bone   marrow   (fig.   1-32). 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 (arterial) blood is due to the combination of oxygen and hemoglobin. Dark 1-24 BLOOD SAMPLE LIQUID (PLASMA) "BUFFY COAT" RED BLOOD CELLS 55% 45% HM3F0131 Figure 1-31.—Blood sample illustrating blood components. HM3F0132 PLATELETS WHITE BLOOD CELLS RED BLOOD CELLS PERIPHERAL BLOOD SMEAR Figure 1-32.—A blood smear showing red blood cells, white blood cells, and platelets.