They  secrete  a  yellow,  waxy  substance  called cerumen  that  protects  the  eardrum. CIRCULATORY  SYSTEM The  circulatory  system,  also  called  the VASCULAR  SYSTEM,  consists  of  the  heart, blood  vessels,  and  lymphatic  system.  It  is  the primary fuel supplier of the body. The transpor- tation media is the blood. This system is a closed circuit.  At  no  place  does  it  have  access  to  other tissues  of  the  body  except  at  the  capillaries. OSMOSIS, the transfer of fluids through the plasma membrane from an area of lower concen- tration  of  particles  to  an  area  of  higher  concen- tration, is the method of feeding body tissues and eliminating  waste  materials.  This  occurs  in  the capillaries,  the  smallest  of  the  blood  vessels. 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, con- tributes  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  ERYTHRO- CYTES,   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 dis- eases  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 diox- ide 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  re- moved  from  the  blood  stream. White   blood   cells   (WBCs),   or   LEUKO- CYTES,   are   almost   colorless,   nucleated   cells originating in the bone marrow and in certain lyrn- phoid 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  engulf- ing 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. 3-20