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  the  con- tinuously  forward  moving  column  of  blood.  The one on the right has three flaps, or cusps, and is called  the  TRICUSPID  VALVE.  The  one  on  the left has two flaps and is called the MITRAL, or BICUSPID, VALVE. The outlets of the ventricles are supplied with similar valves. On the right the pulmonary  valve  is  at  the  origin  of  the  pulmonary artery, and on the left the aortic valve is at the origin  of  the  aorta. Physiologically,  the  heart  acts  as  four  inter- related  pumps.  The  right  atrium  receives  deox- ygenated  blood  from  the  body  via  the  superior and  inferior  vena  cavae.  It  pumps  this  blood through the tricuspid valve to the right ventricle. The  right  ventricle  pumps  the  blood  past  the pulmonary  valve  through  the  pulmonary  artery to  the  lungs  for  oxygenation.  The  left  atrium receives  the  oxygenated  blood  from  the  lungs through  four  pulmonary  veins  and  pumps  it  to the left ventricle past the mitral valve. The left ventricle pumps the blood to all areas of the body via  the  aortic  valve  and  the  aorta. 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 lit- tle muscle fiber because the blood flows from the atria   to   the   ventricles   under   low   pressure. However, the walls of the ventricles, which com- prise 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 the peripheral systemic  circulation  than  into  the  lungs  located only  a  short  distance  from  the  heart. The heart acts by contraction and relaxation. It  contracts  with  a  wringing  motion,  forcing  blood into the arteries. Each contraction is followed by limited  relaxation  or  dilation.  Cardiac  muscle never completely relaxes, it always maintains a degree of tone. Contraction of the heart is called SYSTOLE and the period of work. Relaxation of the  heart  with  limited  dilation  is  called DIASTOLE  and  the  period  of  rest.  A  complete CARDIAC CYCLE is the time from onset of one contraction, or heart beat, to the onset of the next. The contractions of the heart are stimulated and  maintained  by  the  SINOATRIAL  NODE, commonly called the PACEMAKER of the heart, which is a group of hundreds of cells in the up- per part of the right atrium that sets off electrical impulses,   causing   both   atria   to   contract simultaneously. The normal heart rate, or number of  contractions,  is  about  72  beats  per  minute. The BLOOD PRESSURE is the pressure the blood  exerts  on  the  walls  of  the  arteries.  The highest  pressure  is  called  SYSTOLIC  pressure, because it is caused when the heart is in systole, or  contraction.  A  certain  amount  of  blood pressure is maintained in the arteries even when the  heart  is  relaxed.  This  is  the  DIASTOLIC pressure, because it is present during diastole, or relaxation  of  the  heart. Normal blood pressure can vary considerably with age, weight, and general condition of the in- dividual. For young adults the systolic pressure is between 120 and 150 mm of mercury, and the diastolic  pressure  is  between  70  and  90  mm  of mercury.  Women  have  a  lower  blood  pressure than  men.  The  difference  between  systolic  and diastolic   pressure is   known   as   PULSE PRESSURE. BLOOD  VESSELS The blood vessels of the body fall into three distinct  classifications: 1.  Distributors—arteries  and  arterioles 2.  Exchangers—capillaries 3.  Collectors—veins  and  venules The ARTERIES are elastic tubes constructed to withstand high pressure. They carry blood away from  the  heart  to  all  parts  of  the  body.  The smallest  branches  of  the  arteries  are  called arterioles. The  AORTA  is  the  large  tubelike  structure arising  from  the  left  ventricle  of  the  heart.  It arches upward over the left lung and then down along the spinal column through the thorax and the  abdomen,  where  it  divides  to  send  arteries down   both   legs   (fig.   3-31).   The   CORONARY ARTERIES  are  branches  of  what  is  generally called the ascending aorta, and they supply the heart  with  blood. There are certain branches of the aorta with which  you  should  be  familiar,  since  these  often must be compressed to control hemorrhage. You will  find  a  discussion  of  pressure  points  in  the hemorrhage  section  of  the  chapter  in  this  manual entitled “First Aid and Emergency Procedures.” 3-22