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,   or   bicuspid,   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. Heart Functions The heart acts as four interrelated pumps. The right atrium receives deoxygenated blood from the body via the   superior   and   inferior  vena  cava.  It  pumps  the deoxygenated 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, where it is oxygenated. 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's   constant   contracting   and   relaxing forces   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 or “the period of work.” Relaxation of the heart is called  diastole  or “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. 1-26 AORTIC ARCH LEFT PULMONARY ARTERY RIGHT PULMONARY ARTERY PULMONARY TRUNK BRANCHES OF LEFT PULMONARY VEIN AORTIC SEMILUNAR VALVE MITRAL VALVE LEFT VENTRICLE INTERVENTRICULAR SEPTUM MYOCARDIUM (HEART MUSCLE) PAPILARY MUSCLE DESCENDING AORTA INFERIOR VENA CAVA (FROM TRUNK AND LEGS) RIGHT VENTRICLE TRICUSPID VALVE RIGHT ATRIUM PULMONARY SEMILUNAR VALVE SUPERIOR VENA CAVA (FROM HEAD AND  ARM) FROM LUNG FROM LUNG TO LUNG TO LUNG LEFT ATRIUM HM3F0133 Figure 1-33.—Frontal view of the heart—arrows indicate blood flow.