The heart is somewhat centrally located. Two thirds of the heart is on the left side of the sternum. It is the size of your fist, weighing in at a whopping pound and a half.  Although it appears small, its importance cannot be understated.

     The heart is divided into four chambers: the left and right atria, and the left and right ventricle.  The atria are on the upper half of the heart, and the ventricles make up the lower portion.  The dividing wall between the left and right sides is the septum.  There are four valves of the heart, which control the blood flow.  The object of the blood is to circulate oxygen for the growth and development of cells.  The blood is composed of red and white blood cells, platelets, lymph, plasma, and water.  The red blood cells contain hemoglobin, which is the chemical compound that carries the oxygen.  The white blood cells are part of the immune system.  Platelets are used when blood clots, to stop the bleeding. The platelets emit the blot clotting substance, fibrinogen, into its active form, fibrin.  Lymph is the interstitial fluid in the blood.  The plasma is the remaining portion of the blood, the mixture of glucose and water in which the blood cells is suspended.

     Blood enters the heart in the left atrium, from the superior and inferior vena cava.  The superior vena cava is the vein that collects the blood returning from the upper body, and the inferior vena cava returns blood from the lower body.  The deoxygenated blood of these two veins enters the heart in the right atrium during the systole part of the heartbeat.  In this phase of the heartbeat the atria are filling up and the ventricles are contracting in order to pump the blood.  The pulmonary and aortic valves open to allow the blood to leave the ventricles.  During the diastole part of the heartbeat, the atria contract to pump the blood into the ventricles.  To allow the blood to leave the atria and enter the ventricles, the tricuspid and mitral valves of the heart open to allow the blood to pass through.  Both of these phases of the heartbeat take a combined eight tenths of a second.