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.