Leukocytes are derived from stem cells in the bone marrow. There are five types of white blood cells. Eosinophils and basophils have lesser roles in the system. Substances released from platelets initiate blood clotting. The human heart is divided into right and left halves.
Blood exits each ventricle through a semilunar valve.
Heart muscle cells are serviced by the coronary circulation, two arteries that branch directly off the aorta. As the atria fill, the ventricles are relaxed. The ventricles contract, the atrioventricular valves close, and blood flows out through the semilunar valves. Mechanisms of Contraction Because of the close junction of cardiac muscle cells, they contract in unison. The nervous system adjusts rate and strength.