Beginning of Essay
is the course taken by the blood through the arteries, capillaries, and veins and back to the heart. In humans, the heart is made up of four chambers the right and left auricles, or atria, and the right and left ventricles. The right side of the heart pumps oxyge ....

Middle of Essay
.... valve, which completely closes during contraction of the ventricle. In its passage through the lungs, the blood is oxygenated, then it is brought back to the heart by the four pulmonary veins, which enter the left auricle. When this chamber contracts, blood is forced into the left ventricle and then by ventricular contraction into the aorta. The bicuspid, or mitral valve prevents the blood from flowing back into the auricle, and the semilunar valves at the beginning of the aorta stop it from flowing back into the ventricle. Similar valves are present in the pulmonary artery. The aorta divides into a number of main branches, which in turn divide ....