Blood flow through the body is designed so that oxygen-rich blood is seperated from the waste carrying deoxygenated blood. The internal structure of the heart allows for this seperation to be maintained, with blood in need of oxygen being sequestered and passed onto the lungs (also allowing it to eliminate carbon dioxide), while that which carries a hogh amount of oxygen being held seperate and passed onto the rest of the body. This is one of mammals major evolutionary advances which may very well have contributed to their success (and reptiles demise!). This seperation of blood allows for a much more efficient system of circulation compared to the other, "heart-bearing" animals (except birds, who also have four chambers). Below is a description of the flow blood follows within the body.

Blood that has a low oxygen concentration and a high carbon dioxide concentration enters the right atrium through the two venae cavae and coronary sinus.

As the right atrial wall contracts, the blood passes through the tricuspid valve and enters the chamber of the right ventricle. When the right ventricular wall contracts, the tricuspid valve closes (due to increased pressure in the chamber), and the blood moves through the pulmonary valve and into the pulmonary trunk and its branches. From these vessels, the blood enters the capillaries associated with the alveoli of the lungs. Gas exchanges occur between the blood into the capillaries and the air in the alveoli. The freshly oxygenated blood, which is now low in carbon dioxide concentration, returns to the heart through the pulmonary veins, which lead to the left atrium. (NOTE: deoxygenated blood usually flows in veins, oxygenated in arteries, but the pulmonary vessels are different. This is because arteries flow AWAY from the heart, and veins TOWARDS the heart. As the left atrial wall contracts, the blood moves through the bicuspid valve and into the chamber of the left ventricle. When the left ventricular wall contracts, the bicuspid valve closes (pressure changes) and the blood passes through the aortic valve and into the aorta and its many branches throughout the body.

BLOOD SUPPLY TO THE HEART

Blood is supplied to the tissues of the heart by the first two branches of the aorta, called the right and left coronary arteries. Their openings lie just beyond the aortic semilunar valve.

Because the heart must beat continually to supply blood to the body tissues, the myocardial cells require a constant supply of freshly oxygenated blood. The myocardium contains many capillaries fed by branches of the coronary arteries, allowing the heart to get access to the oxygenated blood first. The larger branches of these arteries usually have interconnections between vessels that provide alternate pathways for the blood, a helpful adaptation which allows survival despite partial clogage.

The blood that has passed through the capillaries of the myocardium is drained by branches of the cardiac veins, whose paths roughly parallel those of the coronary arteries. These veins join the coronary sinus, an enlarged vein which is on the posterior surface of the heart and empties into the right atrium.

BLOOD SUPPLY TO THE BODY The heart pushes blood through the body in two different flows. The right atrium receives blood from the body in need of oxygen (and needing to eliminate carbon dioxide) and the right ventricle delievers the blood to the lungs for gas exchange. The left atrium receives the oxygenated blood from the lungs, and the left ventricle delievers it to the body. This systemic blood flow has numerous sub pathways. Arteries branch of from the aorta into the upper body feeding the head and the arms. Blood leaving the heart by way of the descending aorta passes on to: a) the liver (via the hepatic artery and the hepatic portal system); b) the kidney (the renal artery); c) the stomach and the digestive tract; d) the rest of the trunk and the legs. Each of these pathways are seperate paths which then double back to the heart.