This is accomplished via nerve impulses and contractions of muscles. The heart is loosely enclosed in a fibrous, protective, bag-like enclosure called the pericardium. The heart is comprised of 4 chambers. The top portion houses the atrium, of which there are right and left valves. The bottom portion of the heart contains the chamber known as the ventricle, of which there are also right and left valves.
Total health depends on the delivery of clean, oxygenated blood to the cells.
The heart is an amazing mechanism which helps the body maximize oxygen utilization. When unoxygenated blood is delivered to the lungs by way of the pulmonary arteries, it is circulated and mixed with oxygen breathed in from the outside air. This oxygenated blood returns to the relaxed left atrium of the heart, in a phase known as diastole. It passes into the left ventricle which then sends the fresh blood forth throughout the body via the arteries in a process known as systemic circulation, because it travels throughout the entire system. The coronary arteries are the blood vessels which supply blood to the heart.