The human heart is a muscular organ that is composed of four compartments; two small upper compartments called the left and right atria, and two large lower compartments called the left and right ventricles. The heart is essentially a pumping station that permits blood to flow efficiently throughout the body within blood vessels.
Let's trace the path of blood flow into the heart from three entry points, which are located inside the right atrium. The right atrium serves as an entry point for deoxygenated blood (blood that is low in oxygen and high in carbon dioxide) from every part of the body with the exception of the lungs. Deoxygenated blood reaches the right atrium through the following three veins in the body: (1)the superior vena cava, a large vein which returns blood from parts of the body superior (located higher up) to the heart; (2)the inferior vena cava, also a large vein which brings blood from parts of the body inferior (located below) to the heart; and (3)the coronary sinus, which returns deoxygenated blood from the blood vessels that normally supply oxygen to the walls of the heart muscle tissue. When contraction of the right atrium occurs, the increase in hydrostatic pressure within the right atrium forces the entire volume of deoxygenated blood to pass through the right atrioventricular valve (tricuspid valve) and into the right ventricle. Then strong muscular contractions in the right ventricle force the blood out through the pulmonary arteries to the lungs. The pulmonary semilunar valve, located in the opening where the pulmonary artery leaves the right ventricle, impedes the backflow of blood into the heart. In the lungs, the deoxygenated blood passes through capillaries (very small, thin-walled blood vessels that connect arteries to veins) where gas exchange takes place-the deoxygenated blood picks up oxygen and simultaneously releases carbon dioxide. Subsequently, blood becomes oxygenated and returns to the heart through four pulmonary veins, which empty into the left atrium. Contraction of the left atrium combined with an increase in hydrostatic pressure force the oxygenated blood through the left atrioventricular valve (bicuspid or mitral valve) and into the left ventricle. Very strong muscular contractions in the left ventricle pump the oxygenated blood out of the heart and into the aorta (the largest artery in the body). From the ascending aorta, the blood is forced into the coronary arteries, arch of the aorta, thoracic aorta, and abdominal aorta so that it can eventually reach all of the body parts with the exception of the lungs. A valve in the aorta, the aortic semilunar valve located at the opening between the left ventricle and the aorta, prevents the blood from flowing back into the heart itself.
The path of blood flow from the heart to the lungs and back to the heart is generally referred to as the pulmonary circulation. Another path of blood flow from the heart to the various organs of the body and back to the heart is known as the systemic circulation. Finally, the flow of blood through the blood vessels that serve the heart muscle tissue is commonly known as the coronary circulation.