The heart is usually found in the left to middle of the chest with the largest part of the heart slightly to the left. It is about the size of a fist. The heart is surrounded by the lungs.
It is divided into four chambers, the two upper atria and the two lower ventricles. Atria (singular, atrium) are the thin-walled blood collection chambers of the heart. Atria pump the blood into the ventricles. Ventricles are the heart chambers which collect blood from the atria and pump it out of the heart. The four chambers of the heart are shown in Figure. Each of the four chambers of the heart have a specific job, these are:
• The right atrium receives oxygen-poor (deoxygenated) blood from the body this blood enters from the superior vena cava and the inferior vena cava
• The right ventricle pumps oxygen-poor blood through the pulmonary arteries and toward the lungs. In the lungs, carbon dioxide is released from the blood and oxygen is picked up.
• The left atrium receives oxygen-rich (oxygenated) blood from the lungs through the pulmonary veins.
• The left ventricle pumps oxygen-rich blood out of the heart to the rest of the body through the aorta.
On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker and stronger than the wall surrounding the right ventricle because the left ventricle needs to exert enough force to pump the blood through the body. The right ventricle only needs to pump the blood as far as the lungs, which does not require as much contractile force.
Valves in the heart maintain the flow of blood by opening and closing in one direction only. Blood can move only forward through the heart, and is prevented from flowing backward by the valves. Such movement of the blood is called unidirectional flow. There are four valves of the heart:
• The two atrioventricular (AV) valves ensure blood flows from the atria to the ventricles, and not the other way. The AV valve on the right side of the heart is called the tricuspid valve, and the one on the left of the heart is called the mitral, or bicuspid valve.
• The two semilunar (SL) valves are present in the arteries leaving the heart, and they prevent blood flowing back from the arteries into the ventricles.