Structure of the Human Heart

The human heart is a muscular pump made of cardiac muscle. It is divided into a right side and a left side by a wall called the septum, which stops oxygenated and deoxygenated blood from mixing. Each side has two chambers, so the heart has four chambers in total:Atria (singular atrium). The two upper chambers receive blood from the veins. They have thin walls because they only need to pump blood a short distance, into the ventricles below.Ventricles. The two lower chambers pump blood out of the heart into the arteries. They have thicker, more muscular walls. The left ventricle has the thickest wall, because it must pump blood at high pressure all around the body. The right ventricle only pumps blood to the nearby lungs, and a lower pressure there avoids damaging the delicate lung capillaries.Valves make sure blood flows in one direction only. The atrioventricular valves lie between each atrium and ventricle: the tricuspid valve on the right and the bicuspid (mitral) valve on the left. They are attached by tendinous cords to the ventricle walls, which stop them turning inside out when the ventricles contract. The semilunar valves lie at the bases of the aorta and the pulmonary artery.Major blood vessels. The vena cava and pulmonary vein bring blood into the atria; the pulmonary artery and aorta carry blood away from the ventricles.The heart muscle itself is supplied with oxygenated blood by the coronary arteries, which branch from the aorta and run over the surface of the heart.