Our heart is a two-sided pump. Its function is to guarantee the blood flow between the lungs, where blood is oxygenated, and the body tissues, where oxygen is used as nutriment for the tissues' cells.
Our heart is made up of four chambers. The chambers are compartments like small rooms whose function is to store the blood and then push it outside.
The two upper chambers, the atria, receive the blood returning into the heart through the veins. The two lower ones, the ventricles, pump the blood to the body through the arteries.
The right atrium receives all the returning blood from the upper and lower parts of the body. It then transfers this blood through the tricuspid valve to the right ventricle, which then pumps it through the pulmonary valve out to the lungs. In the lungs, carbon dioxide is exchanged for oxygen, then the blood returns to the left atrium, which transfers it though the mitral valve into the left ventricle.
The left ventricle then pumps the blood through the aortic valve out to the body through the arteries, where the blood supplies tissues with oxygen and removes carbon dioxide. The blood, now depleted of oxygen, is returned to the right atrium by the veins. The left part of the heart needs to exert a stronger force than the right part; therefore the left cardiac walls are thicker and stronger.
The atrioventricular valves separate the upper from the lower chambers; one of these is called the mitral valve, which divides the left atrium and the left ventricle, and the other is the tricuspid valve, found between the right atrium and the right ventricle. The tricuspid valve is so-called because it has three leaflets, while the mitral valve is called bicuspid, because it has just two leaflets. The mitral valve has a complex structure: its cusps are retained by the cordae tendinae, which are linked to the papillary muscles. These structures avoid the prolapse of the valve edge into the atrium.
The outflow valves separate the heart from the two main arteries: the aorta and the pulmonary artery. The aortic valve separates the left ventricle from the aorta, the main artery that carries blood to the body. The pulmonary valve separates the right ventricle from the pulmonary artery. The aortic and the pulmonary valves are said to be semilunar because of their leaflets' shape, which are similar to a half moon. The aortic and pulmonary valves are thin structures, without any muscles; they open and close only thanks to blood pressure gradients.
The heart valves close and open in a pulsatile way, filling and emptying the atria and the ventricles. They work as doors, letting in a precise quantity of blood flow in only one direction, and preventing blood backflow.