The valves of the heart serve the cardiovascular system by controlling the direction of the blood flow during circulation and determining the exact moment in which the blood passes between the chambers of the heart as well as in and out of it entirely. There are two atrioventricular valves that separate the heart left and right atria from their respective ventricles and two semilunar valves which control the release of pulmonary and aortic designated blood leaving the heart. The veins also have small valves within them that help push the slower moving blood back through the tissues and into the large cardiac vessels known as the superior vena cava and inferior vena cava.
The atrioventricular valves function to prevent the backflow of blood from the ventricles to the atria during cardiac systole. They attach to the ventricular walls via the chordae tendineae which in turn attach to the papillary muscles so that the cusps are held tense and don’t invert into the atria. This extended attachment of the chordae tendineae and the papillary muscles is known as the subvalvular apparatus.
Opening and closing of the valves is governed by the gradient pressure across the valves themselves and the first heart sound can be heard when the atrioventricular valves close. The first of the two atrioventricular valves is known as the mitral valve or the bicuspid valve, due to the fact that it is comprised of two cusps, which together look like the bishop’s hat that is called a mitre. It is situated between the left atrium and the left ventricle. The other atrioventricular valve is the tricuspid valve which as its name suggests has three separate cusps. It sits between the right atrium and right ventricle, thus preventing the backflow of atrial blood accumulation when it passes on to the ventricle.
The semilunar valves can be seen at the base of both the main arterial outlets that are the pulmonary artery leading to the lungs and the aorta which branches out to the peripheral tissues. These valves allow blood from the ventricles to pass on into the vessels and close immediately after to prevent any backflow, which causes the first part of the second heart sound.
The aortic valve separates the left ventricle and the aorta and has three cusps. During ventricular systole, the pressure in the left ventricle rises and when it surpasses the pressure in the aorta, it opens, releasing the blood accumulated into the vessel. Once the systole ends, the ventricular pressure rapidly drops below that of the aorta, forcing the valve shut again. In this way, the blood can only move up the aorta into its arch and then on to its subsequent branches around the body. Closure of the aortic valve is related to the second part of the second heart sound.
The pulmonary valve or pulmonic valve lies between the right ventricle and the pulmonary artery and like the aortic valve, it is also a tricuspid valve. It opens during ventricular systole, with the same pressure mechanism as the aortic valve and closes, producing the third part of the second heart sound.
Lastly, it is worth mentioning the variations of the cusps that encompass the cardiac valves. Each valve has three cusps, save the bicuspid valve which only has two. The pulmonary cusps are situated to the left, the right and anteriorly. The aortic valve has a left, a right and a posterior cusp. The tricuspid valve has an anterior, a posterior and a septal cusp. Finally, the mitral valve has an anterior and a posterior cusp. The cusps are named according to their position, so it is important to mention the valve you are talking about and not just it’s individual cusps in order to avoid confusion.