Four cardiac valves help to direct flow through the heart. Heart valves cause blood to flow only in the desired direction. If a heart without heart valves were to contract, it would compress the blood causing it to flow both backward and forward (upstream and downstream). Instead, under normal physiological conditions, heart valves act as check valves to prevent blood from flowing in the reverse direction. In addition, heart valves remain closed until the pressure behind the valve is large enough to cause blood to move forward.
Each human heart has two atrioventricular valves that are located between the atria and the ventricles. The tricuspid valve is the valve between the right atrium and the right ventricle. The mitral valve is the valve between the left atrium and the left ventricle. The mitral valve prevents blood from flowing backwards into the pulmonary veins and therefore into the lungs, even when the pressure in the left ventricle is very high. The mitral valve is a bicuspid valve having two cusps and the tricuspid valve has three cusps.
The other two valves in the human heart are known as semilunar valves. The two semilunar valves are the aortic valve and the pulmonic valve. The aortic valve is located between the aorta and the left ventricle and when it closes, it prevents blood from flowing backward from the aorta into the left ventricle. An aortic valve is shown in Fig. 2.8. The pulmonic valve is located between the right ventricle and the pulmonary artery and when it closes it prevents blood from flowing backwards, from the pulmonary artery into the right ventricle.
Papillary muscles are cone shaped projections of myocardial tissue that connect from the ventricle wall to the chordae tendineae, which are tendons connected to the edge of atrioventricular valves. The chordae
tendineae are fine fibrous cords of collagen. These structures allow the AV valves to open and close, but they constrain the valves and prevent them from prolapsing, or collapsing backwards into the atria. Functionally, the papillary muscles and chordae tendineae are part of the valve complex with which they are associated. When the papillary muscles contract, they help maintain systolic close of the mitral or tricuspid valve.
Chordae tendineae rupture and papillary muscle paralysis can be consequences of a heart attack. This can lead to bulging of the valve, excessive backward leakage into the atria (regurgitation) and even valve prolapse. Valve prolapse is the condition under which the valve inverts backwards into the atrium. Because of these valve problems, the ventricle does not fill efficiently. Significant further damage, and even death, can occur within the first 24 h after a heart attack because of this problem.
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