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Emerging from the right ventricle at the base of the pulmonary trunk is the pulmonary semilunar valve or the right semilunar valve. The pulmonary valve is comprised of three small flaps of endothelium reinforced with connective tissue. When the ventricle relaxes, the pressure differential causes blood to flow back into the ventricle from the pulmonary trunk. This flow of blood fills the pocket-like flaps of the pulmonary valve, causing the valve to close and producing an audible sound ("Lub"). Unlike the atrioventricular valves, there are no papillary muscles or chordae tendineae associated with the pulmonary valve.

Located at the opening between the left atrium and left ventricle is the mitral valve    , also called the bicuspid valve    or the left atrioventricular valve    . Structurally, this valve consists of two cusps compared to the three cusps of the tricuspid valve. In a clinical setting, the valve is referred to as the mitral valve, rather than the bicuspid valve. The two cusps of the mitral valve are attached by chordae tendineae to two papillary muscles that project from the wall of the ventricle.

At the base of the aorta is the aortic semilunar valve, or the aortic valve    , which prevents backflow from the aorta. It normally is composed of three flaps. When the ventricle relaxes and blood attempts to flow back into the ventricle from the aorta, blood will fill the cusps of the valve, causing it to close and producing an audible sound ("dub/dup").

In [link] a , the two atrioventricular valves are open and the two semilunar valves are closed. This occurs when both atria and ventricles are relaxed and when the atria contract to pump blood into the ventricles. [link] b shows a frontal view. Although only the left side of the heart is illustrated, the process is virtually identical on the right.

Blood flow from the left atrium to the left ventricle

The left panel of this figure shows the anterior view of the heart with the different valves, and the right panel of this figure shows the location of the mitral valve in the open position in the heart.
(a) A transverse section through the heart illustrates the four heart valves. The two atrioventricular valves are open; the two semilunar valves are closed. The atria and vessels have been removed. (b) A frontal section through the heart illustrates blood flow through the mitral valve. When the mitral valve is open, it allows blood to move from the left atrium to the left ventricle. The aortic semilunar valve is closed to prevent backflow of blood from the aorta to the left ventricle.

[link] a shows the atrioventricular valves closed while the two semilunar valves are open. This occurs when the ventricles contract to eject blood into the pulmonary trunk and aorta. Closure of the two atrioventricular valves prevents blood from being forced back into the atria. This stage can be seen from a frontal view in [link] b .

Blood flow from the left ventricle into the great vessels

The left panel of this figure shows the anterior view of the heart with the different valves, and the right panel of this figure shows the location of the mitral valve in the closed position in the heart.
(a) A transverse section through the heart illustrates the four heart valves during ventricular contraction. The two atrioventricular valves are closed, but the two semilunar valves are open. The atria and vessels have been removed. (b) A frontal view shows the closed mitral (bicuspid) valve that prevents backflow of blood into the left atrium. The aortic semilunar valve is open to allow blood to be ejected into the aorta.

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Source:  OpenStax, Human biology. OpenStax CNX. Dec 01, 2015 Download for free at http://legacy.cnx.org/content/col11903/1.3
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