Why does the blood ejected into the pulmonary trunk or aorta normally not flow back through the semilunar valve?
by 
Robbie
ANSWERS: 1
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Blood ejected into the pulmonary trunk or the aorta is coming directly from the right and left ventricles respectively. Contraction of these chambers of the heart builds up a large pressure gradient in the ventricle relative to the vasculature. Because the pressure in the heart is so much higher, the blood pushes its way out through the valve during contraction of the heart (systole). At the end of systole, this valve snaps shut, preventing any backflow when the pressure equalizes. The valve is a very springy device which requires pressure to open - pressures greater than found during relaxation of the heart (diastole). Interestingly enough, many forms of heart disease and dysfunction result when this valve fails to close or open properly!
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