Answer:
at the base of the aortic and Pulmonary trunk
Step-by-step explanation:
Structurally the mammalian heart is made up of blood vessels for the empty of the blood from the heart chambers -the ventricles. Due to the forceful contraction of the blood against gravity into theses vessels,there are tendency for the back flow, from the vessels back into the ventricles.
These may lead to mixing of deoxyganted with oxygenated blood.
Aorta is the largest artery in the heart.It takes oxygenated blood from the heart into the systemic circulations.To prevent the back flow of the pumped blood after the L.ventricular systole,layers of valves shut up,and stop the flow of blood back into the ventricles.. These are the semilunar vlaves at the bases of the Aorta. The sound produced by this shutting up is called the dupp sound.
Likewise deoxygenated blood flow into the pulmonary circulation en-route the lungs is transported by the Pulmonary trunks.Therefore to prevent the back flow of the deoxygenated blood back into the pulmonary trunk, thes semilunars valves are closed,after R. ventricular systole
These two valves ensures that R and L Ventricular contraction for the ejection of oxygentated and deoxygented blood occurs without backlow and therefore no mixing of the two types occurs.