What prevents backflow of blood inside the heart during contraction?

$(a)$ Name the following:
$(i)$ Part where air is filtered by fine hair and mucus.
$(ii)$ Part which terminates in balloon-like structures.
$(iii)$ Balloon-like structures where exchange of gases takes place.
$(iv)$ Part which separates the chest cavity from the abdominal cavity.
$(b)$ Why is the rate of breathing in aquatic organisms much faster than in terrestrial organisms?

State the function of the following in the alimentary canal:
$(i)$ Liver $(ii)$ Gall bladder $(iii)$ Villi.

Choose the event that does not occur in photosynthesis.

$(i)$ Why are the valves needed in the heart?
$(ii)$ Leakage of blood from vessels reduces the efficiency of the pumping system. How is the leakage prevented?

What prevents backflow of blood inside the heart during contraction?