What prevents backflow of blood inside the heart during contraction?

$(i)$ 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?

Just as $CO_{2}$ is removed from the blood in the lungs,nitrogenous wastes such as urea or uric acid are removed from blood in the liver.

Which of the following statements is (are) true about the heart?

Match the terms in Column $(A)$ with those in Column $(B)$:

Column $(A)$	Column $(B)$
$(a)$ Trypsin	$(i)$ Pancreas
$(b)$ Amylase	$(ii)$ Liver
$(c)$ Bile	$(iii)$ Gastric glands
$(d)$ Pepsin	$(iv)$ Saliva

On which type of food does salivary amylase act in the buccal cavity,and what is the name of the initial product formed due to the action of this enzyme?