Blood must pass through the ........ to reach the left side of the heart.

Which part of the circulatory system serves to supply blood to the heart muscle itself?

Explain the advantage of the complete partition of the ventricle in birds and mammals,which leads to double circulation.

In which of the following is a single circulation of the heart observed?

What is meant by double circulation? What is its significance?

Given below are two statements regarding the circulation of blood in fishes.
Statement $I$: The heart of a fish contains only deoxygenated blood.
Statement $II$: Fishes show single circulation,and blood passes through the heart only once during each cycle.
In light of the above statements,choose the correct answer from the options given below.