Answer:

for completing one round of circulation in the body the blood passes twice through the heart this is called double circulation.

Answer:

When during a single cycle, the blood goes twice in the heart this is called "double circulation".

Why:—

1:It prevents oxygenated blood mixing with deoxygenated blood.

2:It maintain the temperature of body.

Explanation:

Mammals required double circulatory system as their is great need of oxygen and mammals can't tolerate blood mixing.

As in fishes which have two chambered heart can tolerate blood mixing. Note: Mammals have four chambered heart.