Describe double circulation in human beings.
(a) Why is it necessary ?
(b) How are arteries different from veins ?

Explanation

(a) Double circulation of blood means, that the blood enters twice in the heart during its once circulation in other parts of the body. First time oxygenated blood from lungs enters the left atrium and second time de-oxygenated blood enters the right strium through two main vena cava. Double circulation of blood is necessary to separate the oxygenated blood from the de-oxygenated blood to provide more energy for effecient working of body organs.
(b)