(b) The movement of blood twice through the heart is known as double circulation. Double circulation is necessary in human beings as it allows separation of oxygenated and de-oxygenated blood for a more efficient supply of oxygen to the body cells. This efficient system of oxygen supply is very useful in warm-blooded animals such as human beings. As we know, warm-blooded animals have to maintain a constant body temperature (by cooling themselves when they are in a hotter environment and by warming their bodies when they are in a cooler environment). Hence, they require more O2 for more respiration, so that they can produce more energy to maintain their body temperature. Thus, the circulatory system of humans is more efficient because of the double circulatory heart.