Double circulation in mammals refers to the circulation of blood through the heart twice in one complete circuit of the body. This is a crucial adaptation that allows mammals to maintain a high metabolic rate, which is necessary for their active lifestyle and warm-bloodedness.

Explanation:

1. Single Circulation:
To understand double circulation, it is important to first understand single circulation. In single circulation, found in most fish and amphibians, blood flows through the heart once in a complete circuit of the body. Deoxygenated blood from the body is pumped to the gills or lungs for oxygenation, and then oxygenated blood is distributed to the rest of the body.

2. Double Circulation:
In mammals, however, the circulatory system is more complex, with two separate circuits of circulation:

a) Pulmonary Circulation:
The first circuit is the pulmonary circulation, which involves the flow of blood between the heart and the lungs. Deoxygenated blood from the body enters the right atrium and is pumped into the right ventricle. From the right ventricle, the blood is sent to the lungs where it picks up oxygen and gets rid of carbon dioxide. Oxygenated blood returns to the heart and enters the left atrium.

b) Systemic Circulation:
The second circuit is the systemic circulation, which involves the flow of blood between the heart and the rest of the body. Oxygenated blood from the left atrium enters the left ventricle and is pumped out to the rest of the body through the aorta. This oxygenated blood supplies oxygen and nutrients to the body's tissues and organs. Deoxygenated blood returns to the heart through the superior and inferior vena cava, entering the right atrium and completing the cycle.

3. Advantages of Double Circulation:
Double circulation allows for separate oxygenation of blood and delivery of oxygenated blood to the body's tissues. This ensures that oxygen-rich blood is efficiently delivered to the body's organs, enabling mammals to have a higher metabolic rate and maintain a constant body temperature. The separation of oxygenated and deoxygenated blood also prevents the mixing of the two types of blood, ensuring that oxygen is effectively transported to the tissues.

Overall, double circulation in mammals is a highly efficient system that allows for optimal oxygenation of blood and delivery of oxygen to the body's tissues. This adaptation is essential for mammals' active lifestyle and high metabolic rate.