**Double Circulation**

Double circulation refers to the circulatory system found in mammals and birds, where blood flows through two separate circuits: the pulmonary circuit and the systemic circuit. This arrangement allows for efficient oxygenation of blood and effective distribution of nutrients throughout the body.

**Pulmonary Circuit**
The pulmonary circuit is responsible for oxygenating blood. It starts in the right ventricle of the heart, which pumps deoxygenated blood to the lungs through the pulmonary artery. In the lungs, the blood receives oxygen and releases carbon dioxide through the process of gas exchange. Oxygenated blood then returns to the heart through the pulmonary veins, entering the left atrium.

**Systemic Circuit**
The systemic circuit is responsible for distributing oxygenated blood to the body's organs and tissues. From the left atrium, blood flows into the left ventricle, which is the main pumping chamber of the heart. The left ventricle contracts, forcing the oxygenated blood into the aorta, the largest artery in the body. The aorta branches into smaller arteries, which deliver the oxygenated blood to various organs and tissues. Within these organs and tissues, oxygen is exchanged for carbon dioxide and other waste products. Deoxygenated blood, now carrying waste, returns to the heart through veins and enters the right atrium.

**Advantages of Double Circulation**
1. Efficient Oxygenation: Double circulation ensures that blood is pumped to the lungs for oxygenation and then distributed to the rest of the body. This allows for a continuous and efficient supply of oxygen to organs and tissues, enabling their proper functioning.
2. Separation of Oxygenated and Deoxygenated Blood: By separating oxygenated and deoxygenated blood, double circulation prevents mixing and ensures that only oxygenated blood is delivered to organs and tissues. This is crucial for maintaining high oxygen levels in the body.
3. Pressure Regulation: The double circulation system allows for independent regulation of blood flow and pressure in the pulmonary and systemic circuits. The pulmonary circuit has lower pressure since it only needs to pump blood to the lungs, while the systemic circuit has higher pressure to ensure efficient distribution of oxygenated blood throughout the body.
4. Adaptation to High Metabolic Demands: Mammals and birds have high metabolic rates, requiring a constant supply of oxygen and nutrients. The double circulation system helps meet these demands by efficiently oxygenating blood and delivering nutrients to various organs and tissues.

In conclusion, double circulation is an important adaptation found in mammals and birds that allows for efficient oxygenation of blood and effective distribution of nutrients throughout the body. It ensures that oxygenated and deoxygenated blood are separated and that blood flow and pressure are regulated in both the pulmonary and systemic circuits.

When blood passes two times through heart, then it is called double circulation. For example:human body has double circulation system.