The heart of reptiles is three chambered except for crocodile which has a four chambered heart.

Double circulation refers to the circulation of blood in two separate pathways: pulmonary circulation and systemic circulation. This type of circulation is found in organisms that have a complete separation of oxygenated and deoxygenated blood, allowing for more efficient oxygen delivery to the body.

- Pulmonary Circulation:
Pulmonary circulation is the movement of blood between the heart and the lungs. It is responsible for oxygenating the blood and removing carbon dioxide. In this pathway, deoxygenated blood from the body enters the right atrium of the heart and is then pumped into the right ventricle. The right ventricle contracts, sending the blood to the lungs through the pulmonary artery. In the lungs, oxygen is acquired, and carbon dioxide is released. The oxygenated blood returns to the heart through the pulmonary veins and enters the left atrium.

- Systemic Circulation:
Systemic circulation is the movement of oxygenated blood from the heart to the rest of the body and back. In this pathway, oxygenated blood from the left atrium passes through the left ventricle, which then contracts to pump the blood into the aorta. The aorta branches into smaller arteries, which supply oxygenated blood to various organs and tissues. In the capillaries, gas exchange occurs, with oxygen being delivered to the cells and carbon dioxide being picked up. Deoxygenated blood then returns to the heart through the venules and veins, entering the right atrium to begin the process again.

- Evolutionary Aspect:
The presence of double circulation first appeared in reptiles. Reptiles are cold-blooded animals (ectotherms) that require external heat sources to regulate their body temperature. Double circulation allowed for a more efficient delivery of oxygenated blood to the body tissues and organs. This was particularly advantageous for reptiles as they began to colonize diverse environments, including terrestrial habitats. The evolution of double circulation in reptiles marked a significant advancement in cardiovascular efficiency and contributed to their success as a group of organisms.

- Conclusion:
In summary, double circulation is the presence of both pulmonary and systemic circulation in an organism. It allows for the separation of oxygenated and deoxygenated blood, enabling more efficient oxygen delivery to the body. This type of circulation first appeared in reptiles and has since been retained and further evolved in mammals, including humans.