Why do birds and mammals require a more complex heart?

Birds and mammals are endothermic or "warm-blooded" animals. This means that they can maintain their internal body temperature regardless of the temperature of their environment. This is possible due to a high metabolic rate, which generates heat.

To support their high metabolic rate, birds and mammals need a more complex heart. The heart pumps oxygen-rich blood to the tissues, and it also pumps carbon dioxide-rich blood to the lungs. The heart of a bird or mammal has four chambers: two atria and two ventricles. The atria are the upper chambers, and the ventricles are the lower chambers.

The right atrium receives carbon dioxide-rich blood from the body. This blood then flows to the right ventricle, which pumps it to the lungs. In the lungs, the carbon dioxide is exhaled, and the blood is oxygenated.

The oxygen-rich blood from the lungs flows to the left atrium. This blood then flows to the left ventricle, which pumps it to the body. The oxygen-rich blood delivers oxygen to the tissues, and it also removes carbon dioxide.

The four-chambered heart of birds and mammals allows for efficient circulation of blood. This is necessary to support their high metabolic rate and maintain their internal body temperature.

In contrast, ectothermic or "cold-blooded" animals, such as reptiles and amphibians, have a simpler three-chambered heart. This is sufficient for their lower metabolic rate and variable body temperature.