How does blood circulation differ between mammals and fish?

In mammals, the circulatory system is characterized by double circulation, which means that blood flows through the heart twice in one complete circuit around the body. There are two distinct circuits: the pulmonary circuit, where deoxygenated blood is pumped from the right side of the heart to the lungs for oxygenation, and the systemic circuit, where oxygenated blood is distributed from the left side of the heart to the rest of the body. This efficient system supports high metabolic rates and active lifestyles, allowing mammals to maintain their body temperature and engage in complex behaviors.

On the other hand, fish have a single circulation system. In fish, the heart pumps deoxygenated blood directly to the gills, where it gets oxygenated before flowing to the rest of the body. The blood passes through the heart only once per complete circuit, which is less efficient compared to the double circulation seen in mammals. This single circulation is adequate for the aquatic environment in which fish live, as they rely on water flow over their gills to supply oxygen.

Understanding these differences in circulation is crucial because it highlights how each group of animals has adapted its circulatory system to meet its physiological demands based on its habitat and lifestyle.