The single circulatory system in fish limits their physical activity in several ways. Due to the lower blood pressure after the blood passes through the gills, there is a slower delivery of oxygen and nutrients to muscle tissues. This results in a reduced capacity for sustained high-energy activities. The mixing of oxygenated and deoxygenated blood further reduces the efficiency of oxygen delivery to tissues, limiting the aerobic capacity of fish. Consequently, while fish can perform quick bursts of speed, their overall endurance and capacity for prolonged physical activity are lower compared to mammals with a double circulatory system.

The evolutionary advantages of a double circulatory system in mammals are manifold. Firstly, it supports higher metabolic rates, necessary for their active lifestyles, by ensuring efficient oxygen and nutrient delivery to body tissues. The separation of pulmonary and systemic circuits in this system allows for higher blood pressure in the systemic circuit, enhancing the delivery of oxygen and nutrients to body tissues. This system also reduces the risk of oxygen depletion in vital organs, as oxygen-rich blood is delivered directly from the heart to the body. Additionally, the double circulatory system is more adaptable to various activities and environmental conditions, providing mammals with a versatile physiological foundation to thrive in diverse habitats.

The double circulatory system plays a significant role in thermoregulation in mammals. By maintaining a higher and more controlled blood pressure, this system ensures efficient blood flow to the skin, facilitating heat loss when necessary. The extensive capillary networks in the skin can dilate or constrict, aiding in temperature regulation. For instance, in cold environments, blood flow to the skin is reduced to minimize heat loss, while in hot environments, increased blood flow to the skin aids in heat dissipation. This ability to regulate body temperature is crucial for mammals, allowing them to maintain a stable internal environment (homeostasis) despite external temperature fluctuations. The separation of oxygenated and deoxygenated blood also ensures that the organs receive blood at an optimal temperature, further enhancing metabolic efficiency.

Oxygenation efficiency in mammals is notably higher than in fish due to several factors. In mammals, the lungs serve as the site of gas exchange, providing a large surface area and extensive capillary networks that facilitate efficient oxygen uptake and carbon dioxide release. The separation of oxygenated and deoxygenated blood in the double circulatory system ensures that tissues receive blood with high oxygen content. This separation is achieved through the four-chambered heart, which prevents mixing of the two blood types. In contrast, fish rely on gills for oxygenation, where blood is oxygenated in a single pass and often mixes with deoxygenated blood, leading to a lower overall oxygen content in the blood circulating through the body.

In fish, the blood vessels are structured to facilitate single circulation. The major vessels include the ventral aorta, which carries deoxygenated blood from the heart to the gills, and the dorsal aorta, which distributes oxygenated blood throughout the body. The capillary networks in the gills are crucial for gas exchange. In mammals, the blood vessels are more complex, aligning with the double circulatory system. Arteries carry oxygenated blood from the heart to the body, while veins return deoxygenated blood to the heart. Capillaries in mammalian systems are extensively branched, providing a vast surface area for efficient nutrient and gas exchange. This complexity allows for higher blood pressure and more effective transport of oxygen and nutrients, vital for sustaining the higher metabolic demands of mammals.