Answer:
Birds possess a circulatory system that is both closed and complete, featuring a double-loop design. In such a closed system, their blood only circulates through vessels; in a double circulatory system, the blood of birds flows through the heart twice; and in a complete system, arterial blood remains separate from venous blood.
The hearts of birds are comprised of four distinct chambers (two atria and two ventricles). Venous blood, which is rich in carbon dioxide, returns to the right atrium via veins, while oxygenated arterial blood from the lungs enters the left atrium. During simultaneous contractions, blood from the atria is pumped into their corresponding ventricles (right atrium to right ventricle; left atrium to left ventricle). Each ventricle then propels blood into the arteries.
The pulmonary artery connects to the right ventricle, carrying venous blood to the lungs, while the aorta connected to the left ventricle distributes arterial blood throughout the body.
The avian circulatory system shares similarities with that of mammals, with slight variations. For example, bird red blood cells are oval and nucleated, whereas mammalian red blood cells lack nuclei and are round. Additionally, in birds, the aorta departs from the left ventricle to the right, contrasting with mammals, where it tends to go left.
This dual and complete circulation allows birds to access more oxygen, which in turn provides them with the energy they require for flight and helps regulate their body temperature (homeothermy).
Moreover, birds generally have larger hearts relative to their size compared to mammals. This larger heart size is likely necessary to fulfill the high metabolic demands of soaring through the air. Among birds, smaller species tend to have even larger hearts when measured against their body mass than larger birds do. Hummingbirds, for example, possess the most substantial hearts relative to their size, owing to the energy-intensive nature of hovering.
Additionally, bird hearts typically pump a greater volume of blood per unit time than those of mammals, meaning the cardiac output (blood volume ejected per minute) in birds usually exceeds that of similarly sized mammals. Cardiac output is influenced by both the heart rate and the stroke volume (the amount of blood propelled with each heartbeat).
Explanation:
