Which vertebrate has the most well-developed respiratory system?

Birds have a highly efficient respiratory system that allows them to meet the high oxygen demands required for flight. Unlike mammals, which have a one-way flow of air through their lungs, birds have a two-way flow of air. Air passes through a system of air sacs before entering the lungs, where it is then moved across the gas exchange surface by a unidirectional flow of blood. This results in a more efficient exchange of gases and a higher rate of oxygen uptake.

In addition, birds have a high density of capillaries in their lungs, which allows for efficient gas exchange. They also have a unique respiratory system in which air is constantly flowing through their lungs, allowing for a continuous supply of oxygen to their tissues.

While mammals also have a highly efficient respiratory system, birds have adaptations that allow for even greater efficiency and oxygen uptake. Reptiles and amphibians have less efficient respiratory systems, with some relying on cutaneous respiration (gas exchange through their skin) in addition to their lungs.