Science Porn

[Anomalocaris]

the latex cast of the respiratory system of a chicken:


Notice there is hardly a lung like shape visible in this whole thing. This is because unlike in mammals, where the lungs are pretty much all there is of the respiratory system, and most of the breathing action is confined to the chest cavity, in birds the lungs are but a small part of a much more elaborate respiratory system that penetrates into the major bones and extends throughout the body.

Where as in mammals, the breathing action resembles the action of a bellow, with air drawn in to inflate the lungs, and then gets pushed out through the same wind pipe, in birds the breathing action is one way only. Air gets drawn into the lungs, and from the lungs it gets pushed into a system of air sacs distributed throughout the body and limbs of the bird. Finally the waste air is pushed out through an entirely different set of ducts and passages. As a result, in mammals the breathing is inefficient because air move in and out along the same passage, and waste air on the way out mixes with fresh air coming in, so one never gets all the waste air out, and incoming air is never quite fresh. Furthermore because the lungs can't completely deflate during bellow action, there is always a body of dead air in the lungs which breathing action can't move. But in birds fresh air in drawn in in a separate loop, and is never contaminated by mixing with waste air. So every bird breath is completely fresh. Since the movement of air in bird respiration is one way only, all of the air is moved with each breath, there is never any stagnant dead air in the respiratory system.

The fundamentals difference between the respiratory system of birds and mammals is why birds dominates the high metabolic lifestyle of flying in the modern atmosphere, and why bird's ancester the dinosaurs could outcompete mammals during the Mesozoic.