Are bird lungs more efficient than mammal lungs?

Chris - I only discovered how different the respiratory system of birds is when I started to actually teach this to the Natural Sciences students at the University of Cambridge a few years ago and it's ingenious what goes on.

Birds need a very efficient respiratory system, because they have such high metabolic rates, in order to sustain the enormous work output that they do when they fly.

So how do their chests work? Well they have a very different system to the lungs that we do. We have lungs which are like two pairs of balloons that you blow air into, they inflate and then they recoil down, blowing the air out again. Birds have a one-way flow of air through their lungs. They don't have the tiny air sacs - called alveoli - like we do. They have tiny tubes called air capillaries that the air flows through continuously. The benefit of doing that is that you always have fresh air flowing through the lung, maintaining a very high concentration of oxygen up against the bloodstream and therefore, you maximise the gradient for diffusion, pushing oxygen into the blood.

How do they do this? Well if you were to dissect a bird, what you would see is they have these combinations of lung tissue for want of a better word and air sacs. Now the air sacs are in various parts of the body. They are called anterior and posterior air sacs as two groups.

When the bird breathes in, it moves various bones and muscles in order to increase volume of these air sacs anteriorly and posteriorly, so they draw air into them. But, first of all, the air flows into the trachea and it goes into the posterior air sac. The anterior air sacs, which also increase in volume, actually get filled up by the air that's already in the bird's lung. So in other words, they pull air through the lung tissue into their anterior air sac. Then when the bird breathes out, exhales, it squeezes on these air sacs. This time, the posterior air sac empties into the lung tissue and the anterior air sac empties into the bird's trachea and then out through its nostrils and its beak. So in this way, you've always got air going in one direction through the lung. It's always fresh air and therefore, you've always got a very high oxygen gradient taking air into the tissue.

These air sacs are also making use of spaces inside some of the bird's bones including its humorous' shoulder bones and also its vertebrae. And so, that means that the bones are very, very light which the bird needs as an adaptation for flight.