Mammalian tidal respiration
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Suboptimal design > Mammalian tidal respiration
Mammals have a lung ventilation system that is tidal: we breathe in and out through the same tubing. Deoxygenated air is therefore mixed with the fresh, and some of it is re-breathed.
Birds however have a different system. It is 'through-flow': the air follows a circular path round the body to the lungs and out again, so that oxygen-rich incoming air is not mixed with the deoxygenated outgoing air. It has been calculated that the avian system is therefore ten times more efficient than the mammalian one. That is, mammals -- including us humans -- have far less efficient lungs than birds.
What makes this doubly odd is that there are many mammals whose lifestyles mean efficient breathing is essential: long-distance runners like wolves and African hunting dogs, and sprinters like cheetahs. And really oddly, the mammalian equivalent of birds, bats. What is it about the bat lifestyle (say, a pipistrelle), compared to the bird lifestyle (say, a nightjar), that means it deserves lungs so inefficient? And conversely, the avian system is found in hawks and albatrosses, pigeons and peregrines. Fair enough. But it's also used in kiwis, penguins and ostriches. What on earth was the designer up to?!
Interestingly, in the case of bats, they have perhaps the most efficient respiratory system of any mammal. It means that bats can make a good go at a bird-like way of life. But it is good in spite of the aforementioned problem. It overcomes the tidal system's disadvantages. But there's the rub: if bats were designed from scratch, why include a disadvantage and use add-ons to fix it? Especially if there is a more efficient system (the bird one) to hand? If mammals were cars, they all would have one-litre four cylinder engines, regardless of use... and the racing cars -- bats -- have had a turbocharger added.
