How birds breathe
If you tried to revive a bird by mouth-to-mouth resuscitation you would not succeed. Not because the bird has a beak and you a mouth, but because in birds respiration (the removal of air from the lungs) is an active process.
You cannot rely on the bird lungs’ elasticity as in the mammal and blow air into them, expecting the air to come out again by itself. Bird lungs have a fixed volume and in many other ways differ from those of a mammal. Not only, for example, does the gas exchange organ in birds occupy twice as much space in the body as it does in mammals, it also operates on a considerably more efficient system, for it extracts on average 30% of the oxygen in the inhaled air, whereas the mammal manages to use only about 24% of the inhaled air to get its oxygen from.
The anatomy of the bird lung is extremely complicated and designed to produce a one-way-flow of gas with little mixing of inhaled with exhaled air. Gas exchange happens to take place in both phases of the breathing cycle, i.e., inhalation as well as exhalation. How does the bird lung achieve this (a favourite question for undergraduates in exams on comparative physiology)?
Each lung in a bird is connected to anterior and posterior blind-ending air sacs, which may even extend into some bones, making the latter lighter and enlarging the intake volume of the air. The two air sacs (one posterior and one anterior) are filled alternatingly in successive inspirations during two respiratory cycles, with in and out-flow of the air being subjected to oxygen extraction by the capillary meshwork that is in contact with the bird lung’s alveoli.
Because of the special design of the lung and pressure differences between its posterior and anterior components during inspiration and expiration, air is channelled in such a way through the lung that it passes across the gas exchange area continuously and in one direction alone: it’s a “bagpipe” in the bird’s chest and quite different from the less complicated tidal in-and-out flow system of the mammalian lung!
The actual gas exchange mechanism of the bird lung can be likened to on a cross-current exchange system, in which blood flows across the air-capillaries in a series of channels. However, do birds with their efficient, but “fixed-volume lungs”, then have to breathe heavier than mammals, because the latter owing to their lungs’ elasticity can vary the intake volume of air during breathing? It depends. Birds certainly can breathe very, very fast and more than 300 breaths per minute have been recorded in the humming bird, the smallest of all birds. Just to think of such heavy breathing: it can leave you breathless, can’t it?
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2016.
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