Lungs Need Breathing Room

Though nature seems to strive towards the perfect balance of form and function, she doesn't always achieve it. Sometimes there's good reason: a new study suggests that if human lungs were designed to optimally perform their most important task--exchanging oxygen and carbon dioxide between the blood and air--they'd be prone to disaster.

Gas exchange is the raison d'etre of the lung. It requires several square kilometers worth of membranes bunched into tiny sacs and packed up into the human torso. To reach the tiny sacs called alveoli where gas exchange takes place, air must flow through a series of ever-smaller tubes called bronchioles.

Although necessary to bring in air, the bronchioles take up space in the lungs that could otherwise be used for gas exchange. The best lung design would seemingly strike a perfect balance between the need to have bronchioles wide enough to minimize airflow friction while leaving the maximum volume available for alveoli.

Unexpectedly, that's not what happens in our lungs, Bernard Sapoval of the Ecole Polytechnique in Palaiseau, France, and his colleagues report in the 12 February issue of Nature. Our bronchioles are wider than they need to be to minimize resistance, and Sapoval's team thinks they know why: safety. The researchers' calculations show that if lungs were optimized for gas exchange, even a minor constriction of a bronchiole would enormously increase the resistance to airflow--which could mean the difference between effortless inhalation and gasping like an asthmatic. Asthma, in fact, is a prime example of why a little excess volume doesn't hurt; during an asthmatic attack, the bronchioles constrict, making breathing effortful. If our bronchioles started out narrower, the same amount of constriction would make breathing almost impossible.

If the optimal design for gas exchange has a dangerous side-effect, says Hiroko Kitaoka of Osaka University in Osaka, Japan, perhaps Savopal and his colleague's quantitative approach can find the best compromise. It may be that nature has found it already. As Sapoval says, "our lungs are built on the safe side."

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Hiroko Kitaoka's website