Biology Reference
In-Depth Information
are huge, yet very light, and positioned right where a bird's center of gravity must be for
efficient flight. Birds have an extremely specialized respiratory system, unique in the an-
imal kingdom, that allows them to extract much more oxygen from the air they breathe
in than we can. To accomplish this, avian lungs are very small and flat, and rigid, not ex-
pandable. That turkey's lungs were there, fitted perfectly against the bird's back ribs, but
so flat that they didn't look at all the way you expected.
When birds breathe in, the tide of air that flows through the trachea is split. A small
portion of it goes straight into the lungs for immediate exchange of oxygen and carbon
dioxide. From there it flows into air sacs in the forward part of the body. Like bellows,
when the bird exhales, these anterior air sacs push the oxygen-depleted air back into the
trachea and out through the bird's nostrils or mouth. A larger part of the inhaled air goes
from the trachea straight back to posterior air sacs, and when the bird exhales, these sacs
send the fresh air through the lungs and out through the trachea.
So all the air in the lungs at any given moment is fresh, unlike the air that remains and
becomes mixed with carbon dioxide in the alveoli of mammalian lungs. The air sacs are
huge in living birds, but most of the space they fill is behind the relatively heavier organs
in the bird's center of gravity.
When birds die, the weight of their organs and skin, along with air pressure from out-
side the bird, instantly flattens most of the air sacs, so you wouldn't have been able to
easily detect them in the turkey unless you put a straw down its trachea and blew — then
theairwouldflowthroughthelungstoinflatetheairsacs,whichlooklikeclearballoons.
Q When I saw the IMAX movie
Everest
, I noticed big black birds like crows up
near the summit, where virtually all the people needed oxygen tanks. How can birds
breathe at such high altitudes?
A
The trick isn't in breathing at high altitudes; it's in actually getting oxygen from those
breaths into the blood and getting carbon dioxide out when there is so little oxygen in the
atmosphere. Bird lungs, a crisscrossing matrix of tiny air tubes and parallel capillaries,
are exquisitely designed for this. Air passes through the tiny tubes in a direction counter-
currenttothebloodflowingthroughcapillaries,makingitextremelyeasyforthebloodto
pick up oxygen from the air tubes as the tubes pick up carbon dioxide from the blood to
exhale.Everyairtubeinavianlungsisinterconnected;birdlungshavenodead-airspaces
such as the alveoli, where gas exchange takes place in mammalian lungs.
The whole point of the respiratory system is to get the right amount of oxygen to all
the cells, which happens in conjunction with the circulatory system. Strong flyers tend to
have smaller red blood cells than flightless birds and weak flyers. This difference is im-
portantbecausethesmalleracellis,therelativelylargeritssurfaceareaforgasexchange.
