Respiratory System
of Birds
Differences between
avian and mammalian respiration. Respiration in birds is much different than in mammals.
Birds have a larynx, but it is not used to make sounds.
Instead, an organ termed the
"syrinx" serves as the "voice box." Birds
have lungs, but they also have air sacs. Depending upon the species, the bird
has seven or nine air sacs. The air sacs include:
·
Two
posterior thoracic
·
Two
abdominal
·
Two
anterior thoracic
·
Two
cervical (these are not present in some species)
·
One
interclavicular
The air sacs of
birdsextend into the humerus (the bone between the shoulder and elbow), the
femur (the thigh bone), the vertebrae and even the skull. Birds do not have a diaphragm;
instead, air is moved in and out of the respiratory system through pressure
changes in the air sacs. Muscles in the chest cause the sternum to be pushed outward.
This createsa negative pressure in the air sacs, causing air to enter the respiratory
system. Expiration is not passive, but requires certain muscles to contract to
increase the pressure on the air sacs and push the air out. Because the sternum
must move during respiration, it is essential that it is allowed to move freely
when a bird is being restrained. Holding a bird "too tight" can
easily cause the bird to suffocate. Because birds have air sacs that reach into
the bones, and have no diaphragm, respiratory infections can spread to the
abdominal cavity and bones. Bird lungs do not expand or contract like the lungs
of mammals. In mammalian lungs, the exchange of oxygen andcarbon dioxide occurs
in microscopic sacs in the lungs, called 'alveoli.' In the avian lung, the gas
exchange occurs in the walls of microscopic tubules, called 'air capillaries.' The
respiratory system of birds is more efficient than that of mammals,
transferring more oxygen with each breath. This also means that toxins in the
air are also transferred more efficiently. This is one of the reasons why fumes
from teflon are toxic to birds, but not to mammals at the same concentration.When
comparing birds and mammals of similar weight, birds have a slower respiratory
rate. Respiration in birds requires two respiratory cycles (inspiration,
expiration, inspiration, expiration) to move the air through the entire
respiratory system. In mammals, only one respiratory cycle is necessary.
These
air sacs fill a large proportion of the chest and abdominal cavity, and also
connect to the air spaces in the bones. Two primary bronchi, leading from the
trachea, and a number of secondary bronchi, leading from the primary bronchi,
feed air into the abdominal
air sacs . Some of the
secondary bronchi, which spread over the lower surface of each lung, channel
air to theanterior air sacs. Also
leading from the primary bronchi are between 7 and 10 dorsal bronchi, which
spread over the back and sides of each lung, and ventral bronchi, which spread
over the front of each lung. Finally there is a network of tertiary bronchi,
which connects the ventral bronchi with the dorsal bronchi. Arising directly
from the tertiary bronchi are air
capillaries, which are
responsible for the gas exchange between the ventilation system and the blood
that transports oxygen to the body cells where it is needed for respiration.
The
passage of air through a bird's lungs is similar to water flowing through a
sponge: Air flows directly through the lungs and into the adjacent air sacs.
The theory is that the air sacs might very well function like bellows to drive
air through the lungs. Unlike mammalian lungs, in which the volume of air
changes with each inhalation and exhalation, avian lungs maintain a constant
volume of air. Because of the air sacs, the lungs maintain a constant volume of
air. Because of the air sacs, the lungs inflate but do not deflate to take in
more oxygen: they hold air.
If
its lungs inflated and deflated with every breath, a bird in flight would be
constantly gaining and losing altitude. Besides the advantage of keeping them
on course, the avian respiratory system enables birds to maintain a higher
energy level than mammals, because mammals lose energy just by breathing.
Although the rate of respiration varies somewhat with the age of the bird, it
is, in general, inversely proportional to body size -- the smaller the bird,
the faster its breath rate.
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