Day 15: The Respiratory System
How Birds Breathe
There are different theories on
exactly how birds breathe and many scientists are
still studying the process. Respiration in birds is
much different than in humans and other mammals.
These differences are adaptations for flight and for
singing. The bird's lung is relatively small in
proportion to its body size when compared to that of a
mammal; they are only half the size of the mammalian
lung. A mammal's lungs are made up of millions of
tiny balloons, called alveoli, which expand and
contract as the animal breathes. A bird's lungs, on
the other hand, are not elastic - they do not change
size when the bird breathes. The bird's lungs are
composed of air chambers whose walls are made of a
thin layer of squamous epithileum surrounded by
capillaries. Specialized elastic structures called air
sacs are connected to the lungs and act like furnace
bellows to draw air through the lungs - very much like
a furnace forces air through the ductwork of a house.
As air passes through the ductwork of the lungs,
oxygen in the air is exchanged for carbon dioxide in
the blood of capillaries winthin the chamber
walls.
Anatomy of the Air Sacs
The bird has two sets of air sacs. The caudal air
sacs include the abdominal air sac and the caudal
thoracic air sacs. The cranial air sacs include the
cervical air sac, clavicular air sac, and the cranial
thoracic air sacs. Air sacs even extend into the
bones. When the cavity of a bone is at least
partially filled with an air sac, the bone is said to
be pneumatized. Birds who fly have a more extensive
system of air sacs, including the pneumatization of
more of their bones.
Compression or expansion of the air sacs occurs
when the size of the body cavity in which they are
housed changes. Cavity size is controlled by muscle
movement. The largest of the air sacs, the abdominal
air sac, lines the inside of the abdominal cavity and
surrounds the abdominal organs like a coat. As a bird
becomes more active, it requires more oxygen.
Increased movement forces a greater degree of
compression and expansion of its body cavities, and in
turn inflates and deflates more of its air sacs. This
not only forces more air through the lungs, but also
makes the bird's relative weight lighter. When a bird
takes off for flight, the exaggerated movement of its
wings creates an air current which fills its air sacs,
including those within its bones, and makes the bird
light enough to fly. The air current created is
referred to as "flight wind". The abdominal muscles
are largely responsible for breathing while at
rest.
A bird can also use its air sacs to sing by forcing
air through its vocal organs like a bagpipe. Some
birds can sing while they fly! This is due in part to
the bird's ability to sing during inspiration as well
as expiration (like whistling), as well as an
incredible degree of muscle control.
Movement of Air
There are many theories about the pathway which air
takes in the bird's respiratory system. It is a
subject that scientists are still researching. The
following is a very simplified explanation of one of
the theories. The theory suggests that a breath of
air is drawn through the trachea and mesobronchus into
the posterior air sacs (abdominal and caudal thoracic)
when chest muscles draw the ribs forward and lower the
sternum. Upon expiration, air is forced from the
posterior air sacs into the lungs where gas exchange
takes place. When the bird takes a second breath, the
air in the lungs is sucked into the cranial air sacs
-caudal thoracic, cervical, and clavicular. The
cranial air sacs act as a holding chamber which
provides a small back flow of air into the lungs
during expirations. The second expiration forces the
air in the cranial air sacs out through the trachea.
Thus airflow through the bird's respiratory system is
mostly a unidirectional circular path which requires
two breaths to complete. The small amount of back
flow from the cranial air sacs during expiration
provides the lungs with a constant flow of air.
Constant airflow supplies birds with more oxygen from
the air than is possible for mammals to obtain. This
is a necessary adaptation in birds for maintaining
their high metabolic rate and for flying.
Text by Janet Sinn-Hanlon and Dawn Gorski
Illustrations by Dawn Gorski
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