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.

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