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Breathing

Black-and-white Warbler (Courtesy NEBRASKAland Magazine/NGPC)

Mammals and birds have different ways of breathing.

The flow of air through the mammal respiratory system

In mammals, movements of the diaphragm (a partition of muscle and tissue between the chest and the abdominal area in humans) will cause air to enter and exit the lungs.

• Breathing in (also called inspiration) draws oxygen-rich air into the lungs.
• Within the lungs, carbon dioxide (CO2) waste from the body is exchanged for oxygen (O2).
• Breathing out (also called expiration) pushes air back out of the lungs.

During this process, mammal lungs are never completely emptied of air.

Bird Breathing
Because flight is a very energetically expensive activity, birds need a much more efficient respiratory system. They have lungs, but they've also developed numerous air sacs through which air circulates. Usually birds have nine air sacs.

Air sacs are very thin-walled – just one or two cells thick – and are very fragile. Some are tucked into the body cavity and some are inside some of the bird's hollow bones. These air sacs allow a continuous flow of air through the respiratory system, without any wasted space.

Birds lack a diaphragm; air flow is regulated by movements of the sternum and ribs.

The flow of air through the avian respiratory system

1. On first inhalation (breath in), air flows through the nostril into the trachea (wind pipe) and bronchi (tubes leading into the lungs) and primarily into the posterior (rear) air sacs.
2. On the first exhalation (breath out), air moves from the posterior air sacs into the lungs.
3. With the second inhalation, air moves from the lungs and into the anterior (front) air sacs.
4. With the second exhalation, air moves from the anterior air sacs back into the trachea and out.

This continuous flow of air maximizes the efficiency of gas exchange to provide plenty of oxygen – vital to the heart, muscles, and brain during flight.

The bird's highly-developed respiratory system works very well with its four-chambered heart, which is similar to that of mammals.