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