How Do Birds Breathe?

Birds have mastered the art of respiration for long flights without fatigue.

Birds require a respiratory cycle just like mammals in order to supply their muscles and tissues with oxygen and to remove built-up CO2 from the body. Unlike mammals, however, birds do not use just the lungs to respire (breathe) in a simple in-and-out process. In fact, birds require two respirations to pass oxygen all the way through the body, and use a more complicated system of air sacs to aid this process.

Anatomy

While birds do have lungs, they lack a diaphragm to help push air in and out. Instead, muscles along the ribcage and sternum contract and shrink the entire body cavity to push air out. Additionally, birds have a series of air sacs (seven or nine, depending on species) which aid in this process by filling or emptying with air, changing the pressure in the body cavity to either pull air in or push it out.

Difference from Mammals

Unlike mammals, who primarily rely on exchange of gasses in the alveoli (microscopic air pockets in the lungs), birds have developed "air capillaries," which resemble tiny veins riddled throughout the lungs and air sacs. Also different from mammals is the fact that while mammals require only one respiration (or one inhalation and one exhalation) to push oxygen throughout the entire body, birds complete the process in two cycles, or two respirations.

First Respiration

Air is inhaled through the nostrils and passes first into the abdominal air sacs (rather than the lungs). The first time the bird exhales, air in these air sacs moves down through the bronchial tubes (which continue to divide into smaller and smaller capillaries) and into the lungs. The bird's lungs are lined with alveoli like ours, but they are much smaller compared to lung size, with a greater surface area.

Second Respiration

When the bird breathes in a second time, air is forced from the lungs and into the lower sets of air sacs, and is finally expelled from the body on the second exhalation. The gas exchange occurs in both the alveoli of the lungs and the air capillaries lining the air sacs and bronchial tubes.

Efficiency & Sensitivity

Because oxygen absorption is not limited to the lung capacity, birds can absorb a relatively huge amount of oxygen with each breath. And because of the two-part respiration cycle, fresh air is almost constantly cycled through the lungs, delivering a steady supply of oxygen at all times. This system, while far more efficient than mammalian respiration, makes birds more sensitive to chemicals in the air.

References

• My Toos: Bird Breathing

Resources

• Earth Life: Breathing in Birds