Respiration

Birds respire by a method of crosscurrent flow, ie: flow at a 90 degree angle. There are three sections involved in respiration. These are the anterior air sacs (interclavicular, cervicals, and anterior thoracics), the lungs, and the posterior air sacs (posterior thoracics & abdominals). It takes a bird two full breaths (inhaling and exhaling), to cycle air through.

The air flow through air sacs and lungs is as follows:

* First inhalation: air flows through the trachea and bronchi into the posterior air sacs.
* First exhalation: air flows from the posterior air sacs to the lungs
* Second inhalation: air flows from the lungs to the anterior air sacs
* Second exhalation: air flows from the anterior sacs back through the trachea and out of the body.

In birds, air flows in only one direction. Because of this, birds are able to diffuse more oxygen into their blood. Unlike humans and other mammals, there is no mixing of oxygen rich air and carbon dioxide rich air. Thus, the partial pressure of oxygen in a bird's lungs is the same as the environment. This is also why you would more likely see a bird on Mount Everest, than say a mouse.

Birds
Reproduction
Respiration
Birds and Humans