Respiratory System

Bird respiration is entirely pulmonary and involves the respiratory tract (nares, nasal sacs, glottis, larynx, trachea and the voice-producing syrinx), the lungs, and an extensive system of air sacs, all working together without any true muscular diaphragm. The trachea divides into two bronchi, which branch progressively into fine air capillaries interwoven with the pulmonary blood capillaries, inside solid, spongy lungs that are fixed rigidly to the ribs and cannot expand or contract themselves. Because the lungs are non-dilatable, breathing is instead driven by nine interconnected air sacs (paired cervical, a single interclavicular, paired extraclavicular, paired clavicular, paired thoracic and paired abdominal sacs, one of which even extends into the hollow humerus bone) that act as bellows: as the sternum is drawn toward the vertebral column by body-wall muscle contraction, the elastic ribs bend inward, shrinking the body cavity and forcing air out (an active process), while relaxation lets the cavity spring back and draw air back in (a passive process) - the reverse of the pattern in mammals. This air-sac arrangement lets air flow through the lungs during both inhalation and exhalation, making gas exchange unusually efficient and also helping the bird maintain a high, stable body temperature. The syrinx, a chamber supported b …