Ornithology | Respiratory system
Due to having the high metabolic rate required for flying, birds have a high oxygen demand. They meet this by having a respiratory system more efficient than that of a mammal or a reptile. Birds ventilate their lungs by means of posterior and anterior air sacs (seven or nine) which act like bellows, but do not play a direct role in gas exchange. The air sacs of birds extend into the humerus (the bone between the shoulder and elbow), the femur (the thigh bone) and the vertebrae. The lungs have a fixed volume and are the site of gas exchange, the air passing through on its way to the air sacs and on its way back from the air sacs.
There are three distinct sets of organs involved in respiration—the anterior air sacs (interclavicular, cervicals, and anterior thoracics), the lungs, and the posterior air sacs (posterior thoracics and abdominals).
The posterior and anterior air sacs expand during inhalation. Birds can breathe through the mouth or the nostrils. During inspiration, air entering these openings passes through the pharynx and then into the trachea (or windpipe). The trachea is generally as long as the neck. Half of the inhaled air enters the posterior air sacs, the other half passes through the lungs and into the anterior air sacs. The sacs contract during exhalation. The anterior air sacs empty directly into the trachea, the posterior air sacs empty via the lungs, the lungs expel this air via the trachea.
Since during inhalation and exhalation fresh air flows through the lungs in only one direction, there is no mixing of oxygen rich air and carbon dioxide rich air within the lungs as in mammals. Thus the partial pressure of oxygen in a bird's lungs is the same as the environment, and so birds have more efficient gas-exchange of both oxygen and carbon dioxide than do mammals.
Avian lungs do not have alveoli, as mammalian lungs do, but instead contain millions of tiny passages known as parabronchi, connected at either ends by the dorsobronchi and ventrobronchi. Air flows through the honeycombed walls of the parabronchi and into air capillaries, where oxygen and carbon dioxide are traded with cross-flowing blood capillaries by diffusion. A diaphragm is absent in birds; the entire body cavity acts as a bellows to move air through the lungs. The active phase of respiration in birds is exhalation, requiring effort of the musculature.
Parabronchi are millions of tiny passages in the lungs of birds that constitute the primary sites of gas exchange between air and blood for birds; delivering oxygen and removing carbon dioxide. It is not an air sac. This function in mammals is performed by the alveoli, which is an air sac. Bird air sacs are anatomical structures (typically nine) unique to the dinosaur and bird respiratory system that allow unidirectional flow of air through the lungs. The nine flexible air sacs act like bellows to move air through the almost completely rigid lungs. "A portion of the air sac actually integrates with the skeleton, forming air pockets in otherwise dense bone."
Air flows from the parabronchi into air vesicles, called atria, which project radially from the parabronchi. These atria give rise to air capillaries, where oxygen and carbon dioxide are traded with cross-flowing blood capillaries by diffusion.
The syrinx is the sound-producing vocal organ of birds, located at the base of a bird's trachea. As with the mammalian larynx, sound is produced by the vibration of air
flowing through the organ. The syrinx enables some species of birds to mimic human speech, and in some songbirds, the syrinx can produce more than one sound at a time.