Respiratory System

Lungs are the internal respiratory organs of amphibians, reptiles, birds, and mammals. The lungs, paired invaginations located in one area of the body, provide a large, thin, moist surface for gas exchange. Lungs work with the circulatory system, which transports oxygen from inhaled air to all tissues of the body. The circulatory system also transports carbon dioxide from body cells to the lungs to be exhaled. The process of inhaling and exhaling is called pulmonary ventilation.

Besides these similarities, there is a great variety in the respiratory systems of terrestrial vertebrates. Frogs, for instance, have balloon-like lungs that do not have a very large surface area. Diffusion across the frog's moist skin supplements the gas exchange through the lungs. Birds have about eight thin-walled air sacs attached to their lungs. The air sacs take up space in the entire body The human respiratory system. Illustration by Hans & Cassidy. Courtesy of Gale Group.
cavity and in some of the bones. When birds inhale, air passes through a tube called the bronchus and enters the air sacs located in the posterior (rear) of the animal. At the same time, air in the lungs moves forward to air sacs located in the anterior (front). When birds exhale, the air from the anterior air sacs moves to the outside, while air from the posterior sacs moves into the lungs. This efficient system moves air forward through the lungs both when the bird inhales and exhales. Blood in the capillaries of the lungs flows against the air current, which again increases respiratory efficiency. Birds are capable of flying at high altitudes, where the air has a low oxygen content, because of these adaptations of the respiratory system.