Just as a car engine must take in the oxygen it needs to burn fuel for energy, an animal needs to take in oxygen to carry out cellular respiration. A waste product of respiration is carbon dioxide, and the body needs to dispose of it. Gas exchange—the body's transfer of oxygen and carbon dioxide— is the main function of the respiratory system.

Animals have a variety of adaptations that allow respiration to take place. The simplest and smallest animals obtain oxygen by direct diffusion. Other invertebrates, such as earthworms, may breathe through their skin, directly diffusing oxygen and carbon dioxide through a network of blood vessels. Or they may exchange gases through a system of tracheae, as in insects.

Specialized respiratory organs appear in more complex organisms. Water-dwelling animals exchange gases through gills. Land-dwelling animals have lungs for gas exchange.

Close examination of the human respiratory system reveals that the lungs contain tiny sacs called alveoli. Alveoli are the transfer sites of oxygen to the bloodstream and carbon dioxide back to the air.

Gas exchange is made possible by differences in the partial pressures of oxygen and carbon dioxide in the blood and in the air. If the partial pressure of a gas in air is higher than the partial pressure of the dissolved gas, the gas will diffuse from the air to the liquid.

The transport of oxygen is made possible by hemoglobin, a protein-iron complex that associates with oxygen.

The transport of carbon dioxide involves hemoglobin as well, although carbon dioxide also dissolves in the blood plasma.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education