Inhalation and Exhalation The bronchi, primary and secondary, and the air sacs do exchange gas with the blood during avian respiration. Their only function is to help move the air through the actual gas exchange organs, the lungs. To achieve the unidirectional flow of air through the lungs, the inhaled, oxygen-rich air passes through the primary bronchi directly into the posterior air sacs, which are being expanded as a result of the movements of inhalation. While this is happening, the air that is already in the gas exchange areas of the lungs is being pulled out of them and into the anterior air sacs, which are also experiencing expansion during inhalation. Replacing this air in the lungs is a portion of the oxygen-rich air from the primary bronchi that does not enter the posterior air sacs, but flows into the posterior lung regions and is directed forward to the anterior areas. During exhalation, both the posterior and anterior air sacs are compressed and their contents emptied. The air from the posterior air sacs enters the gas exchange areas of the lungs, not yet having lost any of its oxygen, and flows in a posterior to anterior direction. The air from the anterior air sacs, loaded with the carbon dioxide which had diffused out of the blood capillaries, enters the primary bronchi and is exhaled by the animal. The cycle then repeats itself with the next inhalation. During the cycles of inhalation and exhalation, the air is always passing through the gas exchange areas of the avian lungs in a single posterior to anterior direction. The air passing through the gas exchange areas of the lung also always contains a relatively constant and high percentage of oxygen that can diffuse into the blood inside the lung's blood capillaries. The blood entering these capillaries is rich in carbon dioxide, which diffuses out of the blood and into the air as it passes through the lung on its way to the anterior air sacs, and eventually out of the animal.
Studying Gene Flow Scientists from many disciplines are currently studying migration and gene flow in a variety of ways. For decades, ornithologists and marine biologists have been placing identifying tags or markers on members of different species of birds, fishes, and marine mammals to determine the range of their migratory habits in order to understand the role of migration and subsequent gene flow in the biology of their subjects. These studies have led, and will continue to lead, to important discoveries. Most studies of migration and gene flow, however, relate to human beings. Many of the important discoveries concerning the role of gene flow in the evolution of life come from the continuing study of the nature of genes. A gene, in cooperation with such molecules as transfer ribonucleic acid (tRNA) and related enzymes, controls the nature of an organism by specifying amino acid sequences in specific functional proteins. In recent decades, scientists have discovered that what they previously believed to be single pure enzymes are actually groups of closely related enzymes, which they have named "isoenzymes" or "isozymes." Current theory holds that isozymes can serve the needs of a cell or of an entire organism more efficiently and over a wider range of environmental extremes than can a single enzyme. Biologists theorize that isozymes developed through gene flow between populations from climatic extremes and enhance the possibility of adaptation among members of the species when the occasion arises. The combination and recombination of isozymes passed from parent to offspring are apparently determined by deoxyribonucleic acid (DNA). Investigation into the role of DNAin evolution is one of the most promising avenues to an understanding of the nature of life. A classic example of the importance of understanding migration and gene flow in the animal kingdom is the spread of the so-called killer bees. In the 1950's, a species of ill-tempered African bee was accidentally released in South America. The African bees mated with themore docile wild bees in the area; through migration and gene flow, they transmitted their violent propensity to attack anything approaching their nests. As the African genes slowly migrated northward, they proved to be dominant. Further research into migration and gene flow promises to provide information indispensable to the attempt to unravel the mysteries of life. Coupled with the concept of mutation, gene flow is a crucial component of evolution.
