Fish face a number of challenges in obtaining oxygen, because water is extremely oxygen deficient in comparison to air. Freshwater contains 95 percent less oxygen than does air, and saltwater has, in general, only 20 percent of the oxygen content found in freshwater.

Most fish have, therefore, had to develop an extremely efficient system for extracting oxygen from the water. In the majority of the species, water is drawn in through the mouth, whereupon it passes over the gills and out through the operculum, which are platelike structures that cover the gill cavity. Oxygen diffuses into the blood in the gills, and it is picked up by hemoglobin in the red blood cells (as it is in humans). The gills of most fish are so efficient that 80 percent of the water's oxygen content is removed. Carbon dioxide, a metabolic byproduct, diffuses out of the fish's body at the gills as well.

Some fish have evolved alternative methods for dealing with this problem and have bypassed the need for oxygen-bearing water altogether. The most well-known of these is the lungfish, which utilizes oxygen extracted directly from the air at the surface of the water.