Figure 2 - Comparison of gills and lungs.

Gases diffuse between water and capillaries in gills, which are immersed in the flow of water. In contrast, diffusion of gases between inhaled air entering the lungs and capillaries occurs within the lungs. Circulation in the fish is accomplished by a two-chambered heart, whereas a four-chambered heart is used to separate the systemic and pulmonary circulations in mammals.