Oxygen is used as the terminal acceptor of the electron transport system which is found in mitochondria. This means oxygen must diffuse into all cells, even in large multicellular organisms. In addition, carbon dioxide is produced during cellular respiration by decarboxylation reactions which are the exact reverse of carboxylation reactions occurring in photosynthesis. This gas must be removed because if it accumulates it will change the internal environment of the organism which must remain relatively stable for proper function. Carbon dioxide combines with water to form carbonic acid which will change the pH of blood and tissue fluids interfering with enzyme reactions and causing a multitude of difficulties. Consequently, highly developed structures have evolved for gas exchange.
Oxygen cannot diffuse directly to the mitochondria, it must become dissolved in the fluid environment of the cell. Regardless of the size or living conditions of the animal, all gas exchange requires the gas to go into solution. Because the volume of a cell increases as a cube function and the surface area as a square function, the size of cells is limited. This is necessary as eventually not enough surface area would be available for exchange between the cell and its environment. By becoming multicellular, surface area is increased. With division of labour, some cells become gas exchange surfaces. As the organism increases in size, more respiratory surface is needed. This has results in the development of infolded (invaginated) and outfolded (evaginated) respiratory organs. In general, lungs have developed as invaginations and gills as evaginations.
Because oxygen is about 20 times less soluble in water than air (210 cm3 of O2/litre of air, 4 to ll cm3 of O2/litre of water) aquatic animals have developed a wide variety of mechanisms for extracting it from water. Most of these depend on keeping the concentration of oxygen as high as possible on the outside of the respiratory membrane and as low as possible on the inside, so inward diffusion occurs as rapidly as possible. Respiratory pigments combine with oxygen so that the concentration is kept low in the surrounding fluid. The flow of oxygen-rich water is often in the opposite direction of oxygen-poor blood to keep the concentration gradient as high as possible. To facilitate diffusion, respiratory membranes are very thin and, consequently, fragile. Gills are often protected by a covering which sometimes becomes involved in pumping water across them. Terrestrial animals also require oxygen to be dissolved in water in order to be absorbed, and organisms with lungs, therefore, must keep them moist. Because lungs are the result of invagination, they are internal and well protected.