42.19 (7) The digestive, circulatory, and respiratory systems cooperate to bring nutrients and oxygen to cells, and to remove wastes and carbon dioxide. In humans, the respiratory surface is about 100m2 while the rest of the body surface is about 2m2.
Oxygen molecules (O2) are soluble in air, water, and lipids (ie, in membranes) but they cannot dissolve directly from the gaseous state (air) into and through the lipid layer of cell membranes. O2 must first dissolve in water, then diffuse to and through the lipid membranes of cells to enter cell cytoplasm. Because oxygen must first dissolve in water before it can diffuse through cell membranes, the surfaces of animal cells that are the gas exchange organ must be wet. All animals must obey this physical ‘rule’.
Because it must be wet, the gas exchange surface of terrestrial animals allows water to evaporate from it directly back into the air thus causing water loss to the animal, water that must be replaced somehow to avoid desiccation. The upside to evaporation from the gas exchange surface is that it removes a lot of heat from the animal, cooling it.
42.20 (7), 42.21 (8) Getting oxygen from water -- aquatic animals use gills.
Animals that live in water don’t have to worry about loss of water from their gas exchange surfaces. But they have two problems that are different from terrestrial, non-aquatic critters – water is very viscous compared to air, and oxygen is much less soluble in water than in air (~70-fold less) so it is hard for water-dwelling animals to expose the gas exchange surface to enough water to get enough oxygen for survival. Many aquatic animals simply don’t expend very much energy on it. Because it’s so hard to get lots of oxygen they have adapted to a lower-energy strategy to support their ‘lifestyle’, one they can maintain on low oxygen concentrations.
The strategy for energetic aquatic animals is to have a gas exchange organ with a very large surface...