The total exchange surface area is relatively large. The lining of the alveoli is only one cell thick. The concentration of CO 2 is higher in the air than in the alveolar capillaries. Water contains much less O 2 than air per unit volume. Gills have less surface area than lungs.
Water is harder to pump than air.
Exchanging gases with water causes substantial heat loss. Tracheal systems for gas exchange are found in crustaceans earthworms insects jellyfish vertebrates 27. Water can support the delicate gill features. Most fish actively pump water over their gills. Keeping membranes moist is no problem.