Question

What are the requirements of a surface to be highly efficient for gaseous exchange? How has this efficiency increased from fish to mammals?

Answer 1

The process by which oxygen and carbon dioxide move between the bloodstream and the lungs is known as gas exchange. This is the primary function of the respiratory system, and it is critical for maintaining a constant supply of oxygen to tissues while also removing carbon dioxide to prevent its accumulation.

Complete answer:
Diffusion across the outer surface of large animals is insufficient to maintain gas exchange. They created a variety of respiratory surfaces, all of which increase the surface area for exchange, allowing for larger bodies. Thin, moist epithelial cells cover the respiratory surface, allowing oxygen and carbon dioxide to exchange. Only when those gases are dissolved in water can they pass through cell membranes.

Respiratory surfaces are thin and have a large surface area, which are structural adaptations that maximize the gas exchange rate. To keep their plasma membranes intact, all living cells must be immersed in water. The respiratory surfaces of both terrestrial and aquatic animals are moist, and after dissolving in water, $O_2$ and $CO_2$ diffuse across these surfaces. The respiratory surface must supply oxygen and expel carbon dioxide for the entire body and several solutions to the issue of providing a large enough surface have emerged.

An endotherm has a larger respiratory surface area than an ectotherm of comparable size.

Note: Some small multicellular organisms, such as flatworms, can also exchange enough gas across the skin or cuticle that surrounds their bodies. However, in most larger organisms with low surface-area-to-volume ratios, specialized structures with convoluted surfaces such as gills, pulmonary alveoli, and spongy mesophyll provide the large area required for effective gas exchange.