Where does O2 diffusion occur in fish?

The correct answer is the lamellae on filaments of gill arches, which are specialized structures in fish gills. Gills extract oxygen from water, and this process relies on the large surface area provided by the lamellae. Each gill filament is lined with lamellae, which are thin, flat structures that facilitate efficient gas exchange. As water flows over these lamellae, oxygen dissolved in the water diffuses across the thin epithelial layer into the blood vessels within the lamellae, where it is then transported to the rest of the body.

The significant surface area and the close proximity of blood and water in the lamellae enhance the diffusion process, allowing fish to meet their oxygen requirements effectively while constantly pumping water over their gills. This efficient mechanism is essential for supporting the metabolic needs of fish in an aquatic environment.