Explain how a fish is adapted for gas exchange?

A fish had gills, made up of gill filaments containing lamellae, which provide a very large surface area for oxygen to diffuse out of the water as it moves over them. The blood vessels are in very close proximity to the lamellae, allowing a short diffusion pathway. A fish also utilises a countercurrent system, where the blood flows in the opposite direction to the water across the gills. This allows a concentration gradient to be maintained, so oxygen moves into the blood along the entire length of the gill.