How is the structure of the gills suited to their function?

Gills enhance diffusion between aquatic creatures' blood and the surrounding water. They have a large surface area to increase the area available for gas exchange and a lot of capillaries to increase the bloodflow through the gills. The cells in the capillaries are flattened, which further increases their surface area and the rate of gas exchange. Additionally, their blood flows in the opposite direction as the water flows over the gills, which maximizes the diffusion gradient between the two.

https://fishbio.com/field-notes/fish-biology-behavoir/an-efficient-exchange