Final answer:
Fish use gills to extract oxygen from water, where the dissolved oxygen diffuses from the water to their bloodstream, allowing them to breathe underwater.
Step-by-step explanation:
Fish use their gills to filter oxygen out of water. Gills are thin tissue filaments that are highly branched and folded, providing a large surface area for oxygen uptake. As water passes over the gills, dissolved oxygen moves from the higher concentration in the water to the lower concentration in the blood through a process called diffusion. This way, oxygenated blood can be transported to other parts of the fish's body. Over the course of evolution, fish gills have adapted from filtering food to extracting oxygen, allowing fish to become larger and more active.