Fish breathe by extracting oxygen from water through specialized organs called gills. They take in water through their mouths and pump it over the gills, where oxygen from the water diffuses into the fish's bloodstream and carbon dioxide is expelled back into the water. The gills have thin filaments with a large surface area and a network of capillaries, enabling efficient gas exchange. Blood flows in the opposite direction to the water over the gills, maximizing oxygen absorption through a countercurrent exchange system. This process allows fish to extract a large percentage of oxygen from the water to support their respiration underwater.