What happens to the concentration of oxygen during gas exchange in fish gills?

During gas exchange in fish gills, oxygen concentration in the water decreases. This occurs because fish extract oxygen from the water as it flows over their gills. The gills have a large surface area and are rich in blood vessels, allowing for efficient diffusion of oxygen into the fish's blood. When the water passes over the gill membranes, the oxygen moves from an area of higher concentration (the water) to an area of lower concentration (the blood) due to diffusion. As a result, the concentration of oxygen in the water decreases while it increases in the blood stream of the fish.

This process is essential for the survival of fish, as it enables them to take in the oxygen they need for cellular respiration and other metabolic processes. Hence, the correct response highlights the depleting levels of oxygen in the water during this essential exchange process.