How do fish use gills to breathe?

Answered by James Kissner

Fish use gills to breathe by taking water into their mouth and passing it over their gills. The gills, located just behind the fish’s head on each side, are responsible for extracting dissolved oxygen from the water and releasing carbon dioxide back into it. This process allows fish to extract the oxygen they need to survive.

The gills of a fish are quite large and contain thousands of small blood vessels. This design maximizes the surface area available for oxygen exchange. As water flows over the gills, oxygen molecules diffuse from the water into the blood vessels, while carbon dioxide molecules diffuse from the blood vessels into the water. This exchange of gases is made possible by the thin walls of the blood vessels and the high concentration of oxygen in the water.

To illustrate this process, let me provide an example. Imagine a fish swimming in a freshwater stream. As it opens its mouth, water rushes in and flows over its gills. The oxygen dissolved in the water comes into contact with the gill filaments, which are finger-like projections on the gills. These filaments are covered in tiny structures called lamellae, which further increase the surface area available for gas exchange.

As the oxygen-rich water passes over the gill filaments, oxygen molecules diffuse across the thin walls of the blood vessels and enter the fish’s bloodstream. At the same time, carbon dioxide, which has built up in the fish’s bloodstream, diffuses out of the blood vessels and into the water. This exchange of gases allows the fish to obtain the oxygen it needs to survive while simultaneously getting rid of waste carbon dioxide.

It’s important to note that fish need a constant supply of well-oxygenated water to ensure efficient gas exchange. In some cases, fish may have adaptations to help them in low-oxygen environments, such as the ability to gulp air from the surface or the presence of specialized gill structures.

Fish use their gills to extract oxygen from water and release carbon dioxide. The large size of the gills, along with the thousands of blood vessels and thin walls, maximize the amount of oxygen that can be absorbed from the water. This remarkable adaptation allows fish to breathe underwater and thrive in their aquatic habitats.