In water with limited oxygen, fish have the remarkable ability to absorb about 75% of the oxygen that passes through them; this is double the percentage of oxygen that our lungs extract from the air we breathe.
Fish can breathe in several different ways, but the most common method is through the use of gills. These branched organs typically consist of four pairs, all enclosed within the gill cavity. The cavities are protected by the gill cover, also known as the operculum, which plays a crucial role in underwater breathing, just like the gills themselves.
When most fish breathe, they first close their gill covers and take in a gulp of water. Then, they open the gill covers, creating a pressure difference that pulls water through the gills, which contain thread-like filaments distributed evenly along the gill arches. These filaments are covered by countless small blood vessels known as capillaries, along with small extensions called gill lamellae, which further increase the surface area of the gills.
As water passes over these capillaries, the membrane is thin enough for the fish’s red blood cells to extract dissolved oxygen from the water into the bloodstream. Just like when we breathe through our lungs, this process releases carbon dioxide, which is expelled from the water through the open gill covers. This technique is only effective underwater. On land, the pressure difference created by opening and closing the gill covers is not strong enough to draw in sufficient air.
Fish breathe much more frequently than we do.
Inside the gill lamellae, blood flows in the opposite direction to the water, creating a countercurrent system that optimizes gas exchange. In fact, gills can absorb about 75% of the oxygen that passes through them; this is double the rate at which our lungs extract oxygen from the air we breathe. Fish also breathe much more frequently than we do. On average, an adult breathes 12 to 18 times per minute, while most fish pull water through their gills from 20 to 80 times per minute.
These numbers could even make a marathon runner feel outmatched. Thanks to this rapid, frequent, and efficient breathing rhythm, fish process significantly more oxygen than we do.
