Why fish are better at breathing than you are

In 2019, Eliud Kipchoge finished

a Vienna marathon in one hour, 59 minutes, and 40 seconds. This staggering time broke

the two-hour barrier that most runners previously

deemed impossible. However, some researchers

weren't as surprised. Recent studies investigating if humans

could maintain such a high pace for the length of a marathon

had found that elite runners can take in twice as much oxygen

as non-runners. And it’s likely that this superhuman

ability played a role in Eliud’s victory. But when it comes

to breathing efficiently, not even the best runners in the world

can compete with the average fish. That’s right— you’re looking at some

of the best breathers on Earth; which actually makes sense when you

consider how little oxygen there is in their aquatic environments. Fish can breathe in a variety of ways, but the most common

is through the use of gills. These branching organs typically

come in four pairs, all enclosed in gill chambers. These chambers are protected

by opercle, or gill covers, which are just as essential to underwater

breathing as the gills themselves. When most fish take a breath, they first close these covers

and take a gulp of water. Then, they open their opercle, creating a pressure differential

that pulls water through the gills, which are composed of thread-like

filaments spaced evenly along a gill arch. These filaments are covered in countless

small blood vessels called capillaries, in addition to tiny extensions

known as gill lamellae which further increase

the gill’s surface area. When water passes over these capillaries, the membrane is thin enough

for the fish’s red blood cells to pull dissolved oxygen from the water

into the bloodstream. And just like when we breathe

with our lungs, this process releases carbon dioxide, which passes out into the water

through the open gill cover. This technique only works underwater— on the surface, the pressure differential

created by opening and closing gill covers isn’t strong enough to pull

in sufficient air. But under the surface,

it’s remarkably efficient. Inside the lamellae, blood flows

in the opposite direction to the water, creating a counter-current system

that optimizes gas exchange. In fact, gills can absorb roughly 75%

of the oxygen passing through them; that’s twice the percentage of oxygen

our lungs extract from a breath of air. Fish also breathe much more frequently

than we do. The average adult human breathes

12 to 18 times a minute, while most fish pull water

over their gills anywhere from 20 to 80 times a minute. Those numbers would raise

even a marathoner’s eyebrows. Thanks to this fast, frequent,

and efficient breathing fish process far more oxygen than we do. It's also why some species

can live at great depths. Bodies of water get most

of their oxygen from the surface, where O2 dissolves into the water

and begins circulating. But further down, there are

oxygen minimum zones, with concentrations as low as

0.5 milligrams of oxygen per liter. To get the oxygen they need, fish living at these depths rely

on increased gill ventilation and hearts that pump high volumes of

oxygenated blood throughout the body. Even with these tricks, sometimes there's

still not enough oxygen to go around, forcing fish to adapt

more extreme solutions. For example, the Australian lungfish

lives in a habitat where the water level drops precipitously

in August and September, making it almost impossible

to survive with gills alone. Fortunately, these fish

have gills and lungs. Using their long, thin limbs, they can lift their mouths above the

surface and take deep breaths of air, allowing them to survive out of the water

for several days. And other species of lungfish can survive

above ground even longer in cocoons of mud and mucus. Fortunately, for most fish these extreme

adaptations aren’t necessary. After all, 71% of Earth is covered in H2O, giving these aquatic animals plenty

of room to flaunt their gills’ skills.