What if you had gills?

You take a big breath of air. Your lungs fill up.

A fish stays underwater all day. It breathes with gills. What if you had gills too?

Hold your breath and duck under the water at the pool. In a little while, you have to come up for air. A fish never has to come up. It stays underwater all its life, breathing with its gills.

So what if you had gills? Could you swim with the fish all day? We can't grow gills, but we can follow the clues: how much oxygen water really holds, and how gills get it out.

OpenStax notes that all aerobic organisms require oxygen for their metabolism, and that lungs and gills both work by letting oxygen diffuse from the surroundings into the blood. Lungs do this with air; gills do it with water.

This thought experiment gives a person working gills and asks whether they could live underwater. It follows three threads: what 'breathing water' really means, how much oxygen water holds compared with air, and how fish gills are built to cope. Known biology is separated from reasoned ideas.

You breathe in air. Air has oxygen in it. Your body needs oxygen to live.

Your lungs take the oxygen out of the air. Then your blood carries it all around your body.

The air you breathe is roughly one fifth oxygen: about 21 percent, OpenStax says. When you breathe in, air travels down tubes deep into your lungs, which branch into tiny bubble-like sacs called alveoli.

The walls of the alveoli are only one cell thick, and tiny blood vessels wrap around them. Oxygen slips across into your blood, and your blood carries it around your body.

The atmosphere is about 21 percent oxygen (OpenStax). In human lungs, respiratory bronchioles end in alveolar ducts; each duct ends in about 100 alveolar sacs, each with 20 to 30 alveoli 200 to 300 microns across. Gas exchange happens only in the alveoli, whose walls, typically one cell thick, touch capillary walls also one cell thick.

That thin, moist barrier lets oxygen diffuse from air into blood quickly. OpenStax notes that any respiratory surface must be kept moist for gases to dissolve and diffuse, which is true of lungs and gills alike.

Now pretend you have gills. You swim down into the water.

But water has much less oxygen than air! Your gills would have to work very hard.

Gills don't use the oxygen that is part of water itself. Water molecules contain oxygen atoms, but animals can't use those, the USGS explains. Instead, gills take in oxygen gas dissolved in the water, and there is very little of it: up to about ten oxygen molecules for every million molecules of water.

So a person with gills might find water a hard place to get enough oxygen, especially a big, busy body like ours. That's an idea, but it's based on how little oxygen water holds.

The USGS points out that the oxygen atom within each water molecule is not what aquatic organisms use; they rely on dissolved O₂, at most about ten molecules per million water molecules. Summer surface waters typically hold more than 8 mg/L, and below 2 mg/L water is defined as hypoxic.

Compare that with air at about 21 percent oxygen. It is reasonable to infer that a large, warm-bodied animal with human oxygen demand would struggle to meet it from water through gills of human size; that inference is ours, not a measured result.

Gills look like tiny feathery combs. Water flows over them.

Oxygen moves from the water into the fish's blood. Lots of folds give lots of space for oxygen to get in.

Gills are made of thin threads of tissue that branch and fold again and again. All that folding gives them a huge surface, so lots of oxygen can get in at once.

As water flows over the gills, oxygen moves from the water, where there is more of it, into the blood in tiny vessels, where there is less. Scientists call this kind of movement diffusion. Carbon dioxide moves the other way, out into the water. Then the fish's blood carries the oxygen around its body.

OpenStax describes gills as thin tissue filaments, highly branched and folded, giving a large surface area. As water passes over them, dissolved oxygen diffuses from water (higher concentration) into the blood of capillaries in the lamellae (lower concentration), and carbon dioxide diffuses out.

Diffusion alone suffices only for organisms under about 1 mm or with very flat bodies, such as flatworms; larger animals evolved gills or lungs plus circulation. Gills are not only for fish: OpenStax lists them in mollusks, annelids and crustaceans, and earthworms and amphibians also exchange gases through moist skin.

The axolotl is an animal with feathery gills on its head. It lives in water all its life.

It has lungs too! Sometimes it swims up for a gulp of air.

Meet the axolotl, a salamander that keeps its frilly, feathery gills even when it is grown up. The San Diego Zoo says axolotls stay in the water their whole lives, breathing with their gills.

Here's the surprise: axolotls also grow working lungs, and every so often they pop up to the surface for a quick gulp of air. So an animal can have gills and lungs at the same time. Water and air each have their own way to deliver oxygen.

The axolotl is neotenic: it keeps juvenile features, including external gills, a tail and a body fin, into adulthood, and remains aquatic for life (San Diego Zoo Wildlife Alliance). Although it develops functional lungs, it uses its prominent feathery gills to breathe underwater and may surface occasionally to gulp air.

The axolotl shows that gills and lungs are not mutually exclusive. Water's low and variable oxygen content, which the USGS notes falls as water warms, helps explain why some aquatic animals can also take air when they need to; that link is our reasoning.