Why Can’t We Breathe Underwater?

This is one of those sentences that one doesn’t know whether to write as an interrogative (to turn it into a question) or as an exclamation (as a complaint). It would certainly be very interesting to be able to dive and dive without care, enjoying the incredible treasures of the underwater world.

A question of gills

These structures are called gills and are present, for example, in fish. Other alternatives for breathing underwater can be observed in inactive invertebrates with low oxygen needs.

The functioning of the gill system is relatively simple compared to that of our respiratory system. Fish open their mouths to swallow water and let it out through the sides of their body.

In the case of cartilaginous fish (such as sharks, rays, or manta rays), the process is carried out through the so-called gill slits. The rest, known as bony fish (sardines, anchovies, trout, or hake, for example), expel water through the lateral opercula, a tongue-like structure that covers and protects the gills and that normally marks the rear limit of the head.

Whether they are bony or cartilaginous fish, water always passes through the gill arches, structures on which some lamellae can efficiently extract the oxygen dissolved in the water. If we look closely, the water flows in one direction: it enters through the mouth loaded with oxygen and exits through the sides of the body, dragging carbon dioxide with it.

Why our lungs don’t work for that

The way our respiratory system works is completely different, so our lungs face two major limitations when it comes to breathing underwater.

The first, and most important, is that they are unable to efficiently extract oxygen from water. We might think that water contains a lot of oxygen since in its molecular structure we find two hydrogen atoms for each oxygen atom (that is why we represent it as H₂O). The problem is that this oxygen is not accessible to us or to the fish.

The oxygen we need is known as molecular oxygen (represented as O₂) and is present in both air (at a concentration of 21%) and water (at a ratio of around 1% compared to the same volume of air ). However, our lungs cannot efficiently access the O₂ in water, while the intricate design of the gill filaments is capable of extracting 80% of this dissolved oxygen.

The second problem our lungs would have to deal with is precisely the density of water. Assuming we could obtain sufficient oxygen directly from it, the path is bidirectional, meaning that water would have to enter and exit through the same place (as air does). Our respiratory system is not strong enough to mobilize all this liquid and the lungs would quickly become flooded with water poor in oxygen.

On the other hand, the air is very light and we can make it circulate without problems through pulmonary breathing, cyclical inhalations, and exhalations (remember that it is very important to write “spiral” with an “s”; if you write it with an “x” it means “die”).

Gas exchange in the alveoli

When air enters through the nasal passages ( preferably ) or the mouth, it passes through the pharynx and larynx and is conducted by the trachea, bronchi, and bronchioles to the inside of the lungs. There it reaches the inside of hollow, sac-like structures known as alveoli ( there are about 300 million of them per lung on average ).

The walls of these alveoli are covered by a single, very thin layer of cells in close contact with the blood capillaries. The concentration of O₂ in the air is so high that it easily passes through the cell membranes into the blood in an attempt to balance the partial pressure on either side of the membrane (between air and blood).

The same thing happens with carbon dioxide, but in reverse: it moves from the blood to the air inside the alveoli to be expelled at the next expiration.

In each respiratory cycle, we bring air in and out of the lungs ( between 5 and 6 liters per minute ), continually renewing it and ensuring the supply of oxygen to our body.

Whales and dolphins cannot breathe underwater either.

But this is not exclusive to humans. Our respiratory system is very similar to that of the rest of amniotes, a word we use to designate all vertebrates whose embryo develops floating in amniotic fluid thanks to a layer that surrounds it called the amnion; that is, reptiles, birds, and mammals. Not even marine mammals (such as whales or dolphins) can breathe underwater, having to come up to the surface more or less frequently to get oxygen.

Amphibians are halfway between the two systems. Most juveniles live in the water, breathing with gills, which they lose as adults to make way for rudimentary lungs that need the support of cutaneous respiration. This is why they have slimy skin.