During Biology today, one of my classmates asked my teacher how, exactly, does a human drown. His logic, based on what we were being taught, was that since oxygen has to be dissolved in the thin film of moisture covering the respiratory surfaces before it could be diffused in, having the water would help, too, wouldn’t it, because the oxygen is already readily dissolved? According to our teacher, though, that is not the case.
So, how do people drown? It is due to both having insufficient Oxygen (O2), and having too much Carbon Dioxide (CO2) in our body.
See, when you hold your breathe, the amount of CO2 in your body builds up, because you’re not breathing, thus, you’re not allowing the CO2 to be diffused out. When that happens, your brain will automatically signal for you to breathe, and you would breathe, involuntarily, no matter what. Why would your brain signal for you to breathe? That’s because CO2 and water together forms carboxylic acid, which gives you H+ ions that will lower the pH. As humans, our bodies are incapable of functioning properly when the pH gets too low, thus, your brain will automatically get you to breathe anything, in an attempt to help you get rid of the CO2, and, in the process, get you more O2.
Which brings us to the second point: O2 has low solubility in water. As such, having a lung full of water actually means less oxygen for us than if we simply had a lung full of air. The other problem that comes with having a lower concentration of O2 in water than in air is that diffusion works via a concentration gradient, where molecules will diffuse from a region with a higher concentration to a region with a lower concentration. This is why we breathe, or, as my teacher calls it, ventilate.
Ventilation allows us to get a new supply of air, helping us to readjust the concentration gradient such that there will be a higher concentration of O2 in the air than in the blood, and a lower concentration of CO2 in the air than in the blood. With water, however, as it is both heavier and denser than air, being human, our lungs are not strong enough. They cannot ventilate water, thus, on top of having an already low concentration of oxygen in the water that we breathe in, we cannot exhale it to replace it with a new load of water that might have slightly more oxygen in it than one that has already been drained as much as it could be by us.
Thus, when you are drowning, if you try to hold your breathe, you would eventually be forced to start breathing, less you die of having your body being acidified, but if you don’t try to hold your breathe, you would not have enough oxygen, and would thus suffocate to death.
A piece advice here: if you don’t know how to swim yet, go and learn now. It will save your life someday.