Why can't humans breathe underwater?

So today, the diversion appears, the interesting question that was posed by my biology teacher nearly two weeks ago, I confess, it did excite me, the large depth and breadth that I could go to with this really did open up many different trails to follow. So here what I have come up with so far, but I am still trying to get more information about the Silurian period in the Archeozoic time, which is nigh on the Precambrian period. I am really interested in finding out where fish and humans actually seperated through the journey of evolution. 

 

The principle of respiration for humans is breathing, without the lungs and diaphragm this would not be possible. The theory of inhalation and exhalation is that the pressure outside the lungs is less than within, so therefore the air rushes out. In water, if a human filled their lungs with water, they would be unable to exhale, firstly because of the lack of difference in density inside and outside the lungs, but also for many other reasons. Kinetic theory, Fick’s Law and many other aspects contribute to the fact that homo sapiens cannot breathe underwater.

The act of breathing begins at the mouth and nose – the two entry points for the air surrounding the face; it travels down your throat and into your trachea where it is divided down the two bronchial tubes. The air is then divided into smaller passages – the bronchioles, from there it goes into the alveoli. The alveoli are each surrounded by a mesh of capillaries and the oxygen diffuses through the alveoli and then capillary wall to bind with haemoglobin in a red blood cell. This oxygenated blood travels to the heart and is then pumped to the extremities of the body to allow respiration to take place. The deoxygenated blood returns to the lungs and the carbon dioxide that was released during respiration diffuses across the two membranes and is then exhaled. The diaphragm plays a huge part in respiration, it increases the pressure in the lungs, forcing exhalation.

Humans inhale a high concentration of oxygen which enables fast diffusion and is increased due to the low density, whereas water, due to being a liquid is denser, so therefore would be more difficult to inhale. Furthermore, exhaling would be nearly impossible due to the weight of water that would press down on the diaphragm, preventing it from contracting and therefore it would be impossible to expel the water that has a low concentration of oxygen to allow the lungs to fill with fresh water. Additionally, water contains a lower percentage of Oxygen compared to the blood that has just been freshly oxygenated so realistically the oxygen in the blood surrounding the alveoli could theoretically diffuse into the water, due to the concentration gradient being reversed. Moreover, the concentration of oxygen in air is higher than in water so therefore diffusion will require less energy because of the high contrast in concentration gradient.

Aquatic respiration is very different to human respiration. Fish respiration takes place through gills, the water flows to their mouth and is then forced out of their gills on either side of their body, this means that the water is filtered through their gills which consist of large amounts of gill filaments that are all in close proximity of each other. The gill filaments are all attached to the gill bar, stacked up to be specific and each filament has lamellae which increase the surface area of the gills. The water flows in the opposite direction than the blood over the gill filaments; this is known as countercurrent flow. The principle for aquatic respiration is that the water flows over the oxygenated blood, allowing oxygen to diffuse into the blood and then the water flows over the deoxygenated blood, allowing for the oxygen and carbon dioxide to diffuse out of the blood and into the water. This means that when the water flows over the oxygenated blood there is a small difference in the concentration of oxygen in the blood compared to the water, but diffusion can still take place so the concentration of oxygen dissolved in the water decreases, so when it proceeds and flows over the deoxygenated blood there is a concerntration gradient that allows for diffusion out of the blood and into water.

The evidence in contrast between human and aquatic respiration helps to explain why humans cannot respire under water.  The different organs and mechanisms used by each animal for respiration also demonstrates where each creature evolved, and how they have adapted to allow from respiration in the optimum ppO₂.  

I found this image online, but it didn't provide as much answers as I would like, as this picture shows, fish and humans seperated many years ago, even before the renowned Jurassic Period. So, the possibility of humans being able to breathe underwater is so far off possible it is almost bordering on ridiculous. Not only the amount of time since seperation, but the distance, as you can see the different branches show how many species have arisen in between fish and humans. 

I have been spreading the word around my school, my viewer numbers have increased considerably, and I am very grateful to the new readers, along with the increasing numbers of those 'across the pond' (Hi american readers!) - I would just like to say thanks to all my readers for reading. 

See you all soon, hopefully, School is crazy but I really enjoy writing this.