Barring other glaring scientific issues the film Waterworld has (like the total loss of all technology because of glacial melting) there is one that should be brought to light. The movie follows a man named “The Mariner” as he wanders through a flooded post-apocalyptic end of days scenario, fighting bandits and criminals and being outcast because of a simple mutation. Kevin Costner’s character in the film is able to breathe underwater because he has gills.
Now, it seems like a pretty simple thing, but the science behind gills ruins the sci-fi flavor of the movie. Gills are an organ that have specially evolved over the course of time to allow animals (fish, mollusks and such) to survive in water. The gills themselves don’t actually allow the fish to breathe water, rather they allow the fish to pull oxygen out of the liquid. Given that we mammals breathe air, roughly 200,000 parts oxygen per million; our respiratory tracts are unable to deal with the 4-8 parts per million that water has. Where Waterworld falls apart is that Kevin Costner is a human being, warm blooded and all that. What that means is his metabolism is at the level of a warm blooded creature: super high. Fish along with all gill equipped animals are cold blooded because it reduces the rate at which oxygen is burned within the body among other things. While the idea is interesting, Costner would not be able to breathe comfortably in water unless he found a way to move water through his gills at 25,000 times the efficiency that fish have.
How gills work is quite interesting. They are basically a series of flaps of flesh with a tight lattice of blood vessels, arranged in a sheet to allow for maximum surface area. The gills are covered with a flap that can be opened and closed at will, allowing the fish to essentially breathe in and out. The blood vessels carry deoxygenated blood directly from the heart of the fish and push it countercurrent to the water that runs up against the gills. Through diffusion, upwards of 70% of the oxygen in the water is transferred to the blood stream of the fish. The issue that fish run into is in the influx of salt/lack of salt in their system from running a constant osmosis (basically a balancing act from zones of high concentration of stuff to zones of low concentration of stuff). The solution that fish have is two-fold: fresh water fish pee. A lot. This helps to remove water from their bloodstream and keep their salt levels higher than the outside liquid. Saltwater fish have cells that allow them to excrete sodium chloride, keeping their salt levels lower than the water around them.
Just as an interesting addition, not all gills are made equal. Sharks have more gills then most fish (upwards of 7 to the usual 4 of a fish) and don’t cover them with flaps. Because of the lack of the flap, most sharks are able to blast water through a special hole in their mouths called a spiracle. Spiracles funnel water through their mouths directly over the gills. The water then mimics currents over the gills while the sharks aren’t in motion. Some sharks are without the spiracle and must remain in constant motion to breathe. This variety is known as an Obligate Ram Ventilator and includes around 2 dozen species of shark. The most famous variety of obligate ram ventilator? The Great White shark.
Citations, because Waterworld should have done their research: