When recently going through a refresher course in SCUBA diving, I was reminded of just how poorly designed, in many ways, humans are for staying underwater. That hasn’t, however, kept people from coming up with complicated apparatuses to compensate for their natural lack of appropriate adaptations. From early diving bells to modern submersibles, humans have been determined to find ways to explore the oceans.
SCUBA diving is one of the most amazing set-ups that allows a freedom to feel very much a part of the underwater environment. But, then again, you are wearing a weight belt, inflatable vest, tank, dangling breathing devices, a mask and usually a wetsuit and flippers. All of this equipment is designed to overcome the challenges of being a natural air breather that now wants to be underwater. While the human body is composed of over 70% water, that still leaves the rest of our bodies filled with a lot of other stuff – including gas. When you put pressure on those gas-filled spaces like our lungs, it compresses them and can cause damage. We have air in all kinds of places in our bodies like our sinuses, our ears, and, of course, airways. When people describe an “ear squeeze” when diving under the water, it’s the painful feeling of that pressure building up.
When we stand on earth, we feel 1 atmosphere of pressure. For every 10 meters (33 feet) you travel underwater, there is an additional atmosphere of pressure. That means that, at 100 meters, the pressure is 10 times as great as it is on the surface. A human body can’t withstand that kind of pressure. Most people can’t go below about 150 feet without injury, and most recreational divers don’t go below 60 feet. When you think about the weight of water and that 1 cubic foot of water weighs about 60 pounds, that means at just 33 feet, you’re under nearly 2,000 pounds of water!
That doesn’t even address the primary issue of breathing underwater and that, aside from pressure, humans are just plain limited in their ability to keep enough oxygen in their tissues long enough to keep their bodies functioning. Without the aid of a SCUBA tank, we can’t hold our breath for very long – unless you hold the world record which is nearly 25 minutes!
So, how do fish do it? First, they have really watery bodies. Their bodies are essentially saturated by water. Second, they don’t have those giant air-filled organs that are problematic for humans. Instead of lungs, they have gills that can absorb oxygen right out of the water. They also have blood that is supercharged with extra hemoglobin that helps them hold on to much more of that oxygen as compared to human blood. Although they don’t have lungs, they do have one important gas-filled organ — a swim bladder. This is a bit like a balloon that can inflate and deflate to help the fish become more or less buoyant. A fish’s swim bladder is designed to inflate and deflate with much more elasticity than our lungs.
But then there are marine mammals. They have lungs, but they are amazingly adapted to stay underwater for long periods and to dive to great depths as well. The beaked whale, the deepest diving of the whales, has lungs that basically collapse, allowing it to travel down to 10,000 feet. In order to hold onto enough oxygen for its body to function, it pushes it out into muscle and hemoglobin-rich blood. Another amazing marine mammal that is also native to the North Pacific is the elephant seal. It exhales 90% of the air in its lungs yet can still hold its breath for two hours. This is possible, in part, because these seals slow down non-essential metabolic activities like digestion while staying underwater for long periods. They can also slow their heart rate down to as few as three or four beats per minute. Additionally, seals are expert gliders, meaning they don’t have to exert much effort to move through the water.
And then there are those impressive humans who challenge their maladaptablity by practicing free diving. Other than weights to get to the bottom and some type of inflatable device to get back to the surface, a freediver dives without equipment. The record depth for freediving is over 800 feet. The many injuries and fatalities experienced by free divers is a testament to the dangers of this practice. For now, I think I’ll stick to SCUBA diving, appreciative for its ability to augment the capabilities of my human body well enough to allow me to stay underwater and experience the ocean in a way that, though encumbered by equipment, still feels like a close encounter.
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