Breathing is the most natural thing in the world, right? Day in and day out. You scarcely notice. Breath in, breathe out. Can anything be more simple and routine?
It requires more knowledge because it's not the same. For over a century we have known that you can't just provide air to divers and expect them to be safe. You need more to guarantee safe diving every time you go down.
Okay, now the fun part. Let's take a quick peek at some basic physics to understand what's going on. Pretend there is a person in front of you and that person gives you a little shove in the chest. You'd fall backward, right? However, if another person were to stand in back of you and push forward at the same time, you would remain still and not fall backwards. This is because the person behind you counteracted the force of the person in front of you.
The result is that you get a bit squeezed by the two opposing forces. When you are under water the same thing happens, with one big difference. The difference is that the force is coming from all sides at once, not just front and back. Another big difference is that the force is coming from within your body. Fortunately, there is 'oppositely directed force' provided by the strength and stiffness of your tendons, muscles and rib cage. Without this force, you'd collapse due to the pressure. How much pressure you ask?
For every 33 feet (10 m) you dive down the pressure goes up by 1 atm (one atmosphere). That's about 14.7 pounds per square inch all over your body surface mass. To balance that pressure, your body pushes outward. Most people don't know that your muscles and ribs are a little flexible. In order to gain enough rigidity to counter-balance the pressure, your ribs collapse inwardly just a little bit.
This collapsing compresses your lung a little bit. Breathing becomes harder at deeper depths because it's harder for your lungs to counteract the additional pressure. The fact that air is easy to compress just adds to the difficulty. Water, for example, takes a lot of force to compress.
The air in your lungs compresses to a degree, as a result. To breath properly your lungs have to expand against the collapsing we discussed earlier. They also have to deal with the compression of the air and the extra air needed to provide the body with enough oxygen.
It's just these issues that modern-day diving regulators and tanks are built to handle. When diving down deeper in the water, the regulator and tank system combines to provide air that is at the ambient pressure of the ocean water. Using modern tanks and regulators, divers can go down to moderate depths and still breath easily.
Once you get into really deep water, things change a bit. The topic of really deep water diving is beyond the scope of this article. Conserving precious oxygen is one of the ways professional divers help their equipment help them. When you are underwater, you're moving in a dense fluid. This slightly increases the amount of oxygen you need.
Professional and or highly skilled scuba divers swim and move in a slow and apparently leisurely manner. They do this to conserve energy and oxygen.
Less breathing conserves oxygen. Another way professionals conserve oxygen is by keeping a cool head. When scuba diving it's not hard to get excited about all of the amazing things you are seeing. A key challenge is to avoid getting any more stressed than necessary when trouble strikes. For example when visibility is low and you get stuck on some underwater debris. If you stay calm your heart rate does not go up nor does your need for oxygen. Stay calm, breathe right, dive safely and live to see another day.