Breathing underwater while scuba diving is not just about inhaling and exhaling. This is true of breathing on ground but when you get underwater, even if you have an air tank on you, you will need to know a lot more about breathing than simply how to inhale or exhale.
Over a hundred and fifty years of studying scuba diving experience has shown that healthy and safe dive takes more than just giving the diver enough air to breathe. Let us try and use some elementary physics to understand why this is true.
Okay, here's a visualization exercise. Try and imagine someone standing in front of you and trying to push you. What happens to you? You will tip and fall backward. Now what if there were two people - one pushing you from the front and one from the back. You will probably remain standing if the two forces are equal but you may get squeezed by them.
This is what happens when you are in the water, except that the "push force" comes from all sides not just two sides. So how does your body cope with all this pressure? Well, it's your rib cage, and surrounding muscles and tendons that provide the rigidity to withstand this enormous pressure, otherwise your body would simply collapse.
Now you will ask, how much pressure can water exert? The answer is bound to surprise you. Every 10 m (33 feet) deep that you dive will increase the pressure around you by 6.7 kg per square inch. This means about one atmosphere or 14.7 lbs per square inch of increased pressure every 10 m!!
To withstand such great pressure, you body should ideally be pushing back. However, the flexibility of the ribs and muscles allows them to collapse slightly before becoming rigid in order to balance the outside pressure. This causes your lungs to compress slightly when you are under water. The deeper you go, the greater the outside pressure and thereby the more difficult it is for your lungs to expand.
There is another factor that is at work. While water needs considerable force to be compressed, air is relatively easier to compress. Therefore, the pressure of water outside compresses the air in your lungs to a certain degree. In order to supply your body with enough oxygen, you have to expand your lungs despite the slight collapse and the air compression.
Modern equipment like diving regulators and tanks are designed in a way that the air is delivered at the pressure levels of the surrounding water to help the scuba diver cope with these underwater issues. This allows scuba divers to breathe easily up to moderate depths but not great depths.
The oxygen requirement underwater is higher owing to the high density of the surrounding water. Skilled scuba divers learn ways to work with the equipment and swim slowly in order to conserve oxygen.
Scuba divers also make sure they remain calm underwater, so that their heart rate is low and their oxygen use is moderate. Excitement due to fascinating underwater sights and stress caused by possible dangers can all affect the need for oxygen.
So, learn to breathe properly and have a safe dive!