Why do scuba divers exhale slowly as they ascend to surface?

Answer 1

The First Stage ..( the bit on the tank) of a diving regulator senses the pressure exerted on the regulator ( and also on the body) to see how much it is being squeezed, it then compensates for the pressure by filling your lungs with more air than they could hold on the surface BUT because the lungs are being "squashed" the air fills your lungs like normal. The only potential problem comes when you ascend..because you have too much air in your lungs as it starts to expand, it could overinflate your lungs to the point they literally blow up!

Answer 2

A Scuba diver will want to descend as quickly as possible in order to get to their target depth having absorbed as little nitrogen as possible. However, the diver needs to continually equalize the pressure in his or her ears to prevent discomfort or damage to the eardrums. How easily the diver can equalize can determine how quickly they can continue to descend.

Aside from that, the fact that when descending the diver weighs only a little more than the water he or she displaces keeps the diver from falling like a rock to the bottom. Once the diver reaches the desired depth, they'll have added air to their buoyancy compensator to become neutrally buoyant -- to weigh exactly the same as the water they displace, and therefore hover in place.

Addition

There are pros and cons to descending quickly. In short, as long as a diver can equalize their ears as well as maintain buoyancy control on the way down, the descent rate does not matter. There is a theoretical physiological benefit (possibly small) to descending fast in that it could help crush some micro-nuclei - which can be thought of as seeding bubbles. So crushing some gives fewer locations for bubbles to form. While this would increase the nitrogen absorption (due to reaching the deepest depth faster), it is not that significant. On the other hand, descending quickly does seem to make divers more susceptible to narcoses at the deeper depths, especially if one does not routinely make deeper dives.