The science behind free diving

Thanks to Luke, who wanted to know about The Science of Scuba Diving and Free Diving - the role of nitrogen and oxygen, pressure, and how the make-up of the body's tissues govern these sports. I'm going to start with Free diving.

Some people like to participate in an activity called free diving. In this activity, a person holds his or her breath and dives beneath the surface of a body of water. People who dive for abalone or pearls, or who dive down while snorkeling may fall into this category. It sounds simple enough; a person takes a breath, dives beneath the surface to some distance, and resurfaces. The body, however, makes several adaptations when a person dives deep beneath the surface of water- these are called the mammalian diving responses.

To talk about this, we first need to have an understanding of pressure, and what air and water pressure can do to air inside a closed system (such as a person's lungs when they are holding their breath). The air above us is pressing down on us all the time. At sea level, about 15 pounds of air are pressing down on every square inch on the tops of our heads and shoulders. Most of the time, we hardly notice air pressure; however, if you've ever gone from low elevation to the mountains and opened a tube of sunscreen, you've seen air pressure in action. The container was closed at a low elevation (and higher air pressure)- this means that the air inside the tube is at this pressure when it is closed. Because the tube is closed, air can neither go in nor go out. When you get to the higher elevation, there is less air pressure because there is a smaller volume of air pushing down on the earth. When the high-pressure tube is opened at this lower air pressure, the air pushes out to equalize the pressure. As it escapes the tube, it often takes some of your sunblock with it. The opposite is also true. If you had an empty plastic water bottle with you in the mountains and took it to the seashore, it would most likely have collapsed on itself by the time you got there. Again, this is because there are a lot more air molecules pushing down on it. This phenomenon is known as Boyle's Law; it states that as pressure increases, volume will decrease (and vice-versa). The same is true for water pressure, but water is a liquid rather than a gas and is much heavier than air. For every 33 feet we go down in the water, another 15 pounds per square inch is pushing down on us. If we filled a balloon with air at the surface and took it 10 ft. below the surface of the water, the balloon would still have the same amount of air inside; its volume would be smaller because there were more molecules pushing on the outside of the balloon.

As a diver goes under the water, water starts pressing on his or her body. The good news is that because we are mostly made of water, water won't squish our bodies flat if we free dive deep into the ocean. While a diver might not notice the difference in pressure as they dive on their arm, there are other parts of the body where the difference in pressure is more noticeable. A free diver may feel pressure on on his or her eardrum. While this may be uncomfortable this pressure can often be released by pushing air from the lungs into the inner ear. (You can do this on dry land if you hold your nose and try to blow from it). The adaptations taking place do not stop here.

Because the diver's lungs are filled with air when he or she goes beneath the surface of the water, they are somewhat like balloons. As a person dives under the water, the air inside their lungs (at atmospheric pressure) is suddenly at lower pressure than the rest of the body. The rest of the body begins to push on the lungs, and they start to shrink. This has disaster written all over it- if the lungs collapsed in on each other, then some of the tissue lining the lungs might become damaged. This would close off part of the diver's lung, even after he got out of the water. Instead of the lungs collapsing, the capillaries (tiny blood vessels) in the lungs start to leak plasma (the fluid in blood) into the tissue lining the lungs. This causes the tissue to expand, filling the space in the lungs that used to be taken up with air. This prevents them from collapsing. Once a diver surfaces, the plasma goes back into the capillaries and they retain their full lung capacity.

Other adaptations include a slowing of the heart rate (called bradycardia) to use less oxygen and the spleen contracting (to push more blood cells around).

More on SCUBA soon:)