On SCUBA, why does your air consuption rate increase with depth?

Question

Obviously, the body needs a certain amount of oxygen to function. When mountain climbers ascend a mountain, they can have a real tough time with the lack of oxygen. At sea level, we get plenty of oxygen. Why, when we descend into the depths, do we need still more exygen? We aren't burning more calories. I read something once about the breathing reflex being triggered by CO2 buildup rather than lack of oxygen. Does this play into this?

I know each breath at depth holds more O2 molecules because of the pressure compressing the gas. But why does our breathing rate not drop proportionately with the increaase in O2 molecules?

Answer 1

The oxygen consumption rate does not change as one dives deeper. However, the pressure exerted by the water above on a persons lungs requires that the air be forced in by a higher pressure. That is why the air consumption rate increases, because the regulator allows more air to enter the lungs at a deeper depth.

Sometime, try this experiment. Weight yourself down by the ankles, standing on the bottom, in the deep end of a pool so that your head is barely underwater. Then try to draw a breath of air through a tube that is long enough to reach to the surface. If you manage to do it at all, you will find it extremely difficult, and you will not be able to sustain the effort for long. That is less than a few feet of water putting that kind of pressure on you.

At 33 feet (app 10 meters) under water, the pressure on your body is twice what it would be on the surface. Therefore, to keep your lungs expanded at that depth with a full breath, you would need twice as much air in them. However, this exttra air fills up the same amount of space. That is because the air is compressed. This air is pressurized (The air molecules are forced closer together. If you dive down to 20 meters, you will use up the air in your tank about 3 times as fast as you would on or near the surface (Twice as fast at 10 meters, and another time as fast at 20.)

If you held a full breath as you rose to the surface from 20 meters down, your lungs would pop like balloons as the air inside of them continued to expand due to the lessening of the water pressure around you.

The air inside of the scuba tank would have the same amount of oxygen as the air you breath right now, app 21%. The rest is mostly nitrogen. If you dive for a very long time, an excess amount of nitrogen can dissolve in your blood. You have nitrogen diffused in your blood right now. All gases can diffuse into a liquid with which they are in contact, and the rate varies according to the pressure of that air. At 10 meters, twice as much nitrogen will diffuse into your blood, because the pressure coming from the tank is twice as great. As you come to the surface, this nitrogen can form bubbles. This is known as the "Bends." You can get the bends if you make several fairly deep dives (Say, 60 or 80 feet) over a period of several days, and then surface to quickly on your final dive. Divers use specific equations to calculate how much nitrogen has been diffused into their blood, based on the depths and the amount of times they dove, and the amount of time between these dives. Some divers have special computers attached to their scuba apparatus to do all of this math for them. Based on these calculations, divers will know at what depths they should pause and for how long before they continue to surface. They must also take care if they are going to be flying or driving through mountains, because on a plane or in the mountains, the air pressure is less than on the surface, and they could still get the bends. These brief pauses during ascent are called "safety stops."

For really deep dives, the air pressure is so high that nitrogen will diffuse into the blood in such a great amount that even safety stops won't be able to help you. In these deep dives, an oxygen/helium mixture is used. Helium does not diffuse into the bloodstream as rapidly as nitrogen, and it is inert, so it can't poison you, like nitrogen can. Pure oxygen is not used because the diffusion problems would be similar to using a normal air mixture. Also, oxygen at too high of a concentration is also poisonous. Nitrogen poisoning is similar to being intoxicated at its least lethal stage. This may not seem like a big deal, but intoxicated people often fail to recognize risks, take unnecessary chances, do stupid things, and make mistakes. This is a big problem when you are even 20 feet underwater. Drowning is a real hazard, and if something goes wrong, you want to be mentally at your very best if you hope to survive.

Mountain climbers have the opposite problem as divers. As one climbs up to higher elevations, the oxygen ratio in the air is not any different than at sea level. It is still at 21%. The problem is that the air is less dense. So each breath is drawing less air molecules, and less oxygen, because of the lower volume of air available. Over time, the body will manufacture more red blood cells and hemoglobin to counteract the thinner air, and permit the most oxygen molecules possible to be trapped. Although oxygen dissolves in blood as well, most of the oxygen we use is captured and transported by the hemoglobin in red blood cells. In a normal person, the hemoglobin is 95% or more saturated with oxygen from the lungs. If it weren't for the hemoglobin in your blood, you would die almost immediately, even if you breath pure oxygen, because the amount of the gas that diffuses into your blood is minimal. Even during strenuous excercise, you still have an adequate amount of oxygen. The reason you breath heavier during excercise is due to the rise in CO2.

You are mostly correct about the breathing reflex being triggered by rising levels of CO2. In normal humans, there are sensors in the Coratid Arteries that detect the levels of CO2 indirectly by sensing the change in PH. This stimulates centers in the brain stem to increase the rate and depth of breathing. I say you are only partially correct because people with emphysema and similar chronic lung diseases always have a higher than normal CO2 level. This basically desensitizes the sensors in the Carotids, and then the brain senses the oxygen levels to determine an appropriate pace of breathing. This is why doctors must be very careful to monitor emphysema patients that are being treated with oxygen therapy. If you put an oxygen mask on an emphysema patient with too high of an oxygen concentration, their brain thinks that they have plenty of oxygen, so it stops telling the lungs to breath, which causes the CO2 levels to rise. This can be fatal if not quickly corrected. However, the fluctuating CO2 levels do not effect the rate that one uses up a tank of air while scuba diving. This is caused only by the difference in pressures at lower depths.
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Edited 03/08/07
The breathing rate does not drop due to the greater number of O2 molecules because our breathing is not regulated by the levels of O2. It is regulated by the amount of CO2, unless one has chronic lung disease, and you won't see any of them diving.

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Answer 2

One doesn't NEED more O2 at depth, but it's there anyway. Each breath at, say 10m, depth requires twice the absolute pressure compared to STP. So, each breath will require twice the # of molecules to fill the lungs. The tank is therefore emptied twice as fast.
Using air for breathing means that at some depth (~50m) the partial pressure of the O2 will equal 1 atmosphere. Anythng above that will result in toxic effects. Ergo, for diving below these depths, special mixtures of gasses are used such as 5% O2 + 95%He.

Answer 3

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