Dense air - breathe less?

[tomcat]

Hey docs,

Here's what I'm thinking:

As we go deeper, water pressure increases and the air going into our lungs becomes denser.

1. o2 partial pressure goes up, implying that there are more oxygen molecules per volume

2. assuming that the body needs the same amount of oxygen (molecule wise) to function properly...

3. we can breathe more slowly/take smaller breaths because the smaller volume of air going into our lungs is compensated by the fact that there are more oxygen molecules in there.

postulation: o2, co2 exchange takes place as per normal and we use up less air per unit of time.

before i go try this, can anyone tell me if i'm anywhere near correct?

tomcat

 

[dmentia]

...if what you say makes sense- although from a physics standpoint it sounds reasonable. I can tell you that on my dives (as a newbie- if that makes a difference) I used air faster the deeper I went.
dmentia

 

[scubadoc]

Hello Tomcat:

You're correct about density. According to one part of Boyle's law, increase the pressure of a fixed volume of gas and the density increases, and vice versa. This part of the law becomes particularly important on deep dives; it predicts that the inhaled air will become denser the deeper one goes. As a result of increasing air density, deep divers often notice greater difficulty breathing.(due to the density).

However, there are some other assumptions in your letter that could be dangerous.

Even though air is compressed in a scuba tank, the percentage of the individual gas components is the same. Any gas taken to depth in a scuba tank will be unaffected as long as it remains in the tank. Once it leaves the tank and enters the diver's lungs it will have the same pressure as the surrounding water, i.e., the ambient pressure. Given this fact and Dalton's law, we see that as air pressure increases or decreases, the partial pressure of each gas will do the same. With increasing depth, the partial pressure exerted by each gas in the air we breathe will increase. This explains DCS, nitrogen narcosis and oxygen toxicity. However, O2 uptake by the hemoglobin molecule is not a function of partial pressure - but of percentage of O2 which remains the same at depth.

Air is used much faster at depth - again due to the changes that take place from the increased pressure. (Boyle's Law)
Decreasing the depth and frequency of breathing in order to conserve compressed air will result in two things: CO2 retention, hypoxia (decreased oxygen concentration), both of which are very dangerous.

 

[tomcat]

Hey Scubadoc,

What you say makes sense. What I don't quite understand is that since ambient pressure is now increased (with respect to at surface), doesn't that mean that more oxygen would have entered the bloodstream (along with nitrogen and other gases) for a smaller volume of gas? The converse of this situation is breathing low pressure air (e.g. at altitude). We need to breathe more and harder because the amount of oxygen in the air is less, even though partial pressures remain the same. So why is it that we can't breathe less when it is high pressure air with more moles of oxygen per volume)

Even if there is a limit of takeup of o2 by haemoglobin, isn't it the case that less volume of gas is now needed to reach those limits (implying having to breathe less)?

Thanks for the headsup Scubadoc!

tomcat

 

[ScubaRaf]

Hi ScubaDoc,

I believe your explanation is 90% correct, but there'e one part I don't agree with. Maybe you can correct me if I am wrong.
The amount of oxygen your hemoglobin will load is a function of the partial pressure of oxygen in the alveoli. It doesn't matter what percentage of the total gas in your lung is oxygen. So at 99 feet, you may still be breathing 21% oxygen, but its partial pressure is MUCH greater. However, since hemoglobin is almost completely filled up by air delivered at one atmosphere (sea level), you don't get much extra loading at 4 atmospheres (99 feet). You do get a bit more dissolved oxygen in your blood, but this is so little to begin with, it doesn't help much either.

So, back to the original question. If you have more oxygen molecules available at the alveolus and you're not consuming any more of them than usual, why can't you hold your breath much longer at depth than you can at the surface? In essence why can't you take fewer breaths and have a really long bottom time?

The answer has little to do with oxygen! The main ventilatory drive we have is CO2 in the blood. Let's say you can hold your breath for 90 seconds at the surface. While your holding your breath, CO2 is being generated by cellular respiration and building up in your body and bloodstream. There more it rises, the more you need feel like you have to take a breath. Your body says, "Breathe! We have to get rid of all of this CO2!"
Now it doesn't matter whether you're at sea level, 99 feet under water, or on top of a 10,000 foot mountain. As long as you're at rest in these places you'll generate the same amount of CO2 in 90 seconds. So why do you have to breath more often at higher elevations? This is because there is not enough oxygen there and your body can sense this. (Here you may still have 21% oxygen, but your hemoglobin is lower than at sea level) So even before CO2 buildup becomes a problem, your body says, "Breathe! We need more oxygen!" This is the hypoxic ventilatory drive which does not play a significant role at sea level (unless you have pulmonary disease) or under water where oxygen partial pressures are relatively high.
When you're 99 feet down, you'll run into the CO2 limit just like you do on the surface so you'll have to breath at the same rate. You'll just be exhaling more air than you normally would, essentially "wasting" it just to get the CO2 out. That's why rebreathers are such an awesome concept. They'll capture the "wasted" air, take out the CO2, add back the little bit of oxygen you did take out of the air and give it back to you, good as new.

Ok way too long, but I hope it answers your question.

Rafael

 

[turnerjd]

Originally posted by ScubaRaf

I believe your explanation is 90% correct, but there'e one part I don't agree with. Maybe you can correct me if I am wrong.
<snip>
Ok way too long, but I hope it answers your question.

Rafael

ScubaRaf,

Good response, I also thought that for once Scubadoc had missed the mark a little, and you having replied, saved me having to type in something similar.

The response wasn't too long at all!! it gave all the necessary info in a way that everyone could understand. Keep at it!

Jon T

PS I noticed this was you first post - welcome!

 

[turnerjd]

Originally posted by scubadoc
However, O2 uptake by the hemoglobin molecule is not a function of partial pressure - but of percentage of O2 which remains the same at depth.

At the risk of confusing myself and others.....

O2 binding to haemoglobin will be dependant upon a Michaelis-Menten affinity type constant (something similar to Km) and there will also be a dependence upon concentration, which for gasses is expressed as partial pressure.

However, at normal pressures (and certainly when diving) this will not be rate limmiting, and will have no abvious effect. (ie if the partial pressure is greater than XBar, then we are OK, but at X*10bar, there is no difference because some other part of the process is slowing it down - think of it like a production line - there is always one part that is slowest, even if what *you* do can be done 10 times faster, the line runs at that particular spead because someone elses job can't be done faster) At altitude, it is different, because now it *is* the rate limmiting step (now you are the slowest on the production line, and holding everyone up), so to get the same level of O2 into ourselves we have to breath harder and faster.

Hope this helps.

Jon T

 

[scubadoc]

Of course, partial pressures have everything to do with controlling the uptake of O2 in the hemoglobin molecule and thanks for jerking me back to Physiology 101! My sentence was transposed!. I was probably thinking in terms of oxygen content as opposed to oxygen pressure.

Oxygen is a gas and its molecules do exert a pressure but, like any other substance, oxygen also has a finite content in the blood, in units of ml O2/dl blood.

Partial pressures are important because they determine the rate of diffusion of a gas, and therefore strongly affect the rate of gas exchange between the blood and alveolar air. The greater the partial pressure of oxygen in the alveolar air, the more oxygen dissolves in the blood (a restatement of Henry’s Law). (Thus the basis for hyperbaric oxygenation for acute blood loss anemia).

At the alveolus, the blood is said to unload carbon dioxide and load oxygen. The efficiency of both processes depends on the length of stay of an erythrocyte in an alveolar capillary, and how long it takes for oxygen and carbon dioxide to reach equilibrium in the capillary blood. When oxyhemoglobin in the blood reaches an area in the tissues with a much lower partial pressure of oxygen (in metabolically active tissues), the oxyhemoglobin unloads its oxygen, which then diffuses into the tissues.

The oxygen carrying capacity of one gram of hemoglobin is 1.34 ml and in the normal individual this is ordinarily reached at sea level partial pressures. Higher partial pressures of oxygen will generally saturate the non-cellular portions of the blood (serum,plasma). PaO2 is a measurement of pressure exerted by uncombined oxygen molecules dissolved in plasma; once oxygen molecules chemically bind to hemoglobin they no longer exert any pressure.

Tissues have to have a certain amount of oxygen per minute in order to live, a need met by oxygen content, not oxygen pressure. (Patients can and do live with very low PaO2 values, as long as their oxygen content and cardiac output are adequate.)
Here is the equation for oxygen content:
CaO2 = (SaO2 x Hb x 1.34) + .003(PaO2)

Many thanks to turnerjd and ScubaRaf for picking up on my error!
(The last time I was wrong was when I thought I was wrong!)

 

[keralucu]

after reading all that. It's fascinating and I think I'll have to read it again about 10 times before it sinks in.

I did try breathing more shallow at depth and found that I felt like i wasn't getting enough O2. I am normally a deep breather so wasn't sure if it was just my subconscious telling me I wasn't getting enough air or if I really needed it. Anyway, from now on I will just breathe as normally as possible...

Back to re-reading these posts!

 

[tomcat]

keralucu,

i know how u feel. i've reread the posts abt 20 times now and i think what the docs are trying to say is that when when pressure is higher than at surface, co2 is the driver for how often we need to breathe. when pressure is lower, o2 tends to be the driver for breathing rates.

in short, it doesn't matter how dense the gas gets, we need about the same volume of air in our lungs for co2 to be expelled because of various gas laws and equilibria of pressure of gases between our blood stream and our lungs.

docs, pls correct me if the summary is wrong.

tomcat