Therefore, if we increase the amount of clean air we move in and out of the alveoli, we will increase the amount of CO2 that is removed from our bodies and reduce the amount of CO2 in the blood.
The following concept is critical: Our brains use the acidity of our blood to control how often we breathe and how deeply we breathe, so that the pCO2 in our blood is maintained at a constant level. Breathing is work and requires a certain amount of effort. If the pCO2 in our blood rises, we breathe deeper and quicker which causes the pCO2 to be reduced. Conversely, if breathing becomes harder the pCO2 in our blood will rise until the stimulus to breathe equals the increased work of breathing.
Elevated CO2
There are a great number of ways in which the amount of CO2 in our bodies can be increased while we are diving. We often work quite hard while diving; this increases the amount of CO2 being produced by our cells. To eliminate this CO2 we have to breathe faster and deeper, but the stimulation for this increased work of breathing requires an elevated pCO2. Therefore, the pCO2 in our blood is always slightly increased when we are exercising because an increased pCO2 is required to drive the increased respiration. We can voluntarily breathe more or less than required to maintain a normal pCO2 (breath holding or hyperventilation) for short periods of time but in this discussion we will assume the diver is not consciously adjusting their breathing. In addition to exercise, there are other reasons our pCO2 can be elevated. When we are stressed, some people breath more rapidly, but less deeply (we move less gas in and out of our lungs with each breath). This can result in a dramatically elevated pCO2 in our bodies. When we exhale, approximately 250ml of gas is left in the alveoli, in the airways in our lungs (including the trachea), in our throat and our mouth. This volume is called the ‘dead space’. When we are breathing normally, we typically move about 500ml of gas per breath. Therefore, when we inhale the first thing that happens is that the 250ml of gas in the dead space moves back into the alveoli. This gas just came from the alveoli and therefore is already saturated with CO2. The next 250ml of gas
is ‘fresh’ and should contain very little CO2. As a result, in normal respiration only 50% of the gas moving past our lips is getting into the alveoli.
Now imagine what happens if we were to pant (breathe rapidly and shallow) and only move 250ml of gas with each breath. When we exhale, 250ml of gas would move from the alveoli into the dead space. When we inhale the same 250ml would move back into the alveoli. NO FRESH GAS would be entering the alveoli and NO CO2 WOULD BE LEAVING THE BODY (no oxygen would be getting in, either). As a result the level of CO2 in the body would rapidly rise. Try it! In less than a minute, you will be forced to take deeper breaths to eliminate the CO2 that has built up in your body. This leads to the first critical response to elevated CO2. We should take slow deep breaths to increase the amount of CO2 being eliminating. Stress/panic often results in shallow breathing and this is one reason divers (and others) are advised to take slow deep breaths when they are feeling anxious.
breathe and how deeply we breathe
Our brains use the acidity of our blood to control how often we
CO2 Build-Up and Diving Now, let’s look at mechanics of the dead space and diving. The gas volume in a standard scuba regulator second stage is dead space (the gas we exhale into it is breathed back into our lungs on the next breath). This volume must be added to the physiologic dead space in our mouth, throat and lungs.
Full face masks are becoming more common in diving but they too can have significant CO2 retention problems due to dead space. Most full face masks have a special section that seals around the mouth to reduce the dead space but if this seal is not working properly, the entire volume of the mask can become dead space and CO2 is likely to accumulate in the body. As mentioned earlier, if the work of breathing increases, the level of pCO2 in our blood will increase. When we are diving, there are MANY things that make the work of breathing harder. All regulators increase the work of breathing because we have to
‘activate’ the regulator. In addition, all regulators provide some obstruction to the flow of gas. A ‘good’ regulator will have a lower work of breathing than a ‘poor’ regulator.
Breathing at Depth
This problem gets even worse when we dive deeper. As we descend, the gas we are breathing is compressed and increases in density. At 100 feet (30m) there are four times as many molecules in every breath we take and the gas is four times denser compared to the surface. This increased density can dramatically increase the work of breathing. This is why a hard-breathing regulator can be acceptable on a shallow dive but almost unusable on a deep dive. I have several old regulators that I use on decompression bottles. On one dive, I mistakenly used one on a bottle that I was going to be breathing off at depth. When I switched to that regulator at depth, it was like trying to suck soup through a small straw!
This problem is so serious that on deep dives the work of breathing actually limits the amount of work the diver can do, and the work of breathing itself becomes a significant source of CO2! Helium is a much smaller and lighter atom than nitrogen and therefore the work of breathing is reduced when nitrogen is replaced with helium.
When we are diving, we often wear equipment that increases the work of breathing. All wetsuits and most drysuits make it more difficult to contract our diaphragm (pushing our stomach out) and expand our ribcage so that we can suck gas into our lungs. If the wetsuit or drysuit is too small, the problem is much worse. Furthermore, the weight belt, harness, buoyancy compensator, cummerbund, etc. can all restrict the movement of our stomach and chest thus making breathing even more difficult. As a result of these many factors, every diver on every dive will have an elevated pCO2 in the blood during the dive. Normally, this small elevation of the pCO2 does not cause the diver any difficulties. However, if anything causes the pCO2 to rise even further, the diver can quickly experience CO2 toxicity and that can rapidly become fatal.
In the next column I will review the signs and symptoms of CO2 toxicity and look at how the problem of CO2 can be much worse when diving a rebreather.
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