Altitude Sickness, Part 2
Climbing Higher
As we climb higher in altitude, there is a corresponding decrease in atmospheric pressure. Pressure arises from never-ending collisions of the molecules that make up the atmosphere, mostly N2 and O2. Thus, a decrease in atmospheric pressure results from a smaller number of molecules per unit volume of air. A lungful of air at 10,000’ has about 70% as many molecules as a lungful at sea level. We get fewer oxygen molecules per breath as we move higher in elevation. We compensate for the loss of oxygen by breathing faster, for both inhalation and exhalation, and at modest altitudes, it is the excess exhalation that starts the process of altitude sickness.
Refer back to the blood equilibrium equation from Part 1:
H2O + CO2 ⇄ H2CO3 ⇄ H3O+ + HCO3–
Remember that this reaction never stops. It runs in both directions at the same time and at the same rate, so that the concentrations of the five chemical species remain in a stable state of equilibrium. It is a natural phenomenon of chemical reactions, that if you add one of the molecules of the reaction to the mix, the whole thing shifts away from the added molecule until the system regains its stable equilibrium. Likewise, if any of the molecules is removed from the reaction, the whole thing shifts toward the removed molecules until the system regains its stable equilibrium. We breathe in faster to collect all the oxygen molecules we need, but at the same time, we also breathe out faster, and this causes excess loss of carbon dioxide. In a sense, fast breathing "sweeps" carbon dioxide out of the body. Notice how this works with the equilibrium reaction. As we remove carbon dioxide by rapid breathing, the reaction shifts to the left, trying to regain the stable state of equilibrium. As a consequence of the shift, the amounts of H2CO3, H3O+, HCO3– all decrease proportionally. The critical part of this is that the system is losing its acidic molecules. This causes a rise in the pH of the blood. (The pH scale is actually a logarithmic function of the concentration of H3O+.) Blood will not tolerate pH values much higher than 7.40 before alkalosis sets in. The condition of diminished carbon dioxide in the blood is known as hypocapnia. The body enters a state of alkalosis, initiating altitude sickness
There are three types of altitude sickness. Acute Mountain Sickness (AMS), High Altitude Pulmonary Edema (HAPE) and High Altitude Cerebral Edema (HACE). AMS is a mild condition, common among hikers above 10,000', and sometimes as low as 8,000'. The symptoms are familiar to all of us: Headache, fatigue, loss of apatite and poor sleep. I have noticed an additional symptom, probably more associated with being hypoxic rather than in a condition of AMS. I camped in Granite Park one evening, and after stopping for the day, I was actually giddy. I felt like I'd had several drinks. This lasted for an hour or so before the headache set in, leading to a poor night of sleep. That was the only time I've experienced the feeling. I think of myself as being not particularly susceptible to altitude sickness, but that's not true. Although I usually don't get headaches if I stay under 13,000', I generally sleep poorly the first night of a trip, sometimes the first two nights, and my apatite is definitely suppressed the first few days. I noticed doing the JMT that I slept very well after the first few days, and there was no missing apatite whatsoever during the second half of the trip.
I want to limit these pieces to smaller bites of information, so I'll stop here for now. I can see two more parts coming. Part 3 will cover HAPE and HACE (briefly, since I'm not a physiologist) and also a bit about acclimatization (erythropoietin and hemoglobin). Part 4 will finally get to Diamox, aka acetazolamide.
