I just returned from a week in the mountains. Being from the nice flat Midwest, the 14,000+ foot elevations had me gasping and dizzy. By the end of the week, there was an obvious difference. While I wasn’t ready to run a marathon, I could move around much easier.
So what changed? I can understand long term differences from living at altitudes (larger lung capacity, stronger heart kind of things), but what about the short term? It doesn’t seem that my lung capacity could have changed that much in a week, or that I suddenly needed less O2.
Your body will try to increase the number of circulating red blood cells in a time like that. People can also ‘get used’ to having low oxygen, for example some people have chronic anemias that, if you were to knock a health person’s hemoglobin level down to what they run at all the time, and knock it to that level in an instant, that formerly healthy person would be on the floor unconscious. I suspect that your body slows down certain aspects of its metabolism, but that’s a WAG.
You need to go up in small changes. For example people who climb Mt. Everest (29,000 feet) gradually go to higher and higher elevations to get their body ready for lower O2 levels.
If a plane dropped you off at 29k feet without getting used to that low oxygen you would die within an hour. Guys who climb that high take a month or so to get used to the O2 level. And even then they can’t really stay above 26k feet very long and hope to live. BTW, the base of Mt. Everest is 17k feet so to go any lower you have to head out of the area.
More efficient at transporting oxygen, true.
But as the percentage of other things, like white blood cells is lower, you could say that your blood is now less efficient at fighting infections.
“Efficient” is generally meaningful only if you define what input/output ration you are measuring.
Endurance athletes sometimes take advantage of this altitude effect. They’ll live and train at an elevated altitude and get their red blood cell count up. Then when they compete at low altitudes, their performance is boosted. To further enhance it, during this training they sometimes live in special houses which have lowered oxygen levels, effectively raising the altitude by a few more thousand feet.
The effect wears off after a while at lower altitudes, so they usually just do it right before a big event (e.g. the Olympics). Athletes that have to periodically compete (e.g. professional cyclists) don’t have the luxury of taking the time to do this, so they tend to use another method (i.e. EPO) to boost their red blood cell count. However, that method, unlike altitude training, is frowned on severely by the sports authorities.
The relative hypoxia of high altitude produces a number of responses:
Red-cell mass increases within a few days, since increased erythropoietin production is triggered almost immediately. More red cells can carry more oxygen. Even before absolute red cell mass increases, there is a relative loss of blood water (dehydration), resulting in hemoconcentration and increased oxygen-carrying capacity per unit of blood moved around.
Increased 2,3-diphosphoglycerate (2,3-DPG) production shifts the oxygenhemoglobin dissociation curve to the right. This is a fancy way of saying that hemoglobin will release oxygen at lower levels of tissue concentration. Remember hemoglobin has to bind oxygen in the lungs and cough it up in the tissues. This production change begins within hours.
Tachypnea (more rapid breathing) starts right away. This lowers the concentration of carbon dioxide in the blood, and that change triggers blood flow shifts in both the lungs and heart. Initially this hypocapnea can make you feel lousy (among other things it alters the pH of the blood) but within hours to days the pH is adjusted back toward normal and this can also help you feel better.
This is vastly oversimplified altitude physiology, but these are some of the changes that occur within hours to days.