A low O2 environment forces the body to try to adjust. Here's what happens when you head for the hills.
At the first hint of hypoxia, or lack of oxygen, oxygen-sensing nerves in the neck tell the body to breathe faster and deeper, allowing increased oxygen intake. Hyperventilation means you're off-gassing carbon dioxide more quickly, too. The lower level of CO2 can lead to a feeling of lightheadedness.
An increased heart rate, which you may be able to feel for the first day or two at altitude, speeds oxygenated blood to the tissues. People who are born and raised at altitude often have larger hearts with more blood vessels.
Many people who venture into the thin air complain about a dry, sometimes debilitating cough. The hacking may be caused by cold, dry air, but studies also have shown that receptors in the airways that incite cough may simply be more sensitive at altitude.
Located where the carotid arteries split into the internal and external carotid arteries, these nerve bundles sense low oxygen in the blood and cause increased respiration. People who have had a carotid endarterectomy, a surgical procedure that removes plaque from the lining of the carotid artery, may have a decreased ability to acclimatize because of damage to the carotid bodies.
Blood picks up oxygen as it flows through the lungs. In situations where a part of the lung isn't well-oxygenated (say, pneumonia), blood vessels in the less-oxygenated part of the lung constrict to allow more blood flow to areas where oxygen is available. At altitude, all areas of the lung can have reduced oxygenation; all of the blood vessels can tighten, which results in high blood pressure in the lung. That, in turn, may cause high altitude pulmonary edema (HAPE), a life-threatening condition in which fluid fills up air pockets in the lungs and prevents oxygen from getting into the blood. HAPE occurs in about one in every 10,000 Colorado skiers.
During hypoxia the kidneys release erythropoietin (EPO), a protein that stimulates red blood cell production. Also, typically noticeable on the second day at altitude, the kidneys react to lower CO2 levels—and the resulting alkaline blood—by increasing urination. (Which is why you always feel like you have to pee just as you get on the chairlift.)
EPO produced in the kidneys tells the bone marrow to up the production of oxygen-carrying red blood cells.
Increased red blood cells deliver more oxygen to the tissues.
Some altitude researchers believe that mild swelling in the brain may be a root cause of acute mountain sickness (AMS, see page 97). One percent of people that ascend above 13,000 feet will experience a build-up of fluid on the brain (high altitude cerebral edema, or HACE); this condition is life-threatening and requires immediate descent to low altitude. Traveling to extreme altitude, like climbing Mt. Everest, can cause temporary decline in cognitive and speaking abilities.
Muscles use oxygen and sugars to create energy. In situations where oxygen isn't readily available, energy is created using a process called anaerobic metabolism. The by-product of anaerobic metabolism is lactic acid, a substance that produces that burning sensation you feel in your thighs after only the second run of the day.
Increased stress hormones and lower oxygen levels in body tissues at altitude (starting around 14,000 feet) can delay wound healing. Even the smallest cut can take a long time to heal, and can become infected more easily.