At high altitude each breath brings in less oxygen, and during exercise the muscles use much more. In both cases the lungs, heart and blood adjust to match oxygen delivery to need.
Thin air
The air at altitude still contains 21 percent oxygen, but the total air pressure is lower, so less oxygen enters the blood. At about 3000 m, SpO2 in a healthy person often falls to around 90 percent.
Acclimatization
Low oxygen makes you breathe faster and deeper within minutes, and the heart rate rises. Over days the kidneys excrete bicarbonate to correct the resulting alkalosis, and over weeks the body makes more red blood cells.
Altitude illness
Acute mountain sickness causes headache, nausea and poor sleep. High altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE) are life threatening, and the key treatment is to descend and give oxygen.
Exercise
During hard exercise, ventilation can rise more than tenfold, first through deeper breaths and then faster ones. In healthy people the lungs are rarely the limit; the heart's output and the muscles' use of oxygen usually are.
At the bedside
Interpret SpO2 in light of altitude, since a reading in the low 90s can be normal in a mountain town. In patients with lung or heart disease, ask about symptoms with exertion and travel, including flights, because problems often appear first there.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).