To compensate, your breathing rate must increase a great deal, even at rest.
Altitude sickness: The primary concern of mountaineers. As altitude increases, the number of oxygen molecules per breath is reduced. At 12,000ft (3,658m), there are 40 per cent fewer per breath. To compensate, your breathing rate must increase a great deal, even at rest. The body can also overcompensate by allowing blood vessels to leak in the brain or lungs.