During the first week of adaptation, a variety of changes take place. Respiratory rate and depth increase in response to lower concentrations of oxygen in the blood, causing more carbon dioxide to be lost and more oxygen to be delivered to the alveoli. The increased respiratory rate begins within the first few hours of arriving at altitudes as low as 5,000 feet. The lost carbon dioxide causes the body to become more alkaline. This adaptation occurs within 24 to 48 hours after hyperventilation starts.
Cells require a constant supply of oxygen so the heart beats more quickly to meet the demand.
Except at extreme altitudes, heart rate returns to near normal after acclimatization. In the lungs, the pulmonary capillaries constrict, increasing resistance to flow through the lungs and raising pulmonary blood pressure. As acclimatization continues, the bone marrow contributes by increasing red blood cell production. New red blood cells become available in the blood within four to five days, increasing the blood's oxygen-carrying capacity. An acclimatized person may have 30 to 50 percent more red blood cells than his counterpart at sea level.