Environments in higher altitudes (HA) are exposed to less oxygen. This is because gravity pulls oxygen downward.
Regarding the respiratory system, upon initial exposure to HA, the resting pulse increases greatly, and the stroke volume per pulse of the heart is decreased. When sleeping, the amount of CO2 in the blood drops significantly. This switches off the drive for the individual to breath. The individual takes the next breath only when he/she also senses a drop in blood O2.
Those who are used to living in lower altitudes will initially experience hypoxia (a deficiency in the amount of oxygen reaching the tissue). During this initial exposure to high altitude (HA), the lowlanders will experience a decrease in physical activity and experience a period of confusion.
In response to this state of hypoxia, the body will attempt to overcompensate by undergoing several physiological alterations. People may experience a respiratory state of hyperventilation that can lead to respiratory alkalosis. Heart rate, blood pressure, and metabolic changes are also common. Collectively, these are known as acclimatization.
After about 3 days (shorter if induced by drugs), the body begins to adapt to the higher elevation. The body compensates for the respiratory alkalosis by renal excretion of bicarbonate, allowing adequate respiration to provide oxygen without risking alkalosis.
When you are at higher altitudes, there the air is much thinner, resulting in less oxygen. This makes it difficult to breathe and will cause shortness of breathe.