At high altitudes, the atmospheric pressure is much less than in the plains. As a result, the amount of oxygen per unit volume of air is lower and less oxygen reaches the lung per breath which is not sufficient to fulfill the body requirements. Thus, to get in more oxygen, our lungs have to make extra effort which causes breathlessness. Those who habitually live on the mountains, however, do not feel breathlessness because they have more hemoglobin in the blood to carry more oxygen.
