Altitude Hypoxia - What is Altitude?

The Earth’s atmosphere is composed primarily of nitrogen (78.08%) and oxygen (20.95%) with small amounts of other gases. The proportion of these gases does not change with altitude but the total amount of air decreases. As one ascends in altitude and moves further and further into the atmosphere, there is a smaller column of air pressing down upon him/her. This is what is meant by a decrease in barometric pressure. Because the proportion of oxygen does not change but the total amount of air decreases, there is less oxygen available with increasing altitude.

To understand the impact of altitude on breathing, it is important to examine how the lungs work. Air always moves from areas of high pressure to low pressure. In order to breathe in, one must decrease the pressure inside his/her lungs so that air can move from the higher pressures of the atmosphere into the lower pressures of the lungs. This pressure difference,which is relatively small even at sea level, is generated by increasing the lung’s volume. As barometric pressure decreases, the pressure gradient driving air into the lungs shrinks. To get the same amount of air as at sea level, one has to increase the rate and depth of breathing. Because the air is thinner at altitude (decreased barometric pressure) but the proportion of oxygen remains the same, less oxygen enters the blood after a breath at altitude compared to sea level. This is the root cause of all altitude related problems.

Click here for an explanation of hypoxia-related symptoms and treatments.