Hypoxic Hypoxia

Hypoxic Hypoxia

Hypoxic hypoxia is the result of low oxygen levels available to the entire body. A blocked airway and drowning are  examples of how the lungs can be deprived of oxygen.

The reduction in partial pressure of oxygen at high altitude is an appropriate example for pilots.

Although the percentage of oxygen in the atmosphere is constant, its partial pressure decreases proportionately as atmospheric pressure decreases. 

As the airplane climbs during flight, the percentage of each gas in the atmosphere remains the same, but there are fewer molecules available. Why?

Daltons Law of Partial Pressure states that each gas exerts a unique pressure at STP. For oxygen, at STP, this is 20% of 760 mm Hg for an Atmosphere, or a partial pressure of 159 mm Hg. 

At 18,000 feet the atmospheric pressure is one half what it is at sea level. 360 mm Hg comes to mind. Take 20% of 360 and the drop in oxygen partial pressure goes to 72 mm Hg.

Your body needs approximately 100 mm Hg partial pressure at the alveolar sacs to replenish to adequate levels of oxygen in the blood. Why the drop from 159 mm Hg to 100 mm Hg at sea level.

Your respiratory system is like a tube that extends from your nose to the surface where a "gas exchange" can take place. In the non-exchange areas (like the trachea) the oxygen from the atmosphere mixes with the carbon dioxide and water vapor present in the "dead" air. This reduces the partial pressure of oxygen to 100 mm Hg. (Ain't this fun!!)

When the air mixes with carbon dioxide and water vapor, in the physical respiratory system track, before it gets to the alveoli, the partial pressure of oxygen is too low to keep you awake for flying. You will pass out in approximately thirty minutes without supplemental oxygen.

The decrease in the partial pressure of oxygen prevents an adequate number of oxygen molecules reaching the alveoli.

This is hypoxic hypoxia.