Effect of Altitude on Alveolar Oxygen Pressure

location

sea level

Isabella, MN

Denver, CO

Pike's Peak, CO

fraction of inspired oxygen O2

0.21

legend

alveolus

alveolar membrane

blood vessel

This Demonstration shows the effects of changes in altitude on alveolar oxygen pressure using the alveolar gas equation:

PaO

2

=

FiO

2

×(

P

ATM

-

pH

2

O)-1.2

PaCO

2

, where

Pa

is the alveolar partial pressure in mmHg,

FiO

2

is the fraction of inspired oxygen, and

pH

2

O

is the vapor pressure of water at 37 °C, 47 mmHg.

As altitude increases, the atmospheric pressure decreases. This causes a decrease in partial pressure of oxygen in the arteries. The slider for

FiO

2

changes the percentage of oxygen in the air from 21% (the normal fraction) to 50% (the maximum amount that avoids lung toxicity). For simplicity,

PaCO

2

, the partial pressure of carbon dioxide, is set to 35 mmHg as a reasonable average value.

The changes in partial pressure of oxygen are shown numerically and graphically. The lines on the graph represent the altitudes at four different locations and the corresponding changes in

PaO

2

as

FiO

2

is increased. The visual model shows an increase in oxygen molecules as

PaO

2

is increased, as well as a color gradient in the blood cells to represent the level of oxygenation in the blood.