The Effects of Altitude on the Respiratory System

Definition:  High altitude causes problems in the respiratory system
because the amount of oxygen is low, causing hypoxic hypoxia.  

Importance and relevance:
	Barometric pressure decreases with altitude, but the fraction of
oxygen in the air remains the same.  There is much less oxygen available
to breath at higher altitudes, so the respiratory system must adjust by
acclimatization.  When the partial pressure of oxygen in the air drops
below 47 mmHg, the body actually receives no oxygen because it is
completely cancelled out by the vapor pressure of water.  
	Hyperventilation is a useful response to this.  Increasing
ventilation increases alveolar partial pressure of oxygen.  Hypoxic
stimulation of the peripheral chemoreceptors trigger hyperventilation.
This lowers the partial pressure of carbon dioxide and causes alkalosis,
but the cerebrospinal sluid counteracts this to raise the pH by removing
bicarbonate.  Ventilation then can increase further.
	Polycythemia, and increase in red blood cell concentration, is
another common reaction to altitude problems.  The increase in hemoglobin
results in an increase in oxygen carrying capacity, bringing the oxygen
concentration of arterial blood to about normal.  However, this also
increases the blood viscosity, which can be a problem if the person goes
down in altitude.
	Other reactions to highh altitude include a shift to the right of
the oxygen dissociation curve, which allows oxygen to be unloaded more
easily.  The number of capillaries per unit volume may increase to better
reach all tissues with enough oxygen.  The maximum breathing capacity
increases because the air is less dense.  Also, pulmonary vasoconstriction
occurs, which raises the pulmonary arterial pressure and the work done by
the right heart.  People who are not acclimatized often get acute mountain
sickness.  Long-term residents get chronic mountain sickness.

A sample problem:

How is hyperventilation triggered and how does it help the respiratory system in high altitudes?

Answers:

Hyperventilation is triggered by the hypoxic stimulation of the chemoreceptors. The increased rate of breathing allows more oxygen to get in the blood per unit time. This is necessary in high altitudes because the partial pressure of oxygen is low.

For more information on this topic, please refer to West , page 137.

Also, check out the following links that may be helpful:

• Outdoor Action Guide to High Altitude: Acclimatization and Illnesses
• The Physiology of Decompression Illness
• And, as always, if you have questions, or if you can suggest a helpful link, please e-mail me.

No!
Please don't make me go to any of those pages!
I want to go somewhere completely different!

This page was written by Valerie Leupp , a student in this course.

BME 403 Pages maintained by the T.A., Douglas Miles.