Altitude Safety

INTRODUCTION

Changes in altitude have a profound effect on the human body. The body attempts to maintain a state of homeostasis or balance to ensure the optimal operating environment for its complex chemical systems. Any change from this homeostasis is a change away from the optimal operating environment. The body attempts to correct this imbalance. One such imbalance is the effect of increasing altitude on the body's ability to provide adequate oxygen to be utilized in cellular respiration. With an increase in elevation, a typical occurrence when climbing mountains, the body is forced to respond in various ways to the changes in external environment. Foremost of these changes is diminished ability to obtain oxygen from the atmosphere. If the adaptive responses to these stressors are inadequate the performance of body systems may decline dramatically, I f prolonged results can be serious or even fatal. In looking at the effect of altitude on body functioning we first must understanding what occurs in the external environment at higher elevations and then observe the important changes that occur in the internal environment of the body in response.

HIGH ALTITUDE

In discussing altitude change and its effect on the body mountaineers generally define altitude according to the scale of high (8,000 Ð'- 12,000 feet), very high (12,000 Ð'- 18,000 feet), and extremely high (18,000+ feet), (Hubble, 1995). A common misperception of the change in external environment with increased altitude is that there is decreased oxygen. This is not correct as the concentration of oxygen at sea level is about 21% and says relatively unchanged until over 50,000 feet (Johnson, 1988).

What is really happening is that the atmospheric pressure is decreasing and subsequently the amount oxygen available is a single breath of air is significantly less. At sea level the barometric pressure averages 760 mmHg while at 12,000 feet it is only 483 mmHg. This decrease in total atmospheric pressure means that there are 49% fewer oxygen molecules per breath at this altitude compared to sea level (Princeton, 1995).

HHUMAN RESPIRATORY SYSTEM

The human respiratory system is responsible for bringing oxygen into the body and transferring it to the cells where it can be utilized for cellular activities. It also removes carbon dioxide from the body. The respiratory system draws air initially either through the mouth or nasal passages. Both of these passages join behind the hard palate to form the pharynx. At the base of the pharynx are two openings. One, the esophagus, leads to the digestive system while the other, the glottis, leads to the lungs. The epiglottis covers the glottis when swallowing so that food does not enter the lungs. When the epiglottis is not covering the opening to the lungs air may pass freely into and out of the trachea.

The trachea sometimes called the "windpipe" branches into two bronchi, which in turn lead to a lung. Once in the lung the bronchi branch many times into smaller bronchioles, which eventually terminate in small sacs called alveoli. It is in the alveoli that the actual transfer of oxygen to the blood takes place.

The alveoli are shaped like inflated sacs and exchange gas through a membrane. The passage of oxygen into the blood and carbon dioxide out of the blood is dependent on three major factors: 1) the partial pressure of the gases, 2) the area of the pulmonary surface, and 3) the thickness of the membrane (Gerking, 1969). The membranes in the alveoli provide large surface areas for the free exchange of gasses. The typical thickness of the pulmonary membrane is less than the thickness of a red blood cell. The pulmonary surface and the thickness of the alveolar membranes are not directly affected related to altitude and affects gas transfer in the alveoli.

GAS TRANSFER

To understand gas transfer it is important to first understand something about the behavior of gases. Each gas in our atmosphere exerts its own pressure and acts independently of the others. Hence the term partial pressure refers to the contribution of each gas to the entire pressure of the atmosphere. The average pressure of the atmosphere at sea level is approximately 760 mmHg. This means that the pressure is great enough to support a column of mercury (Hg) 760 mm high. To figure the partial pressure of oxygen you start with the percentage of oxygen present in the atmosphere which is about 20%. Thus oxygen will constitute 20% of the total atmospheric pressure at any given level. At sea level the total atmospheric pressure is 760 mmHg so the partial pressure of O2 would be approximately 152 mmHg.

760 mmHg x 0.20 = 152 mmHg

A similar computation can be made for CO2 if we know that the concentration is approximately 4%. The partial pressure of CO2 would then be about 0.304 mmHg at sea level.

Gas transfer at the alveoli follows the rule of simple diffusion. Diffusion is movement of molecules along a concentration gradient from an area of high concentration to an area of lower concentration. Diffusion is the result of collisions between molecules. In areas of higher concentration there are more collisions. The net effect of this greater number of collisions is a movement toward an area of lower concentration. In Table 1 it is apparent that the concentration gradient favors the diffusion of oxygen into and carbon dioxide out of the blood (Gerking, 1969). Table 2 shows the decrease in partial pressure of oxygen at increasing altitudes (Guyton, 1979).

Table 1

ATMOSPHERIC AIR ALVEOLUS VENOUS BLOOD

OXYGEN 152 mmHg (20%) 104 mmHg (13.6%) 40mHg

CARBON DIOXIDE 0.304 mmHg (0.04%) 40 mmHg (5.3%) 45mmHg

Table 2

ALTITUCE (ft.) BAROMETRIC PRESSURE (mmHg) Po2 IN AIR (mmHg) Po2 IN ALVEOLI

(mmHg) ARTERIAL OXYGEN SATURATION (%)

0 760 159* 104 97

10,000 523 110 67 90

20,000 349 73 40 70

30,000 226 47 21 20

40,000 141 29 8 5

50,000 87 18 1 1

*This value differs from table 1 because the author used the value for the concentration of O2 as 21%.

The author of table 1 chooses to use the value as 20%.