The Effects Of Altitude On Human Physiology

Essay, Research Paper

The Effects of Altitude On Human Physiology

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 the

diminished ability to obtain oxygen from the atmosphere. If the adaptive

responses to this stressor are inadequate the performance of body systems may

decline dramatically. If prolonged the results can be serious or even fatal. In

looking at the effect of altitude on body functioning we first must understand

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 – 12,000 feet),

very high (12,000 – 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 stays relatively unchanged

until over 50,000 feet (Johnson, 1988).

What is really happening is that the atmospheric pressure is decreasing

and subsequently the amount of oxygen available in 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 40% fewer oxygen molecules per breath at this altitude

compared to sea level (Princeton, 1995).

HUMAN 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 a large surface area for

the free exchange of gases. 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 by a change in

altitude. The partial pressure of oxygen, however, is directly 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%) 40 mmHg CARBON

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

Table 2 ALTITUDE (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 choose to use the value as

20%.

CELLULAR RESPIRATION

In a normal, non-stressed state, the respiratory system transports

oxygen from the lungs to the cells of the body where it is used in the process

of cellular respiration. Under normal conditions this transport of oxygen is

sufficient for the needs of cellular respiration. Cellular respiration converts

the energy in chemical bonds into energy that can be used to power body

processes. Glucose is the molecule most often used to fuel this process although

the body is capable of using other organic molecules for energy.

The transfer of oxygen to the body tissues is often called internal

respiration (Grollman, 1978). The process of cellular respiration is a complex

series of chemical steps that ultimately allow for the breakdown of glucose into

usable energy in the form of ATP (adenosine triphosphate). The three main steps

in the process are: 1) glycolysis, 2) Krebs cycle, and 3) electron transport

system. Oxygen is required for these processes to function at an efficient level.

Without the presence of oxygen the pathway for energy production must proceed

anaerobically. Anaerobic respiration sometimes called lactic acid fermentation

produces significantly less ATP (2 instead of 36/38) and due to this great

inefficiency will quickly exhaust the available supply of glucose. Thus the

anaerobic pathway is not a permanent solution for the provision of energy to the

body in the absence of sufficient oxygen.

The supply of oxygen to the tissues is dependent on: 1) the efficiency

with which blood is oxygenated in the lungs, 2) the efficiency of the blood in

delivering oxygen to the tissues, 3) the efficiency of the respiratory enzymes

within the cells to transfer hydrogen to molecular oxygen (Grollman, 1978). A

deficiency in any of these areas can result in the body cells not having an

adequate supply of oxygen. It is this inadequate supply of oxygen that results

in difficulties for the body at higher elevations.

ANOXIA

A lack of sufficient oxygen in the cells is called anoxia. Sometimes the

term hypoxia, meaning less oxygen, is used to indicate an oxygen debt. While

anoxia literally means “no oxygen” it is often used interchangeably with hypoxia.

There are different types of anoxia based on the cause of the oxygen deficiency.

Anoxic anoxia refers to defective oxygenation of the blood in the lungs. This is

the type of oxygen deficiency that is of concern when ascending to greater

altitudes with a subsequent decreased partial pressure of O2. Other types of

oxygen deficiencies include: anemic anoxia (failure of the blood to transport

adequate quantities of oxygen), stagnant anoxia (the slowing of the circulatory

system), and histotoxic anoxia (the failure of respiratory enzymes to adequately

function).

Anoxia can occur temporarily during normal respiratory system regulation

of changing cellular needs. An example of this would be climbing a flight of

stairs. The increased oxygendemand of the cells in providing the mechanical

energy required to climb ultimately produces a local hypoxia in the muscle cell.

The first noticeable response to this external stress is usually an increase in

breathing rate. This is called increased alveolar ventilation. The rate of our

breathing is determined by the need for O2 in the cells and is the first

response to hypoxic conditions.

BODY RESPONSE TO ANOXIA

If increases in the rate of alveolar respiration are insufficient to

supply the oxygen needs of the cells the respiratory system responds by general

vasodilation. This allows a greater flow of blood in the circulatory system. The

sympathetic nervous system also acts to stimulate vasodilation within the

skeletal muscle. At the level of the capillaries the normally closed

precapillary sphincters open allowing a large flow of blood through the muscles.

In turn the cardiac output increases both in terms of heart rate and stroke

volume. The stroke volume, however, does not substantially increase in the non-

athlete (Langley, et.al., 1980). This demonstrates an obvious benefit of regular

exercise and physical conditioning particularly for an individual who will be

exposed to high altitudes. The heart rate is increased by the action of the

adrenal medulla which releases catecholamines. These catecholamines work

directly on the myocardium to strengthen contraction. Another compensation

mechanism is the release of renin by the kidneys. Renin leads to the production

of angiotensin which serves to increase blood pressure (Langley, Telford, and

Christensen, 1980). This helps to force more blood into capillaries. All of

these changes are a regular and normal response of the body to external

stressors. The question involved with altitude changes becomes what happens when

the normal responses can no longer meet the oxygen demand from the cells?

ACUTE MOUNTAIN SICKNESS

One possibility is that Acute Mountain Sickness (AMS) may occur. AMS is

common at high altitudes. At elevations over 10,000 feet, 75% of people will

have mild symptoms (Princeton, 1995). The occurrence of AMS is dependent upon

the elevation, the rate of ascent to that elevation, and individual

susceptibility.

Acute Mountain Sickness is labeled as mild, moderate, or severe

dependent on the presenting symptoms. Many people will experience mild AMS

during the process of acclimatization to a higher altitude. In this case

symptoms of AMS would usually start 12-24 hours after arrival at a higher

altitude and begin to decrease in severity about the third day. The symptoms of

mild AMS are headache, dizziness, fatigue, shortness of breath, loss of appetite,

nausea, disturbed sleep, and a general feeling of malaise (Princeton, 1995).

These symptoms tend to increase at night when respiration is slowed during sleep.

Mild AMS does not interfere with normal activity and symptoms generally subside

spontaneously as the body acclimatizes to the higher elevation.

Moderate AMS includes a severe headache that is not relieved by

medication, nausea and vomiting, increasing weakness and fatigue, shortness of

breath, and decreased coordination called ataxia (Princeton, 1995). Normal

activity becomes difficult at this stage of AMS, although the person may still

be able to walk on their own. A test for moderate AMS is to have the individual

attempt to walk a straight line heel to toe. The person with ataxia will be

unable to walk a straight line. If ataxia is indicated it is a clear sign that

immediate descent is required. In the case of hiking or climbing it is important

to get the affected individual to descend before the ataxia reaches the point

where they can no longer walk on their own.

Severe AMS presents all of the symptoms of mild and moderate AMS at an

increased level of severity. In addition there is a marked shortness of breath

at rest, the inability to walk, a decreasing mental clarity, and a potentially

dangerous fluid buildup in the lungs.

ACCLIMATIZATION

There is really no cure for Acute Mountain Sickness other than

acclimatization or descent to a lower altitude. Acclimatization is the process,

over time, where the body adapts to the decrease in partial pressure of oxygen

molecules at a higher altitude. The major cause of altitude illnesses is a rapid

increase in elevation without an appropriate acclimatization period. The process

of acclimatization generally takes 1-3 days at the new altitude. Acclimatization

involves several changes in the structure and function of the body. Some of

these changes happen immediately in response to reduced levels of oxygen while

others are a slower adaptation. Some of the most significant changes are:

Chemoreceptor mechanism increases the depth of alveolar ventilation.

This allows for an increase in ventilation of about 60% (Guyton, 1969). This is

an immediate response to oxygen debt. Over a period of several weeks the

capacity to increase alveolar ventilation may increase 600-700%.

Pressure in pulmonary arteries is increased, forcing blood into portions

of the lung which are normally not used during sea level breathing.

The body produces more red blood cells in the bone marrow to carry

oxygen. This process may take several weeks. Persons who live at high altitude

often have red blood cell counts 50% greater than normal.

The body produces more of the enzyme 2,3-biphosphoglycerate that

facilitates the release of oxygen from hemoglobin to the body tissues (Tortora,

1993).

The acclimatization process is slowed by dehydration, over-exertion, alcohol and

other depressant drug consumption. Longer term changes may include an increase

in the size of the alveoli, and decrease in the thickness of the alveoli

membranes. Both of these changes allow for more gas transfer.

TREATMENT FOR AMS

The symptoms of mild AMS can be treated with pain medications for

headache. Some physicians recommend the medication Diamox (Acetazolamide). Both

Diamox and headache medication appear to reduce the severity of symptoms, but do

not cure the underlying problem of oxygen debt. Diamox, however, may allow the

individual to metabolize more oxygen by breathing faster. This is especially

helpful at night when respiratory drive is decreased. Since it takes a while for

Diamox to have an effect, it is advisable to start taking it 24 hours before

going to altitude. The recommendation of the Himalayan Rescue Association

Medical Clinic is 125 mg. twice a day. The standard dose has been 250 mg., but

their research shows no difference with the lower dose (Princeton, 1995).

Possible side effects include tingling of the lips and finger tips, blurring of

vision, and alteration of taste. These side effects may be reduced with the 125

mg. dose. Side effects subside when the drug is stopped. Diamox is a sulfonamide

drug, so people who are allergic to sulfa drugs such as penicillin should not

take Diamox. Diamox has also been known to cause severe allergic reactions to

people with no previous history of Diamox or suffer allergies. A trial course of

the drug is usually conducted before going to a remote location where a severe

allergic reaction could prove difficult to treat. Some recent data suggests that

the medication Dexamethasone may have some effect in reducing the risk of

mountain sickness when used in combination with Diamox (University of Iowa,

1995).

Moderate AMS requires advanced medications or immediate descent to

reverse the problem. Descending even a few hundred feet may help and definite

improvement will be seen in descents of 1,000-2,000 feet. Twenty-four hours at

the lower altitude will result in significant improvements. The person should

remain at lower altitude until symptoms have subsided (up to 3 days). At this

point, the person has become acclimatized to that altitude and can begin

ascending again. Severe AMS requires immediate descent to lower altitudes (2,000

- 4,000 feet). Supplemental oxygen may be helpful in reducing the effects of

altitude sicknesses but does not overcome all the difficulties that may result

from the lowered barometric pressure.

GAMOW BAG

This invention has revolutionized field treatment of high altitude

illnesses. The Gamow bag is basically a portable sealed chamber with a pump. The

principle of operation is identical to the hyperbaric chambers used in deep sea

diving. The person is placed inside the bag and it is inflated. Pumping the bag

full of air effectively increases the concentration of oxygen molecules and

therefore simulates a descent to lower altitude. In as little as 10 minutes the

bag creates an atmosphere that corresponds to that at 3,000 – 5,000 feet lower.

After 1-2 hours in the bag, the person’s body chemistry will have reset to the

lower altitude. This lasts for up to 12 hours outside of the bag which should be

enough time to travel to a lower altitude and allow for further acclimatization.

The bag and pump weigh about 14 pounds and are now carried on most major high

altitude expeditions. The gamow bag is particularly important where the

possibility of immediate descent is not feasible.

OTHER ALTITUDE-INDUCED ILLNESS

There are two other severe forms of altitude illness. Both of these

happen less frequently, especially to those who are properly acclimatized. When

they do occur, it is usually the result of an increase in elevation that is too

rapid for the body to adjust properly. For reasons not entirely understood, the

lack of oxygen and reduced pressure often results in leakage of fluid through

the capillary walls into either the lungs or the brain. Continuing to higher

altitudes without proper acclimatization can lead to potentially serious, even

life-threatening illnesses.

HIGH ALTITUDE PULMONARY EDEMA (HAPE)

High altitude pulmonary edema results from fluid buildup in the lungs.

The fluid in the lungs interferes with effective oxygen exchange. As the

condition becomes more severe, the level of oxygen in the bloodstream decreases,

and this can lead to cyanosis, impaired cerebral function, and death. Symptoms

include shortness of breath even at rest, tightness in the chest, marked fatigue,

a feeling of impending suffocation at night, weakness, and a persistent

productive cough bringing up white, watery, or frothy fluid (University of Iowa,

1995.). Confusion, and irrational behavior are signs that insufficient oxygen is

reaching the brain. One of the methods for testing for HAPE is to check recovery

time after exertion. Recovery time refers to the time after exertion that it

takes for heart rate and respiration to return to near normal. An increase in

this time may mean fluid is building up in the lungs. If a case of HAPE is

suspected an immediate descent is a necessary life-saving measure (2,000 – 4,000

feet). Anyone suffering from HAPE must be evacuated to a medical facility for

proper follow-up treatment. Early data suggests that nifedipine may have a

protective effect against high altitude pulmonary edema (University of Iowa,

1995).

HIGH ALTITUDE CEREBRAL EDEMA (HACE)

High altitude cerebral edema results from the swelling of brain tissue

from fluid leakage. Symptoms can include headache, loss of coordination (ataxia),

weakness, and decreasing levels of consciousness including, disorientation, loss

of memory, hallucinations, psychotic behavior, and coma. It generally occurs

after a week or more at high altitude. Severe instances can lead to death if not

treated quickly. Immediate descent is a necessary life-saving measure (2,000 -

4,000 feet). Anyone suffering from HACE must be evacuated to a medical facility

for proper follow-up treatment.

CONCLUSION

The importance of oxygen to the functioning of the human body is

critical. Thus the effect of decreased partial pressure of oxygen at higher

altitudes can be pronounced. Each individual adapts at a different speed to

exposure to altitude and it is hard to know who may be affected by altitude

sickness. There are no specific factors such as age, sex, or physical condition

that correlate with susceptibility to altitude sickness. Most people can go up

to 8,000 feet with minimal effect. Acclimatization is often accompanied by fluid

loss, so the ingestion of large amounts of fluid to remain properly hydrated is

important (at least 3-4 quarts per day). Urine output should be copious and

clear.

From the available studies on the effect of altitude on the human body

it would appear apparent that it is important to recognize symptoms early and

take corrective measures. Light activity during the day is better than sleeping

because respiration decreases during sleep, exacerbating the symptoms. The

avoidance of tobacco, alcohol, and other depressant drugs including,

barbiturates, tranquilizers, and sleeping pills is important. These depressants

further decrease the respiratory drive during sleep resulting in a worsening of

the symptoms. A high carbohydrate diet (more than 70% of your calories from

carbohydrates) while at altitude also appears to facilitate recovery.

A little planning and awareness can greatly decrease the chances of

altitude sickness. Recognizing early symptoms can result in the avoidance of

more serious consequences of altitude sickness. The human body is a complex

biochemical organism that requires an adequate supply of oxygen to function. The

ability of this organism to adjust to a wide range of conditions is a testament

to its survivability. The decreased partial pressure of oxygen with increasing

altitude is one of these adaptations.

Sources:

Electric Differential Multimedia Lab, Travel Precautions and Advice, University

of Iowa Medical College, 1995.

Gerking, Shelby D., Biological Systems, W.B. Saunders Company, 1969.

Grolier Electronic Publishing, The New Grolier Multimedia Encyclopedia, 1993.

Grollman, Sigmund, The Human Body: Its Structure and Physiology, Macmillian

Publishing Company, 1978.

Guyton, Arthur C., Physiology of the Human Body, 5th Edition, Saunders College

Publishing, 1979.

Hackett, P., Mountain Sickness, The Mountaineers, Seattle, 1980.

Hubble, Frank, High Altitude Illness, Wilderness Medicine Newsletter,

March/April 1995.

Hubble, Frank, The Use of Diamox in the Prevention of Acute Mountain Sickness,

Wilderness Medicine Newsletter, March/April 1995.

Isaac, J. and Goth, P., The Outward Bound Wilderness First Aid Handbook, Lyons &

Burford, New 1991.

Johnson, T., and Rock, P., Acute Mountain Sickness, New England Journal of

Medicine, 1988:319:841-5

Langley, Telford, and Christensen, Dynamic Anatomy and Physiology, McGraw-Hill,

1980.

Princeton University, Outdoor Action Program, 1995.

Starr, Cecie, and Taggart, Ralph, Biology: The Unity and Diversity of Life,

Wadsworth Publishing Company, 1992.

Tortora, Gerard J., and Grabowski, Sandra, Principles of Anatomy and Physiology,

Seventh Edition, Harper Collins College Publishers, 1993.

Wilkerson., J., Editor, Medicine for Mountaineering, Fourth Edition, The

Mountaineers, Seattle, 1992.