Running 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 sulfa 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.