Effects of Altitude on Human Physiology Term Paper

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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 sys>

Transfer interrupted!

tem 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.

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