Theme 3: Energy and the Human Body Background Material

1. Cellular Respiration - The Need For Oxygen
Without energy, human beings can not survive. All living things, including humans, need a constant supply of energy to stay alive. To get this energy, we must have food and a constant supply of oxygen.

Food contains stored chemical energy locked inside it. Each cell in the body must process the food so that it releases its energy in a form that the cell can use. The process by which cells release energy from chemical energy is called respiration. Through respiration, proteins, fats and carbohydrates are all processed to produce energy. However, the primary source of food energy for the cell is the sugar called glucose.

The most efficient form of energy release is aerobic and requires a continuous supply of oxygen. The oxygen is used to "burn" the glucose to release the chemical energy stored in this molecule. In this reaction, glucose is taken apart and the energy is released to form a chemical called adenosine triphosphate (ATP). ATP is another form of chemical energy. ATP is used to drive most of the chemical reactions that occur in the cells of the body.

The waste products of cellular respiration are carbon dioxide and water. Carbon dioxide is a harmful waste product that must be removed from the body and is found in relatively high concentrations in the tissues. It diffuses into the blood and is carried to the lungs where it is eliminated when we exhale. Carbon dioxide is much more soluble than oxygen and readily diffuses into red blood cells. It reacts with water to form carbonic acid that can lower the pH of the blood if not removed from the body.

Cellular respiration is summarized by this equation:

glucose + oxygen ----> energy (ATP) + CO₂ + H₂O

The human respiratory system brings oxygen into the body and provides a site where it can be transferred into the blood stream. The circulatory system then transports oxygen to all the cells of the body where energy reactions take place. The respiratory system also provides a means by which the very toxic carbon dioxide can be eliminated from the body.

2. Gas Exchange
Respiration is the process that keeps things alive. Cellular respirationis the process that occurs in all cells in the human body. All living things need a constant supply of energy. Therefore, their cells must carry out respiration at all times. To do this, most living organisms require a constant supply of oxygen to burn their food. This process releases the energy they need to stay alive but it also produces two waste products - carbon dioxide and water vapor.

To stay alive, humans must be able to take oxygen from the air and get rid of the carbon dioxide into the air. Swapping oxygen for carbon dioxide like this is called gas exchange. Gas exchange in terrestrial animals, including humans, has two basic properties:

1. The gases pass in or out of an organism through a gas exchange surface. The gas exchange surface allows dissolved oxygen and carbon dioxide to pass through it easily. Oxygen and carbon dioxide can only move across the gas exchange surface if they are dissolved in water. Therefore, this surface must be kept moist. 2. Inside the cells the gases are always dissolved in water. Oxygen diffuses into the cell. Once inside the cell, the oxygen is used in cellular respiration to release the energy from glucose. The carbon dioxide produced by cellular respiration must be eliminated. The carbon dioxide dissolves in the water and diffuses across the gas exchange surface.

This process is shown in the following diagram:

Gas exchange between a living cell and its environment always takes place by diffusion across a moist cell membrane. The gases must be in a solution if they are to move across the membrane. There are a number of problems living things, especially terrestrial organisms, have overcome in order to ensure an adequate supply of oxygen. These problems include (1) increase in body size; (2) waterproof body coverings; (3) cells not in direct contact with gas exchange surface.

(1) Increased Body Size
As the volume of an organism increases, it must maintain a large surface area for gas exchange. The problem is very critical for active animals whose rapid utilization of oxygen demands a large amount of oxygen.

(2) Waterproof Body Covering
Our skin, like the body covering of many other living organisms, is essentially waterproof. This characteristic prevents us from drying out. However, a waterproof body covering prevents gas exchange. This limits the gas exchange surface to a specific region of the body making the problem of providing an adequate gas exchange area even more critical.

(3) Cell Location
In larger organisms most cells are buried deep inside the body and are not in direct contact with the gas exchange surface. Diffusion alone can not provide enough oxygen to these cells. In general, diffusion of dissolved gases is only efficient when the distance is less than one millimetre. Some other mechanism must be in place to get oxygen to every cell in the body.

In humans, the mechanism that solves these three problems is the gas exchange or respiratory system and its direct connection to the circulatory system.
JUMP TO: Circulatory system

Human Respiratory System
The human respiratory system consists of two lungs and a set of air passages. The lungs, together with the heart, sit in the thoracic cavity or chest. This is an air tight cavity that:

• provides a large surface area for gas exchange
• has moist gas exchange surface areas
• is associated with the circulatory system to transport oxygen to the cells and pick up carbon dioxide produced by every cell

Human Respiratory System Diagram from "Ultimate Visual Dictionary of Science," Stoddart 1998.

Air is drawn into the human respiratory system through the nose (external nares) and enters the nasal passage.

The nasal passage warms and moistens the air as well as filtering dust and other impurities.

The nasal passage, as well as other structures in the respiratory system, has cells that have tiny hairs (cilia) and secrete mucous.

The mucous helps to clean the air before it enters the lungs. The cilia constantly push the mucous towards the nose.

The air passes through the pharynx and larynx (voice box) and into the trachea. The trachea is an air duct leading from the larynx into the thoracic cavity. It is lined with the ciliated and mucous secreting cells. The cilia beat in waves that carry foreign particles and mucous up the trachea away from the lungs.

Lateral view of the larynx Diagram from "Ultimate Visual Dictionary of Science," Stoddart 1998.

A series of "C-shaped" pieces of cartilage are embedded in the walls of the trachea and they prevent it from collapsing during inhalation (inhaling).

At its lower end, the trachea divides into two bronchi, tubes that lead toward the two lungs.

Each bronchus branches and rebranches forming bronchi and finally tiny tubes called bronchioles.

The smallest of these branches are called the terminal bronchioles. The bronchioles end in tiny air sacs called alveoli where gas exchange takes place. The total surface area of the alveoli is about 100 square metres.

Gas exchange in the alveoli Diagram from "Ultimate Visual Dictionary of Science," Stoddart 1998.

The walls of the alveoli are exceedingly thin and are only one cell thick.

Each alveolus is surrounded by a dense bed of capillaries.

The alveoli are the sites of the actual gas exchange and are regarded as the primary functional units of the lungs.

Oxygen entering an alveolus dissolves in the film of water on its wall and moves by diffusion across the cells into the blood. Oxygen is in higher concentration inside the alveolus than in the blood. Oxygen moves from a higher concentration to a lower concentration by the process of diffusion. On the other hand, the concentration of carbon dioxide is higher in the blood than in the alveoli. The carbon dioxide also moves from a high concentration to a lower concentration by the process of diffusion.

Diffusion is the only way for oxygen to enter the blood stream. This is normally not a problem. However, at higher altitudes the partial pressure of O₂ in the atmosphere decreases and diffusion of oxygen slows down. Mountain climbers may suffer from O2 deprivation because passive transport is insufficient to meet the O₂ demands.

The oxygen binds to the hemoglobin in the red blood cells. The circulatory system is responsible for transporting the oxygen to all of the cells in the body. When the blood enters a tissue capillary bed the oxygen diffuses from the blood across the thin-walled capillary and into the cells of the body. The oxygen moves from a higher concentration in the blood to a lower concentration in the cells. Carbon dioxide diffuses from a high concentration in the cells to a lower concentration in the blood. The venous blood returns to the right side of the heart where it is pumped to the lungs. The deoxygenated blood enters the capillary beds surrounding the alveoli and the process starts all over again.
JUMP TO: Human blood

4. Breathing
The lungs and the heart of mammals are inside a chamber called the thorax. This chamber is made by the ribs, the sternum and the backbone. The floor of the thoracic cavity is a large muscular sheet called the diaphragm.

Running between the ribs are the intercostal muscles. The only way air can get in or out of the thorax to enter the lungs is through the trachea or windpipe, which is kept open by rings of stiff cartilage.

How breathing works Diagram from "Ultimate Visual Dictionary of Science," Stoddart 1998.

Air is drawn into and expelled from the lungs by a mechanical process called breathing. The process generally involves muscular contractions of two regions, the rib cage and the diaphragm. The diaphragm is a large, dome shaped muscle. When it relaxes, it curves upward. When it contracts, it pulls downward and flattens. This movement increases the size of the thoracic cavity.

There are two sets of intercostal muscles - external and internal. The external intercostal muscles are on the outside of the ribs. When they contract, they pull the rib cage up and out making the thoracic cavity large. The internal intercostal muscles are on the inside of the ribs. When they contract, the intercostal muscles pull the ribs down and in, reducing the size of the thoracic cavity.

During inhalation (breathing in), the diaphragm contracts and gets flatter. The external intercostal muscles also contract and pull the rib cage up and out. These two actions increase the size of the thoracic cavity, which in turn reduces the air pressure inside the chest cavity. As a result, air moves from the atmosphere (region of higher pressure) into the lungs (region of lower pressure). The trachea have the rings of cartilage to keep them open as air pressure inside the chest cavity is reduced during the process of inhalation.

During exhalation (breathing out) the diaphragm relaxes and returns to its normal curved shape. The internal intercostal muscles contract pulling the rib cage down and in. These actions increase the pressure inside the thoracic cavity. When the pressure inside the chest cavity is greater than the air pressure in the atmosphere, the air is expelled. In both inhalation and exhalation, the air moves from a region of higher pressure to a region of lower pressure. The pressure changes are caused by the contraction of muscles. The lungs are passive structures and do not take a very active role during breathing. They do, however, have certain elastic properties that help to move air out of the alveoli.

Control of Breathing

Introduction
The respiratory rate varies considerably from person to person. The average resting respiratory rate for adults is between 14 and 20 breaths per minute. Newborn babies may take 40 breaths per minute. Athletes normally have a much lower breathing rate than those that are not in "shape."

At rest, a normal adult male uses approximately 250 millilitres of oxygen per minute. When you exercise, your body requires a great deal of energy. Remember the most effective way for a cell to utilize glucose to release energy is in the presence of oxygen.
JUMP TO: Cellular respiration

During exercise, your body demands a great increase in the oxygen supply. This demand for oxygen may be met in two ways:

1. Pumping the blood faster and supplying more oxygenated blood to the cells.

2. Increasing the concentration of oxygen that the blood is carrying.

As you exercise, you will notice that your breathing rate (number of breaths per minute) and the depth of breathing changes. This helps in the exchange of gases (oxygen and carbon dioxide), bringing more oxygen in contact with the gas exchange surfaces of the alveoli. Oxygen diffuses through the plasma and into the red blood cells combining with hemoglobin, which is about 95 percent saturated with oxygen as it leaves the lungs. At the same time, the heart rate increases and speeds oxygen delivery to the muscle tissues.

The respiratory system can be looked at, simply, as a feedback control system. The simplest feedback systems have the following components: output centre, effectors, and sensors. In the human respiratory system, this feedback system can be summarized as follows:

Output centre: brainstem including both the medulla and pons regions (other parts of the brain are also involved)

Effectors: diaphragm and intercostal muscles (see Breathing...)

Sensors: chemoreceptors - oxygen receptors found in the carotid artery (peripheral receptors) as well as chemical receptors located in the medulla region itself (central receptors)

The respiratory control system functions to generate a coordinated series of inspiratory and expiratory events. The normal rate and depth of respiration may be dramatically affected by wakefulness/sleep, and by reflexes such as swallowing or sneezing. In addition, anxiety and depression can also alter the rate and depth of breathing.

Chemical Control of Breathing
Stimuli that control respiration can broadly be classified as either chemical or behavioral. Scientifically, the mechanisms of chemical control are more completely understood than their behavioral counterparts. Chemical control has evolved to meet the following general needs:

1) to remove carbon dioxide from the body
2) to insure a supply of oxygen for tissue metabolism
3) to help maintain the acid-base balance of the body

1. Carbon Dioxide
An increase in the amount of carbon dioxide in the blood is called hypercarbia. The amount of carbon dioxide in the arterial blood is regulated with extraordinary precision through sensing mechanisms. Central chemoreceptors are located in the brain. A rise in the concentration of carbon dioxide in the arterial blood is detected very quickly. The carbon dioxide crosses the blood-brain barrier easily and acidifies (lowers the pH of) the cerebrospinal fluid (CBF). Cerebrospinal fluid normally has a pH of approximately 7.32. The change in pH in the cerebrospinal fluid is pronounced because the CBF lacks buffers to neutralize the acid. Approximately 85 percent of the respiratory system response to carbon dioxide occurs at the central chemical receptor level.

Peripheral chemical receptors found in the carotid arteries respond to an increase in the levels of carbon dioxide in the blood. The sensors detect increased levels of carbon dioxide and send nervous impulses to the control centre in the brain. The respiratory control centre is located in the medulla oblongata and the pons regions of the brain. A constant stream of information, regarding the oxygen needs and concentration of carbon dioxide, is processed by these regions of the human brain. The respiratory centre responds to these signals by increasing or decreasing the breathing rate to meet the body's demands.

This process is shown below:

As the levels of carbon dioxide return to normal, there is less activity in the respiratory centre in the brain. The reduction in activity in the medulla reduces the activity of the breathing muscles.

2. Lack of Oxygen
Hypoxia is a condition in which there is a decrease of oxygen to the tissue in spite of adequate blood flow to the tissue. Anoxia is a condition in which there is an absence of oxygen supply to an organ's tissues although there is adequate blood flow to the tissue. Anoxia and hypoxia, however, are often used interchangeably to describe a condition that occurs in an organ when there is a diminished supply of oxygen to the organ's tissues.
JUMP TO: Effects of altitude

The respiratory system is driven by a decrease in the pressure of oxygen in the arterial blood. The level of oxygen is detected by peripheral chemical receptors found in the arteries. An interesting fact is that a decrease in the oxygen concentration in the blood is not the most important stimulus for respiration under normal conditions. The major controlling variable is the concentration of carbon dioxide. Under normal conditions, moderate decreases in oxygen content will not stimulate the peripheral chemoreceptors.

The ventilation response to hypoxia, on the other hand, is controlled almost exclusively by the peripheral chemoreceptors. A person suffering from hypoxia has a faster ventilation rate that increases rapidly as hypoxia progresses. Hypoxia is an important respiratory stimulus at extreme altitude.

3. Acidosis
Byron requires tremendous amounts of energy to climb Mt. Everest. The energy comes from the oxidization of glucose and the production of ATP. In addition to energy, his body will produce large amounts of carbon dioxide as a waste product.
JUMP TO: Cellular respiration

Carbon dioxide forms a weak acid when it is dissolved in water. This acid, carbonic acid, has a tendency to lower the pH of the blood and cerebrospinal fluid. As the blood pH decreases (becomes more acidic) there is a corresponding increase in ventilation. Faster ventilation increases the rate at which the body can rid itself of the excess carbon dioxide.

Control of the respiration is effective in regulating the uptake of oxygen, the disposal of carbon dioxide, and in maintaining the pH of the blood.

Effects of Altitude
As this map shows, Byron will have climbed to an altitude of 8,850 m (29, 035 ft) when he reaches the summit of Mt. Everest. Even at Basecamp, the members of the expedition will be living at an altitude of more than 5,334 metres (17,500 feet) above sea level. These altitudes have a dramatic effect on the human body.

Through homeostasis, our body self-regulates itself ensuring all bodily systems, including the respiratory system, are functioning properly. Homeostasis regulates the respiratory and circulatory systems, allowing the body to function at these high altitudes.

In mountaineering, high altitude is generally defined as anything above 2,438 metres (8,000 feet). There are three ranges of high altitude:

• extremely high - above 5,486 metres (18,000 feet)
• very high - 3,658 metres (12,000 feet) to 5,486 metres (18,000 feet)
• high - 2,438 metres (8,000 feet) to 3,658 metres (12,000 feet)

Acclimatization

April 25, 2000 Drinking lots of fluid can help climbers acclimatize.

The major cause of high altitude illnesses is going too high too fast. The human body, as you have studied in Phase 3, can adjust to the decrease in atmospheric pressure at high altitudes. The process of adjusting to high altitude is called acclimatization.
More information on high altitude and the human body.

Byron and the other members of the Everest 2000 expedition will experience a number of changes that will allow them to function at very high and extreme altitudes. These changes include:

• the depth of breathing (respiration) increases (see Breathing)
• resting heart rate eventually returns to near sea level values (see Factors affecting heart rate)
• cerebral blood flow (CBF) increases at high altitudes
• pressure in the pulmonary arteries is increased, "forcing" blood into portions of the lungs which are normally not used during sea level breathing (see Human respiratory system)
• an increased production in red blood cells (the hematocrit increases) (see Human blood)
• an increased production of a specific enzyme (2,3 DPG) that makes it easier for oxygen to be released from the hemoglobin molecule where it is needed (see Human blood)

The trek from Kathmandu (1,300 metres) to Base Camp (5,334 metres) will take 10 to 14 days. This slow ascent to increasing altitude will help prevent problems associated with high altitude.

The following chart shows what can typically happen to people who ascend to increasing altitude.

There are great individual variations in response to surviving at high and extreme altitudes. Factors such as cold, exercise, and genetic predisposition to high altitude (some people simply can not acclimatize) all play a role in the ability of different individuals to function at high altitudes. However, above a certain altitude, acclimatization is not possible. The rate of deterioration increases with increasing altitude. This is why there is only a short window of opportunity for Byron to summit Mt. Everest.

Preventing High Altitude Illnesses
Dr. Virginia Robinson, the Everest 2000 team doctor, will play a key role in preventing High Altitude Illnesses. Dr. Robinson must ensure Byron and other team members acclimatize properly.

The following are guidelines she will use:

• start below 3,000 metres (10,000 feet) and walk up
• altitude should only be increased by 300 metres per day (1000 feet)
• for every 900 metres (3,000 feet) of elevation gained, take a rest day
• "Climb high and sleep low" - Byron will be able to climb more than 300 metres per day as long as he comes back down and sleeps at a lower altitude.

If Dr. Robinson notices the symptoms of high altitude illnesses (see next section), she will ensure that:

• the climbers don't go higher until the symptoms resolve
• if the symptoms do not resolve, or get worse, the climber should descend
• all members of the expedition are properly acclimatized
• the climbers keep properly hydrated. Acclimatization is accompanied by fluid loss and the team must drink lots of fluids to remain properly hydrated (three to four litres per day).
• the individuals produce copious amounts of clear urine
• team members eat a high carbohydrate diet (Food and energy)

Problems Associated with High Altitudes
The distinctions between the various high altitude syndromes are not clear, nor do they necessarily occur in isolation - they form a continuum. The following are the most common problems associated with high altitudes.

Anoxia
A lack of sufficient oxygen in the cells is called anoxia. Anoxia literally means "no oxygen." Mountain climbers must be concerned with anoxia because of the oxygen deficiency associated with the lower partial pressure of oxygen available at high altitudes. Susceptibility to AMS is NOT related to age, sex, or physical conditioning.

Most of us experience a mild form of anoxia in our muscles when we climb stairs. The increased oxygen demand of the cells in providing the energy for the muscles required to climb, ultimately produces a local shortage of oxygen in the muscles. At the same time, there is an increase in the carbon dioxide levels in the blood. The first response of the body is to breath faster. This is called increased ventilation.
JUMP TO: Controlled breathing

At sea level, the body also responds to anoxia by increasing the amount of blood flowing to the muscles. The cardiac output of the individual increases both in terms of increased heart rate and stroke volume. . However, at altitude the stroke volume is actually decreased. For a given workload, your hear rate is increased , your stroke volume is decreased and your cardiac output is the same as it would be at sea level.
JUMP TO: Factors affecting heart rate

Acute Mountain Sickness
The most common form of high altitude illness is known as Acute Mountain Sickness (AMS). The symptoms of AMS include: headache, nausea or vomiting, sleep disturbance, dizziness, shortness of breath, lack of appetite, and fatigue. At elevations over 3,000 metres, 75 per cent of people will have mild symptoms.

Want to know more? Hypothermia prevention High Altitude Sickness High Altitude Medicine Guide

The occurrence of AMS is dependent upon the elevation, the rate of ascent to that elevation, and the genetic makeup of the individual - some people are not as susceptible to AMS. In addition, for reasons not entirely understood, high altitude and lower air pressure cause fluid to leak from the capillaries which can cause fluid build-up in both the lungs and the brain. There is no cure for AMS other than acclimatization or descent to a lower altitude.

Mild AMS is an annoyance, which may resolve itself without special treatment. However, an ataxic patient, that is a patient that can not walk a straight line, no longer has mild AMS. Ataxia probably represents a progression to HACE which may be life threatening.

The rate of ascent to higher altitude is probably the most important determining factor in who gets sick. The major cause of high altitude illness is a rapid increase in elevation without an appropriate acclimatization period. This explains why Byron will, in effect, climb the equivalent of four "Mt. Everests" as he moves up and down the mountain getting his body acclimatized to the extreme altitude at the summit.

On the mountain, Byron may measure the oxygen saturation of his blood by using a pulse oximeter. At 3,000 metres SaO₂ measurements between 85 per cent to 95 per cent are common. Measurements below 85 per cent imply more serious impairment of oxygen exchange.

Several other altitude-related changes contribute directly or indirectly to AMS. The temperature decreases an average of 6.5 degrees C per 1,000 m (3,280 ft) gain in elevation. In colder climates more oxygen is required to maintain body temperature. Ultraviolet (UV) light penetration increases four percent per 300 m (985 ft) gain in altitude, increasing the danger of snow blindness, sunburn and, in the long-term, skin cancer. UV light also reflects off snow and ice and can produce temperatures of 40 degrees C to 42 degrees C in enclosed spaces, such as mountaineering tents, so even heat exhaustion is a danger. Dehydration is often a problem, since insensible losses are increased as a result of hyperventilation and increased work. The only way to get water at high altitudes is to melt snow or ice, which is a time consuming task.
JUMP TO: Coping with the Sun on Mount Everest

Acute Mountain Sickness can be treated with medications but all climbers will have to go to lower altitudes for between one to three days. Twenty-four hours at a lower altitude normally results in significant improvements. The climber should remain at a lower altitude until symptoms have subsided - usually in three days. At this point the person has become acclimatized to that altitude and can begin ascending again. The rule of thumb: "When in doubt - go down."

Severe AMS requires immediate descent to lower altitudes. Supplemental oxygen may be helpful in reducing the effects of high altitude sickness but does not overcome all of the difficulties that may result from the lowered barometric pressure.

Gamow Bag
Scuba divers may suffer from a problem called the bends if they come to the surface too quickly.

The bends are treated by placing the diver in a hyperbaric (decompression) chamber.

Mountain climbers suffering from AMS can be treated in a similar device called a Gamow Bag.

A Gamow Bag is a portable sealed chamber with a pump.

The person with AMS is placed in the chamber and it is inflated. The Gamow Bag full of air effectively increases the concentration of oxygen molecules inside the bag. This simulates a descent to a lower altitude.

This is a temporary measure that can be used to reset a climber's body chemistry to a lower altitude. The effects last for up to 12 hours, which is usually enough time to get him/her down to a lower altitude.

High Altitude Pulmonary Edema (HAPE)
High altitude pulmonary edema (HAPE) is the build-up of fluid in the lungs. The fluid fills the alveoli reducing the gas exchange surface. This prevents the efficient diffusion of oxygen from the air into the blood. The effects would be similar to those experienced by a person with pneumonia. It is the most common cause of death at high and extreme altitudes.

The cause(s) of HAPE are not clearly understood. One theory suggests an uneven flow of blood to the lungs that creates very high blood pressure in some alveolar capillaries. The tiny holes in capillaries in these areas get larger and start to leak plasma (fluid). The fluid builds up in the lungs, which reduces the area for gas exchange.
JUMP TO: Human Respiratory System

The symptoms of HAPE include: shortness of breath, tightness in the chest, marked fatigue, weakness, and a persistent productive cough bringing up a white, watery, or frothy fluid. Confusion and irrational behavior are signs of a lack of oxygen to the brain. As the condition becomes worse, the level of oxygen in the blood stream decreases. If not treated quickly the person will die.

The treatment is immediate descent and evacuation.

High Altitude Cerebral Edema (HACE)
High altitude cerebral edema (HACE) results from swelling of the brain tissue from fluid leakage. This is usually thought of as the more severe form of AMS. HACE, if untreated, is a fatal disorder. HACE is suspected in any person who has difficulty keeping up with the group or has mental changes. For example, a person with HACE will have difficulty following simple instructions.

Symptoms include headache, loss of coordination, weakness, decreasing levels of consciousness, disorientation, memory loss and hallucinations. If not treated, the person will fall into a coma and die. Treatment is immediate descent and evacuation to a medical facility.

Summary
High altitude can have a dramatic impact on the human body. The effect of decreased partial pressure of oxygen at higher altitudes can be fatal if not treated properly. Each individual adapts at a different rate. Acclimatization is often accompanied by fluid loss so the ingestion of liquids is essential (three to four litres per day).

The process of acclimatization is one of the major decisions Byron and his medical advisors must take into account on a daily basis. Planning and awareness can greatly decrease the chances of high altitude sickness. Recognizing early symptoms can result in the avoidance of more serious consequences of high altitude sickness.