ByronSmith.ca Everest 2000 - Educational Program: Phase3

Energy and the Human Body Background Material

Circulartory System: Blood pressure | High Altitude

Blood Pressure
When the left ventricle contracts (systole) it sends blood under high pressure into the aorta and the blood surges into the arteries. The walls of the arteries are elastic, and the pulse wave stretches them. When the ventricles relax (diastole) the pressure in the arteries falls, but the elastic recoil of the previously stretched artery walls maintains some pressure on the blood. There is a regular cycle of pressure in the larger arteries, which reaches its high point during systole and a low point during diastole.

Sphygmomanometer
Measuring blood pressure
From "Ultimate Visual Dictionary of Science," Stoddart 1998.
When a doctor or nurse takes your blood pressure they use a tool called a sphygmomanometer. They attach a cuff to your upper arm and inflate it. Then the doctor or nurse uses a stethoscope to listen for the sound made by the blood trying to force its way through the constricted artery.

They get two readings - systolic and diastolic pressures. The usual blood pressure for adults is approximately 120/70. These numbers are measured in millimetres of mercury (mm Hg). The 120 mm Hg represents the highest pressure during systole. The 70 mm Hg represents the lowest pressure still in the arteries during diastole. Note that these pressures apply to the upper arm only and would be different when taken elsewhere.

If you exercise, your blood pressure will increase. However, it drops back to your normal value soon after you rest.

There are five factors that affect blood pressure:
1. The volume of blood in the body. If the amount of blood drops, the pressure in the system drops because of the decrease in volume.

2. The heart rate. The faster the heart pumps blood, the greater the pressure which is built up. The pressure falls as the heart rate decreases, especially during rest or sleep.

3. The size of the arteries. When the arteries become bigger in diameter, the volume of the vessels increases and the pressure falls. If the arteries get smaller, pressure is built up because of the extra resistance to blood flow.

4. Elasticity. The walls of the arteries must be flexible and elastic. They must be able to expand as a pulse of blood is forced out of the heart. The arteries must also be able to contract once the wave of pulse passes. If they can not stretch in this way, they are described as hardened.

5. Viscosity of the blood. Viscosity refers to the thickness of the blood: thick, sticky fluids flow less readily than thin watery liquids. The balance between the number of red blood cells and the amount of plasma present is one factor that controls the viscosity of blood. This is very important to mountain climbers. Remember, as the climbers acclimatize to the amount of oxygen in the air, their bodies respond by producing more red blood cells.

Blood clots
The increased number of red blood cells helps the climber get more oxygen to their muscles. However, more red blood cells increase the viscosity of the blood and can become a problem. If the blood becomes thick it increases the chances of clotting. A clot that gets dislodged can float around in blood vessels and may block tiny capillaries. If the clot blocks blood vessels in the brain a stroke occurs. If the blood clot plugs capillaries in the heart, the person will suffer a heart attack. Blood clots in the lung capillaries are called pulmonary embolisms.
JUMP TO: Red blood cell production

Blood pressure at altitude
Byron and the other members of the Everest 2000 team will likely experience higher blood pressure as their bodies attempt to adjust to the lower levels of oxygen available at higher altitudes.

Most climbers will experience increased hypertension (blood pressure) in their pulmonary (lungs) circulation (the blood pressure in their pulmonary arteries will increase).

Blood pressure | Circulatory system & high altitude | Top

High Altitude and the Circulatory System
The height of Mt. Everest has been measured at 8,850 m (29, 035 ft) above sea level. The human body can adapt, to a limited extent, to high altitude. However, there is a limit to how high a person can climb before the human body is no longer able to cope with the reduced air pressure.

High altitude affects both the circulatory system and the human respiratory system.

The Atmosphere
The Earth is surrounded by a thick blanket of gases called the atmosphere. The atmosphere regulates the temperature on the Earth's surface. As well, the atmosphere provides most living organisms with life sustaining gases of oxygen and carbon dioxide.

The outer reaches of the atmosphere reach hundreds of kilometres above the surface of the Earth. Most of the gases, however, are squeezed into the lowest 15 kilometres or so in a region called the troposphere. Only the troposphere contains water vapour and enough air for humans to breathe and survive. Even the air in the troposphere gets much thinner as you go up which is why Byron will use supplemental oxygen as he summits Mt. Everest.

Gases of the Atmosphere
The air is composed of many gases. The following chart outlines a few of the gases found in the troposphere:

Gas:

Nitrogen

Oxygen

Argon

Carbon dioxide

Other gases

Percentage by volume:

78

21

0.93

0.03

less than 1%

Another normal component of the atmosphere is water vapour. Its concentration varies greatly, averaging from one to three per cent. Water vapour and carbon dioxide are vital to life on Earth. Both of these gases help keep the planet warm by preventing heat from escaping into space. Water vapour also condenses out of the air to form clouds, and so gives the rain (water) that is critical to all living things.

Definition of High Altitude
In medical terms, there are three categories of altitude which climbers and others must understand.

High Altitude (1,500 to 3,500 metres above sea level). Humans who ascend too quickly to this altitude will have increased breathing rate (ventilation) at rest and decreased exercise performance. There is minor impairment of arterial oxygen and the saturation of oxygen in arterial blood (SaO2) drops to about 90%. Mountain sickness is common in people who make a rapid ascent about 2,500 metres.

Very High Altitude (3,500 to 5,500 metres above sea level). Humans who ascend to this altitude too quickly will likely develop symptoms of a lack of oxygen (hypoxia) to the brain. Extreme hypoxia may occur during exercise, sleep and altitude illness. The maximum saturation of oxygen in arterial blood (SaO2) is lower than 90%. This is the most common range for serious altitude illness.

Extreme High Altitude (above 5,500 metres). Humans who ascend to this altitude and stay above this altitude will experience a progressive deterioration of normal bodily functions. There is marked hypoxia at this altitude and deterioration eventually outstrips acclimatization. A number of serious can develop in people who are not properly acclimatized or simply do not have the genetic predisposition to stay at these altitudes. These disorders are discussed in Phase 4 of the Everest 2000 education program.

The primary cause for the problems experienced by climbers at Extreme High Altitude is not, as you will see, the concentration of oxygen molecules in the atmosphere, but the reduction in air pressure the higher you climb.

Air Pressure
The atmosphere exerts pressure on the Earth in all directions. The pressure is caused by the constant bombardment of air molecules. Air pressure gradually decreases as you go higher in the atmosphere. You may have experienced this when driving through the mountains or if you have been in an airplane - when your ears "pop."

According to the particle theory, as you climb higher in the atmosphere, fewer air particles press against the climber's body. On average, the atmosphere exerts a force of 10 Newtons (about the weight of a one kg object) on every square centimetre on the Earth's surface at sea level.

Air pressure is measured by an instrument called a barometer. The standard still used today is how high a pool of mercury can be lifted by the pressure of the air. At sea level, atmospheric pressure forces the mercury 760 millimetres (760 mmHg) up a thin vacuum tube.

High Altitude and Atmospheric Pressure
As Byron climbs Mt. Everest, he will be experiencing very low atmospheric pressures. For example, at 3,657 metres (12,000 feet) air pressure is only 483 mmHg. This decrease in total atmospheric pressure means, at this altitude, there are 40 per cent fewer oxygen molecules per breath,compared to sea level. It is important to note that the air at this altitude is still 21 per cent oxygen. As the air pressure decreases, the amount of oxygen available in a single breath becomes significantly less.

The movement of gases into and out of the alveoli (air sacs) in the lungs depends on the process of diffusion. Diffusion is the movement of a substance from a region of greater pressure to a region of lower pressure. Each gas in the 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.

At sea level, the partial pressure of oxygen is 159 mmHg. This can be calculated by multiplying the total atmospheric pressure by the percentage of each gas in the atmosphere.

Calculation: P0₂ = 760 mmHg x 0.21 = 159 mmHg

The following chart summarizes the approximate barometric pressure and the partial pressure of oxygen at different altitudes:

Altitude

Barometric Pressure (mmHg)

Partial Pressure of O₂ in the air (mmHg)

0 (sea level)

760

159

3,000 metres
(10,000 ft)

523

110

6,000 metres
(20,000 ft)

349

73

9,100 metres
(30,000 ft)

226

47

Diffusion of oxygen takes place because of the collision of molecules. In areas of higher concentration there is a greater number of collisions. The net effect of this greater number of collisions is the movement from a higher concentration to a lower concentration.

If we add to the chart above, we can see that there is a marked decrease in the amount of oxygen in the alveoli (airs sacs in the lungs) at higher altitudes. This chart also indicates the percentage of hemoglobin saturated with oxygen at different altitudes.

The graph below shows the relationship between Partial Pressure of Oxygen in the Alveoli and the Arterial Oxygen Saturation at different altitudes.

Oxygen saturation

Graph provided by Wilderness Medical Society

The chart and graph clearly show that Byron is making a good decision by using supplemental oxygen (bottled oxygen) as he makes his final attempt for the summit of Mt. Everest. Without supplemental oxygen, only 60 per cent of Byron's hemoglobin would be carrying oxygen as he reached the summit of Mt. Everest (8,850 m or 29, 035 ft above sea level).

To learn more about Red Blood Cells and the role they play in acclimatization to high altitudes, JUMP TO: Red blood cells

Blood pressure | Circulatory system & high altitude | Top

	Phase 3 Overview

	Telecollaborative Project: Getting to Know You (Part 2) � Activity

	Theme 1: Goal Setting, Decision Making � Read your Way to Everest (Part 2) � What would you do? � Results and Submissions

	Theme 2: Energy and the Environment � Implementation � Division I activities � Challenge: Division II to IV � Results and Submissions

	Theme 3: Energy and the Human Body � Implementation � Background � Weather Watch activities � Heart Health activities � Stairway to Everest Challenge � Results and Submissions