The human biological, physiological, cellular and physical make up is affected by our environment and genetics. This is as understated as saying the sky is blue. As a response and a change to environmental conditions, adaptation is vital to the survival of every organism. Adaptation results in the creation of beneficial characteristics and speciation, the creation of an entirely different type of species. When the new adaptive feature is no longer needed, it becomes vestigial.
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Change
An environmental change comes in different forms, it can be biological affecting cells, and it can be entirely physical, in this case, increased altitude.
The earth’s atmosphere serves a protective and structural function. It consists of a layer of gases contained within the earth’s gravity and has a mass of 5.15×1018 kg. With increased altitude, the atmospheric pressure decreases.
At sea level, the percentage of inspired oxygen in the atmosphere is 21%. While this value is unaffected by a change in altitude, it is the partial pressure of oxygen that is affected with increasing altitude.
The atmospheric pressure at sea level is 760mmHg and inspired oxygen is 21%.
The partial pressure of oxygen at sea level would be 760mmHg × 0.21. Therefore, with increasing altitude, the decrease in the partial pressure reduces the partial pressure of oxygen delivered to the tissues.
When the partial pressure of oxygen decreases from 100mmHg to 60mmHg, ventilation increases to match this decrease.
At an elevation of 50000 feet, the partial pressure of oxygen is 18mmHg, a considerably low value when compared with the 159mmHg at sea level. This value at 50000 feet is further diluted by water vapour and the exhaled carbon iv oxide whose partial pressure also decreases with the increased altitude.
The first response to a decreased low partial pressure of oxygen is by the arterial chemoreceptors that induce an increase in ventilation within seconds. This response can increase to five times normal with the first three days of exposure. But this action also causes an opposite cumulative effect because the increased ventilation also causes increased carbon iv oxide expiration that reduces blood pH. A reduction in pH inhibits the respiratory center.
But in the next few days, this opposing action fades away as the body’s need for oxygen overrides it.
Hypoxia: In the unacclimatized person, this decreased partial pressure of oxygen leads to hypoxia caused by decreased oxygen delivery to tissues. At a height of above 12000feet, this leads to drowsiness, lassitude, headache, nausea, and euphoria.
Acute mountain sickness: The symptoms include headache, anorexia, insomnia, and breathlessness. These are also linked to the hypoxic conditions. For adequate acclimatization and survival for mountain climbers, they are encouraged to ‘climb high and sleep low’ to ensure progressive acclimatization.
The range of physiological responses that leads to mountain sickness and pulmonary edema are as follows. The hypoxic conditions automatically cause blood to be shunted away from lesser priority areas in the pulmonary circulation. There is also pulmonary vasoconstriction that leads to pulmonary edema over time as pulmonary blood pressure is increased.
Tachycardia: One of the immediate response to high altitude is an increased heart rate to compensate for the hypoxic conditions.
Increased cardiac output: The cardiac output increases more than thirty percent in response to the increased red blood cell production and high blood volume.
In addition to the pulmonary vasoconstriction, the increased red blood cell production leads to increased blood viscosity. So, the right side of the heart works excessively. The combined effect of these responses in response to the hypoxia might lead to acute pulmonary edema.
Since the cerebral circulation is of high priority, the pulmonary vasoconstriction is countered by local vasodilation of the cerebral blood vessels. This causes an increased blood supply to the cerebral capillaries, elevating the pressure and causing fluid leakage into cerebral tissues. The resulting cerebral dysfunction can be fatal if the person is not returned to sea level.
Increased red blood cells and hemoglobin concentration
One of the principal triggers for increased red blood cell concentration is hypoxia. Erythropoietin is secreted which causes increased red blood cell concentration. As the hematocrit rises, the hemoglobin concentration also rises to facilitate improved oxygen transport.
Improved pulmonary capillary function and increased diffusion capacity.
This increased diffusion of oxygen through the pulmonary membrane to the blood occurs both during exercise and at high altitude and this is in response to the ‘oxygen hunger.’
The combined effect of increased ventilation, high pulmonary arterial blood pressure leads to increased pulmonary capillary blood volume that improves oxygen diffusion to the bloodstream.
Cardiac output
Following the increased cardiac output as an immediate response, over the next few days, the cardiac output decreases to normal since the hematocrit is also high.
Tissue capillary angiogenesis
This is particularly obvious in natives of high altitude regions. To improve the efficiency of oxygen delivery to the tissues, there is an increased growth in the number of systemic capillaries.
Subcellular adaptation
This adaptation is again common to natives of high altitude regions. The number of cellular mitochondria and oxidative enzymes are higher to use the lower partial pressure of oxygen efficiently.
Increased chest size
This chronic adaptation again applies to natives and those who have resided in high altitude regions for too long. Due to the increased cardiac output, the heart size might slightly increase. This effect combined with the increased ventilation might lead to an increase in chest size, especially in relation to overall body mass over time.
“Live-High, Train-Low"
This maxim has often been used as athletes as a ‘performance enhancer.’ The positive effects of dwelling at high at high altitudes produce beneficial advantages for athletes.
The increased red blood cell production, increased cardiac output, improved pulmonary efficiency and myoglobin and mitochondria concentrations help improve athletic performance. But when athletes return to sea level, the body systems ‘de-training’ or returning to normal might nullify these advantageous features. For this reason, some athletes ‘live at high altitude and train at low altitudes.’
Since 1975 the Denver Broncos own the best home record in the NFL. That is some record. But the city is located 5,280 feet above sea level. Visiting teams will have to puff and huff their way through an environment that would be considerably harsh to their body systems while the Denvers enjoy their home advantage. The Toronto Raptors could have used some of this help. Probably.
In May 2007, FIFA banned football matches at more than 2500 feet above sea level. This meant that countries like Bolivia, Ecuador and Colombia would be unable to host home games during FIFA world cup qualifiers. The reason for this ban was not far-fetched especially in light of this ‘advantage’ it gave these countries. In contrast, visiting teams from sea level had to exercise through the harsh conditions caused by the decreased partial pressure of oxygen.
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The ban was revoked in May 2008.
Players have been known to vomit during world cup qualifiers played at high altitude Bolivia. Recall one of the effects of hypoxic conditions at high altitude is nausea.
Human physiology is beautiful and powerful, especially in its attempt to maintain homeostasis regardless of the change in external conditions. Adaptation can also be behavioral. We change, shift and wiggle our way through societal expectation daily. Imagine if the adaptive process didn’t exist, survival would be impossible.
High Altitude football controversy
Denver's Edge: How Altitude Provides Their Teams With The Greatest Home-Field Advantage in Sports
Effect of high altitude on humans
Pressure with height
Atmosphere of the earth
Gas Exchange: Diffusion & Partial Pressure Gradients
Guyton, Arthur C, and John Hall, Textbook of Medical Physiology. Elsevier, 2000.
Oxygen at high altitude
