The composition of the air remains consistent across all elevations, with oxygen making up about 21% of the atmosphere. Despite this constant percentage, the actual amount of oxygen molecules available for the body decreases significantly as elevation rises. This is due to the physical properties of the atmosphere. People visiting or moving to Denver, the “Mile High City,” will encounter a measurable reduction in the effective oxygen supply, necessitating complex and immediate adjustments within the human body to maintain normal function.
Understanding Air Pressure and Available Oxygen
The decrease in available oxygen at altitude is fundamentally a matter of physics concerning air pressure and density. As elevation increases, the column of air pressing down decreases, causing a reduction in barometric pressure. This lower pressure means the air is less dense, and gas molecules, including oxygen, are spaced further apart. Although oxygen still makes up 21% of the total, each breath contains fewer molecules of air overall.
The true physiological measure of oxygen availability is the partial pressure of oxygen (PO2). This pressure represents the driving force that pushes oxygen from the lungs into the bloodstream. According to Dalton’s Law of Partial Pressures, the PO2 is calculated by multiplying the total barometric pressure by the constant 21% oxygen fraction. A drop in total barometric pressure directly results in a proportional drop in the partial pressure of oxygen. This lower PO2 causes the body to experience oxygen deficiency, known as hypoxia.
The Specific Oxygen Quantification for Denver
Denver sits at an elevation of approximately 5,280 feet (1,609 meters), classified as moderate altitude. At this height, the average barometric pressure is significantly lower than the standard pressure measured at sea level. The air pressure in Denver is about 83% of the pressure found at sea level. This reduction in total pressure translates directly into a lower partial pressure of oxygen.
To quantify the difference, the partial pressure of oxygen (PO2) at sea level is approximately 159 millimeters of mercury (mmHg), dropping to about 132 mmHg in Denver. This 17% reduction means the air provides an effective oxygen level equivalent to breathing air that is only about 17.3% oxygen at sea level, rather than the true 21%. This decrease means the body must work harder to capture the necessary oxygen. For visitors, this reduced oxygen availability prompts the body’s initial physiological response.
Short-Term Physiological Responses to Altitude
Upon rapid ascent, the body’s first priority is to detect and counteract the reduced oxygen supply. Specialized sensory organs called carotid bodies, located in the neck’s major arteries, detect the drop in blood oxygen levels (hypoxemia) within minutes. These chemoreceptors send signals to the brainstem, triggering an increase in the rate and depth of breathing, known as hyperventilation.
This immediate increase in ventilation helps raise the oxygen level in the lungs, but it also causes the body to exhale more carbon dioxide than normal. The resulting drop in carbon dioxide lowers the blood’s acidity, a condition called respiratory alkalosis, which can temporarily inhibit the respiratory drive. The cardiovascular system responds by increasing the resting heart rate and the force of contraction, boosting cardiac output to circulate the limited oxygen more quickly.
For an unacclimatized person, these immediate responses can lead to noticeable, though typically mild, symptoms of acute mountain sickness. Common complaints include a mild headache, fatigue, and shortness of breath, particularly during physical exertion. Some individuals also experience disturbed sleep and mild nausea, with symptoms usually beginning within 12 to 36 hours of arrival.
Long-Term Acclimatization
If a person remains at the moderate altitude of Denver, the body initiates long-term adaptations to improve oxygen delivery efficiency. One of the first adjustments involves the kidneys, which work over several days to correct the initial respiratory alkalosis. The kidneys increase the excretion of bicarbonate into the urine, which helps restore the blood’s normal pH balance. This adjustment allows increased ventilation to continue without inhibition, further improving the oxygen level in the blood.
A more profound adaptation involves the hematological system. The reduced oxygen availability stimulates the kidneys to increase production of the hormone erythropoietin (EPO). EPO levels peak within the first one to three days of exposure, signaling the bone marrow to accelerate the production of red blood cells (erythropoiesis). This gradual increase in red blood cells and hemoglobin concentration enhances the blood’s overall capacity to carry oxygen, providing a long-term solution to the lower partial pressure of oxygen. Other slow-developing changes include increased density of capillaries supplying the muscles and a rise in myoglobin concentration, both of which facilitate the transfer and storage of oxygen within the tissues.