Ascending to high elevations often brings with it a noticeable and uncomfortable sensation of breathlessness, a condition known medically as dyspnea. This difficulty in breathing typically begins for unacclimatized individuals at altitudes around 8,000 feet (2,400 meters) above sea level, a threshold commonly defined as the start of high altitude. The primary cause is not a change in the air’s composition but rather a fundamental shift in the physical properties of the surrounding atmosphere. Understanding this phenomenon requires looking beyond the simple idea of “thin air” to the underlying physics.
The Role of Barometric Pressure
The fundamental physical reason for breathing difficulty at high altitude is the progressive drop in barometric, or atmospheric, pressure. Barometric pressure is the weight of the entire column of air pressing down on the Earth’s surface, and as one climbs higher, there is less air overhead, causing this pressure to fall. For example, the total air pressure at the summit of Mount Everest is only about one-third of the pressure measured at sea level.
Air composition remains constant, meaning oxygen still makes up approximately 21% of the total air volume regardless of altitude. However, the lower total pressure means the partial pressure of oxygen (PO2) decreases proportionally. Partial pressure is the individual pressure exerted by a single gas within a mixture, and it is the true measure of oxygen availability for the human body.
Impaired Oxygen Transfer in the Lungs
The low partial pressure of oxygen in the inspired air directly compromises the body’s ability to pull oxygen into the bloodstream. Gas exchange in the lungs relies on a steep pressure gradient, which is the difference in oxygen pressure between the air inside the tiny lung sacs, called alveoli, and the blood flowing through the surrounding capillaries. Oxygen naturally moves from an area of higher pressure to an area of lower pressure.
At sea level, the alveolar oxygen pressure is high enough to create a substantial gradient, allowing oxygen to rapidly diffuse into the blood and fully saturate the hemoglobin. As altitude increases, the inspired partial pressure of oxygen drops, which flattens this necessary gradient. This results in a much slower and less efficient diffusion of oxygen into the blood. Consequently, the blood leaving the lungs is inadequately saturated with oxygen, a state leading to systemic hypoxia, or insufficient oxygen supply to the body’s tissues.
Acute Physiological Responses
Upon sensing the reduced oxygen content in the blood, the body initiates immediate, short-term physiological adjustments to counteract the hypoxia. Specialized sensory organs, known as chemoreceptors, detect the drop in oxygen and immediately signal the brain to increase breathing. This response leads to hyperventilation, where an individual breathes faster and deeper than normal.
This increased ventilation helps to raise the oxygen pressure within the alveoli slightly, but it also causes the body to exhale more carbon dioxide than usual. The rapid removal of carbon dioxide lowers its concentration in the blood, leading to a change in blood pH called respiratory alkalosis. Simultaneously, the sympathetic nervous system activates, causing an immediate increase in heart rate and cardiac output. This faster heart rate, known as tachycardia, serves to circulate the limited oxygen-carrying blood more quickly throughout the body.
Acclimatization: Long-Term Biological Adjustments
If the body remains at high altitude for an extended period, it begins a slower, more profound process of adaptation called acclimatization, which takes days or weeks. One of the most significant long-term adjustments involves the production of red blood cells. The kidneys sense the chronic low oxygen levels and release the hormone erythropoietin (EPO), which stimulates the bone marrow to produce new red blood cells.
This increase in red blood cell count, or polycythemia, enhances the blood’s capacity to transport oxygen, helping to compensate for the lower saturation percentage. The kidneys also work to correct the initial respiratory alkalosis by increasing the excretion of bicarbonate, which helps normalize the blood’s pH and allows the hyperventilation to continue more effectively. Over time, changes also occur at the tissue level, including an increase in the density of capillaries to shorten the distance oxygen must travel to reach the cells.