The human body is tuned to the environment of sea level, where oxygen is plentiful and atmospheric pressure is stable. Ascending to higher elevations introduces environmental changes that challenge nearly every physiological system, affecting performance and health. The body’s response to this stress determines whether an individual adapts or suffers from altitude-related illness.
Defining Altitude Zones
Medical experts categorize elevation into distinct zones. High altitude begins at about 1,500 meters (approximately 4,900 feet), where the first noticeable changes in exercise performance may occur. Very high altitude ranges from 3,500 to 5,500 meters (about 11,500 to 18,000 feet), a zone where acute illness is common and acclimatization is essential for safe activity. Beyond 5,500 meters (18,000 feet) lies extreme altitude, an environment where long-term survival is impossible for humans, even with full acclimatization, due to severely limited oxygen availability.
The Mechanics of Thin Air
The primary challenge at elevation is the reduction in available oxygen, a direct consequence of decreased barometric pressure. As elevation increases, the weight of the air pressing down decreases significantly. Although oxygen still makes up 20.9% of the air volume, the molecules are farther apart, meaning every breath contains fewer oxygen molecules than at sea level.
This reduction in atmospheric pressure directly lowers the partial pressure of inspired oxygen (PiO2), which drives oxygen movement from the lungs into the bloodstream. For instance, at 5,500 meters, the partial pressure of oxygen is only about half of its value at sea level. This state of reduced oxygen availability to the body’s tissues is termed hypobaric hypoxia, the root cause of all altitude-related physiological stress.
The Body’s Physiological Response
Upon rapid ascent, the body immediately attempts to compensate for the sudden drop in inspired oxygen through protective responses. The carotid bodies, small sensors in the neck’s arteries, detect the fall in blood oxygen levels and trigger an increase in the rate and depth of breathing, a process known as hyperventilation. This initial hyperventilation helps raise the oxygen concentration in the lungs, but it also causes a temporary imbalance in the body’s acid-base chemistry by excessively blowing off carbon dioxide.
The cardiovascular system also reacts quickly by increasing the heart rate and the amount of blood pumped per minute, which is intended to maintain oxygen delivery to the tissues. Over the next few days, the kidneys begin to excrete bicarbonate in the urine to correct the initial chemical imbalance caused by hyperventilation. This allows the breathing rate to increase further and more effectively. This sustained physiological adjustment is known as acclimatization.
Over weeks at sustained elevation, the body initiates profound, long-lasting adaptations. Decreased oxygen stimulates the kidneys to produce the hormone erythropoietin, which signals the bone marrow to increase the production of red blood cells. This rise in red blood cells increases the blood’s oxygen-carrying capacity, providing a more permanent solution to the hypoxic environment. Additionally, the density of capillaries within muscle tissue may increase, improving the efficiency of oxygen diffusion from the blood into the working cells.
Recognizing Altitude Illnesses
When the body fails to acclimatize successfully or the ascent is too fast, altitude illness can develop, progressing from mild to life-threatening conditions. The most common condition is Acute Mountain Sickness (AMS), which typically presents like a severe hangover. Symptoms include headache, nausea, fatigue, and difficulty sleeping. These usually appear within 6 to 24 hours of arrival and often resolve naturally if the individual rests and avoids ascending further.
Two more severe and potentially fatal forms of altitude illness require immediate recognition and response. High Altitude Cerebral Edema (HACE) involves fluid accumulation and swelling of the brain, often progressing from untreated AMS. Signs include a severe headache unresponsive to pain relievers, confusion, and a loss of coordination, demonstrated by an inability to walk a straight line heel-to-toe.
The other severe condition is High Altitude Pulmonary Edema (HAPE), which is a buildup of fluid in the lungs. HAPE can occur without prior AMS symptoms, and it is signaled by increasing shortness of breath even when resting, a persistent, dry cough, and chest tightness. In advanced stages, the cough may produce pink or frothy sputum, indicating a medical emergency that requires immediate descent and medical intervention.