Air composition remains constant, with oxygen consistently making up about 20.9% of the atmosphere, regardless of elevation. The feeling of “thin air” is not due to a change in this percentage but rather to a decrease in the absolute number of oxygen molecules available with each breath. This reduced availability is a direct consequence of the diminishing air pressure at higher altitudes, which fundamentally changes how the body acquires oxygen.
Atmospheric Pressure and Oxygen Availability
The drop in barometric pressure is the primary challenge at high altitude. At sea level, this pressure is high. As elevation increases, the air column above shrinks, causing the total barometric pressure to fall dramatically. At 5,500 meters, for example, the barometric pressure is only about half of what it is at sea level.
This overall pressure drop translates directly into a lower partial pressure of oxygen (\(P_{O_2}\)), which is the specific pressure exerted by oxygen molecules alone. Since the percentage of oxygen is constant at 20.9%, the \(P_{O_2}\) decreases in direct proportion to the falling barometric pressure. This lower \(P_{O_2}\) means there is a weaker “driving force” pushing oxygen from the lungs into the bloodstream. Consequently, each lungful contains fewer oxygen molecules, leading to insufficient oxygen delivery to the tissues, a condition known as hypoxia.
Immediate Physiological Adjustments
Upon ascent, the body initiates acute responses to combat the lack of oxygen. Specialized sensory cells called chemoreceptors, primarily located in the carotid bodies near the neck, detect the drop in arterial oxygen content. This detection triggers hyperventilation, an increase in breathing rate and depth. While hyperventilation raises the oxygen level in the lungs, it also causes a rapid decrease in blood carbon dioxide (\(CO_2\)). Low \(CO_2\) leads to respiratory alkalosis, a temporary increase in blood pH. The kidneys eventually excrete bicarbonate to rebalance the blood pH, allowing the hyperventilation to continue without inhibition, a process that takes hours to days. Simultaneously, the heart rate increases to boost cardiac output, circulating the limited available oxygen to the tissues more rapidly.
Long-Term Biological Adaptation
If exposure to altitude lasts for days or weeks, the body shifts to long-term acclimatization. The sustained lack of oxygen stimulates the kidneys to release erythropoietin (EPO). EPO acts on the bone marrow, triggering erythropoiesis, the accelerated production of new red blood cells (RBCs). The increased number of red blood cells enhances the blood’s oxygen-carrying capacity, compensating for the low \(P_{O_2}\). Furthermore, the body makes microscopic changes at the tissue level, such as increasing the density of capillaries to improve blood flow to muscle cells. Cellular machinery also adapts, with changes occurring in mitochondria to improve the efficiency of oxygen utilization inside the cells. These chronic, cellular changes are slower than the initial respiratory and cardiac adjustments, taking weeks to fully develop.
Recognizing Altitude-Related Illnesses
When the body’s compensatory mechanisms are overwhelmed, or if ascent is too rapid, it can lead to altitude-related illnesses. The mildest and most common is Acute Mountain Sickness (AMS), typically presenting with headache, nausea, fatigue, and loss of appetite. These symptoms usually appear within the first day of reaching a new high elevation.
Severe Illnesses
More severe, and potentially life-threatening, forms include High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE). HACE is a progression of AMS where swelling occurs in the brain, indicated by severe symptoms like confusion, disorientation, and ataxia (loss of coordination). HAPE involves fluid accumulation in the lungs, manifesting as breathlessness even at rest, a persistent cough, and chest tightness. Recognizing these symptoms quickly is paramount, as descent is the most effective treatment for all forms of altitude sickness.