When ascending to higher elevations, the air changes significantly. While oxygen remains about 21% of the atmosphere, the challenge at altitude is a decrease in the absolute number of oxygen molecules available with each breath. This reduced oxygen availability directly impacts how the human body functions and adapts.
Understanding Oxygen’s Behavior at Altitude
Lower oxygen availability at higher altitudes stems from a decrease in atmospheric pressure. At sea level, the weight of the atmosphere exerts pressure, pushing air molecules, including oxygen, closer together. As elevation increases, the column of air above shortens, leading to less atmospheric pressure and a reduction in air density.
This reduction in air density means each inhaled breath contains fewer oxygen molecules. The concept of “partial pressure of oxygen” refers to the pressure exerted by oxygen alone within the total atmospheric pressure. At higher altitudes, the total atmospheric pressure drops, which directly lowers the partial pressure of oxygen, making it harder for the body to absorb.
For instance, at sea level, atmospheric pressure is around 760 millimeters of mercury (mmHg), with a partial pressure of oxygen of about 159 mmHg. At 10,000 feet (about 3,000 meters), atmospheric pressure drops to roughly 523 mmHg, and partial pressure of oxygen falls to approximately 110 mmHg. On Mount Everest’s summit, over 29,000 feet, atmospheric pressure plummets to about 253 mmHg, with the partial pressure of oxygen being a mere 53 mmHg, significantly challenging human respiration.
Your Body’s Response to Thin Air
Upon ascending to higher altitudes, the human body immediately recognizes reduced oxygen availability, a condition known as hypoxia. Specialized chemoreceptors in the carotid arteries and aorta detect the drop in blood oxygen levels. This detection triggers rapid physiological adjustments.
One noticeable initial response is an increased breathing rate, called hyperventilation. The body compensates for less oxygen per breath by taking more breaths per minute, aiming to bring more air into the lungs. This increased ventilation helps maintain blood oxygen levels, though it can also lead to a decrease in carbon dioxide.
Simultaneously, heart rate accelerates as the body circulates oxygenated blood more rapidly. The heart pumps harder and faster to deliver limited oxygen to tissues and organs. These immediate adjustments are temporary solutions, helping the body cope as it begins adaptation.
Acclimatization and Preventing Altitude Sickness
Acclimatization is the gradual process by which the body adapts to lower oxygen levels at high altitudes over several days or weeks. This adaptation involves a series of physiological changes that enhance oxygen delivery and utilization. One significant long-term adjustment is an increase in the production of red blood cells, which are responsible for carrying oxygen from the lungs to the rest of the body.
The kidneys play a role in this by releasing erythropoietin (EPO), a hormone that stimulates the bone marrow to produce more red blood cells. Over time, the body also develops a greater density of capillaries, which are tiny blood vessels that facilitate oxygen exchange in tissues. Additionally, cells become more efficient at extracting and using oxygen from the blood, even with reduced supply.
Despite these adaptive capabilities, rapid ascent to high altitudes can overwhelm the body, leading to various forms of altitude sickness. Acute Mountain Sickness (AMS) is the most common, presenting with symptoms such as headache, nausea, dizziness, fatigue, and difficulty sleeping, often resembling a bad hangover. AMS can progress if ignored.
More severe, though less common, conditions include High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE). HACE involves fluid accumulation in the brain, leading to severe headache, confusion, loss of coordination, and potentially coma. HAPE involves fluid in the lungs, causing extreme shortness of breath, cough, and a gurgling sound in the chest. Both HACE and HAPE are life-threatening medical emergencies requiring immediate descent and treatment.
Preventing altitude sickness primarily involves a gradual ascent, allowing the body sufficient time to acclimatize. A common recommendation is to ascend no more than 1,000 to 1,500 feet per day above 8,000 feet, with rest days every 2,000 to 3,000 feet. Staying well-hydrated, avoiding alcohol and sedatives, and consuming a diet rich in carbohydrates can also aid prevention. Recognizing the early symptoms of AMS and descending if they worsen is important for safety.