‘Altitude sickness’ is caused at high altitude due to
the low partial pressure of oxygen.
Understanding Altitude Sickness at High Altitudes
Altitude sickness, also known as mountain sickness, occurs when a person ascends to high altitudes too quickly. This condition is caused by the change in atmospheric conditions present at higher elevations compared to sea level.
Atmospheric Conditions and Partial Pressure of Oxygen
As altitude increases, the total atmospheric pressure decreases. While the percentage of oxygen in the air remains approximately 21% at all altitudes, the absolute amount of oxygen molecules in a given volume of air decreases due to the lower pressure. This leads to a reduced partial pressure of oxygen.
Partial pressure is the pressure exerted by a specific gas within a mixture of gases. According to Dalton's law of partial pressures, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of each individual gas in the mixture.
Mathematically, the partial pressure of oxygen (\(P_{O_2}\)) can be calculated as:
\( P_{O_2} = \text{Fraction of } O_2 \times \text{Total Atmospheric Pressure} \)
At sea level, the total atmospheric pressure is about 760 mmHg. The fraction of oxygen is about 0.21.
\( P_{O_2} \text{ at sea level} \approx 0.21 \times 760 \text{ mmHg} \approx 160 \text{ mmHg} \)
At a high altitude, like the summit of Mount Everest (about 8,848 meters), the total atmospheric pressure is much lower, around 253 mmHg.
\( P_{O_2} \text{ at Everest summit} \approx 0.21 \times 253 \text{ mmHg} \approx 53 \text{ mmHg} \)
This significant drop in the partial pressure of oxygen means that with each breath at high altitude, fewer oxygen molecules enter the lungs and consequently fewer are available to diffuse into the bloodstream.
Why Low Partial Pressure of Oxygen Causes Altitude Sickness
The human body requires a sufficient amount of oxygen to function properly. When the partial pressure of oxygen in the inhaled air is low, the driving force for oxygen to move from the lungs into the blood is reduced. This leads to lower oxygen saturation in the blood, a condition called hypoxemia.
The symptoms of altitude sickness, such as headache, nausea, dizziness, fatigue, and shortness of breath, are the body's response to this lack of sufficient oxygen. In severe cases, it can lead to high-altitude pulmonary edema (HAPE) or high-altitude cerebral edema (HACE), which are life-threatening conditions.
Analyzing the Options
Let's evaluate the given options based on our understanding of high altitude physiology and altitude sickness:
- Option 1: The high partial pressure of oxygen. This is incorrect. At high altitudes, the total atmospheric pressure is lower, and therefore the partial pressure of oxygen is lower, not higher.
- Option 2: The low partial pressure of oxygen. This is correct. As explained above, the reduced total atmospheric pressure at high altitudes leads to a low partial pressure of oxygen, which is the primary cause of altitude sickness.
- Option 3: Low level of haemoglobin. While the body does adapt to high altitude over time by producing more haemoglobin (a process called acclimatization), a low level of haemoglobin is not the *cause* of altitude sickness. In fact, individuals with pre-existing low haemoglobin (anemia) might be more susceptible to altitude sickness, but the root cause at high altitude is the environment's low oxygen availability.
- Option 4: The high partial pressure of carbon dioxide. This is incorrect. While changes in breathing patterns at high altitude can affect CO\(_2\) levels in the blood, the primary environmental factor causing sickness is the low partial pressure of oxygen, not a high partial pressure of carbon dioxide in the air.
Therefore, the main cause of altitude sickness at high altitude is the low partial pressure of oxygen.
Revision Table: Altitude Sickness
| Factor | Condition at High Altitude | Impact on Oxygen Availability | Relevance to Altitude Sickness |
|---|---|---|---|
| Total Atmospheric Pressure | Decreased | Reduces partial pressure of all gases, including oxygen. | Directly contributes to low partial pressure of oxygen. |
| Percentage of Oxygen in Air | Remains ~21% | Percentage is constant, but the absolute amount of O\(_2\) molecules in a given volume decreases. | Does not explain the problem; partial pressure is key. |
| Partial Pressure of Oxygen (PO\(_2\)) | Decreased | Reduces driving force for oxygen diffusion into blood. | Primary cause of altitude sickness. |
| Haemoglobin Levels | Initially normal; increases with acclimatization | Lower oxygen uptake initially; improved with acclimatization. | Low levels can worsen symptoms but aren't the cause; body adapts by increasing it. |
Additional Information: Acclimatization and Altitude Sickness Prevention
Acclimatization is the process by which the body adjusts to the lower partial pressure of oxygen at high altitudes. This process takes time and involves several physiological changes, including:
- Increased breathing rate and depth (hyperventilation)
- Increased production of red blood cells over days/weeks to carry more oxygen
- Changes in the affinity of haemoglobin for oxygen
- Increased capillary density in muscles
- Increased production of certain enzymes
To prevent altitude sickness, it is crucial to ascend gradually, allowing the body time to acclimatize. Recommendations often include:
- Slow ascent, especially above 2500 meters (8000 feet).
- Taking rest days or "altitude days" during ascent.
- Avoiding strenuous activity upon arrival at high altitude.
- Staying well-hydrated.
- Avoiding alcohol and sedatives.
- Considering prophylactic medication like Acetazolamide (Diamox) in some cases, under medical advice.
Understanding the role of low partial pressure of oxygen is key to appreciating why these preventative measures are effective.
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