Final answer:
At high altitudes, like the 5000 meters reached by the mountaineer, arterial PCO2 will decrease and pH levels will increase due to hyperventilation. Initially, this results in respiratory alkalosis. Over time, the body can adjust, but the immediate effect is a rise in pH and a fall in PCO2.
Step-by-step explanation:
When a mountaineer reaches an altitude of 5000 meters, the arterial PCO2 (partial pressure of carbon dioxide) will decrease while the pH levels will increase. This is due to the lower atmospheric pressure at high altitudes, which reduces the partial pressure of oxygen. As the body responds to this hypoxia (inadequate supply of oxygen), the respiratory rate increases, leading to more CO2 being expelled from the body, thus reducing the arterial PCO2. In response to the decreased PCO2, the body will experience a respiratory alkalosis, as CO2 is a component of the body's acid-base balance, and the removal of CO2 raises blood pH levels.
The initial response to high altitude is typically an increase in pH (alkalosis) and a decrease in PCO2 due to hyperventilation. Over time, the body may acclimatize, and the kidneys will help to adjust the buffering capacity of the blood by excreting bicarbonate, which in turn could bring the pH back toward normal. However, this adjustment may take several days.
Therefore, the correct answer to the mountaineer's situation would be: B. The pH will rise and PCO will fall.