A 24-year-old man has passed up to twelve watery stools a day for five days after returning from a trek in Nepal. He is thirsty and weak. His pulse is 118/min and his blood pressure 92/58 mmHg. His hands are cool and capillary refill takes four seconds. His abdomen is soft and not tender. Results on arrival: arterial pH 7.29; arterial carbon dioxide tension 26 mmHg; serum sodium 139 mmol/L; serum potassium 3.1 mmol/L; serum chloride 115 mmol/L; serum bicarbonate 12 mmol/L; serum albumin 41 g/L.
Which one of the following mechanisms best accounts for this man's arterial pH?