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Chemical Control of Breathing and Plasma pH Quiz
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A 22-year-old woman presents with prolonged vomiting. Blood tests show a raised plasma pH consistent with metabolic alkalosis. Which change in potassium handling best explains the expected plasma potassium abnormality?
- A Potassium moves into cells and renal excretion falls, so plasma potassium rises You picked ✕
- B Potassium moves out of cells and renal excretion rises, so plasma potassium falls You picked ✕
- C There is no significant shift of potassium, because vomiting affects only gastric acid You picked ✕
- D Potassium moves into cells and renal excretion rises, so plasma potassium falls You picked ✓
- E Potassium moves out of cells and renal excretion falls, so plasma potassium rises You picked ✕
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The answer is D: Potassium moves into cells and renal excretion rises, so plasma potassium falls
Explanation
Why D is correct
See Metabolic Acidosis and Alkalosis. Option D is correct because metabolic alkalosis tends to cause hypokalaemia, a low plasma potassium, by two mechanisms. First, potassium and hydrogen ions are exchanged across cell membranes, so when the plasma is alkaline, hydrogen ions leave the cells and potassium moves in, which lowers the plasma level (the reciprocal cation shift, meaning that one positive ion moves in as another moves out). Second, the kidneys excrete more potassium. MSD describes gastric suction or vomiting removing volume and hydrochloric acid, causing renal potassium losses as a result of metabolic alkalosis and of aldosterone stimulation by volume depletion, because aldosterone and metabolic alkalosis both cause the kidneys to excrete potassium. Prolonged vomiting also causes direct loss of fluid and acid from the stomach, which is why the woman is alkalotic in the first place. Hypokalaemia matters because it causes muscle weakness. Hypokalaemia can itself sustain metabolic alkalosis, so correcting it is part of treatment. A useful memory aid is that alkalosis pulls potassium into cells and sends it out in the urine.
- Low potassium: metabolic alkalosis tends to cause hypokalaemia.
- Cell shift: hydrogen ions leave the cells and potassium moves in.
- Kidneys: excrete more potassium.
The pairing to remember is that alkalosis causes low potassium, and acidosis tends to cause high potassium.
Why the other options are incorrect
- A. Potassium moves into cells and renal excretion falls, so plasma potassium rises: Potassium moving into cells with reduced renal excretion would keep the plasma potassium roughly stable at best. In metabolic alkalosis the kidneys excrete more potassium, not less, so hyperkalaemia does not follow. Your answer
- B. Potassium moves out of cells and renal excretion rises, so plasma potassium falls: Potassium moving out of cells would raise the plasma level, which is the pattern of acidosis. In alkalosis, hydrogen ions leave cells and potassium enters, so the shift is in the opposite direction. Your answer
- C. There is no significant shift of potassium, because vomiting affects only gastric acid: Systemic alkalosis changes the movement of ions across cell membranes in the whole body, not only in the stomach. Vomiting causes both volume loss and alkalosis, and both change potassium handling. Your answer
- E. Potassium moves out of cells and renal excretion falls, so plasma potassium rises: Potassium shifting out of cells with less renal excretion, leading to hyperkalaemia, is the pattern in acidosis, where excess hydrogen ions enter the cells in exchange for potassium. This patient is alkalotic, so the pattern is reversed. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Which statement best explains why the pH of plasma must be tightly regulated in humans?
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2
A 19-year-old breathing room air at sea level has an arterial carbon dioxide tension of 6.4 kPa and an oxygen tension of 12.0 kPa. The laboratory reference intervals are 4.7 to 6.0 kPa for carbon dioxide and 11 to 13 kPa for oxygen. Which description best fits these arterial gas results?
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