Internal balance of potassium after a meal
After a meal, internal balance is the rapid movement of potassium from the extracellular fluid (the fluid outside cells) into cells, so plasma potassium is held steady within minutes.
Remember
Cells take up most of the extra potassium within minutes, while the kidneys remove the surplus from the body over the following hours: shift now, flush later.
After a potassium-rich meal, potassium absorbed from the gut enters the extracellular fluid and raises the plasma potassium level. Within minutes, most of the extra potassium is moved into cells. This is the internal balance of potassium: potassium shifts between the extracellular fluid and the intracellular fluid (the fluid inside cells).
Cells can absorb a large load quickly because about 98% of the body's potassium is already stored inside them. A rise in potassium stimulates insulin release, and insulin makes the Na-K-ATPase pump (a protein in the cell membrane that uses energy to push potassium into cells) more active. The speed matters: a high potassium level (hyperkalaemia) alters the electrical charge across cell membranes and can cause dangerous heart rhythm problems.
The kidneys then deal with the surplus through the external balance. After a small delay they excrete (remove from the body) potassium in the urine, which takes around 6 to 12 hours. A shift into cells does not remove potassium from the body. Cells store the potassium temporarily, and the kidneys control the total. In short: shift now, flush later.
Key facts
- Timescale
- internal balance acts over minutes, whereas renal excretion takes around 6 to 12 hours.
- Where potassium is stored
- about 98% of body potassium is inside cells and only about 2% is outside cells, so cells can absorb a large load and a small shift out of cells can raise plasma potassium sharply.
- Drivers
- after a meal, potassium moves into cells mainly because of insulin and the Na-K-ATPase pump.
- Total body potassium
- a shift into cells does not remove potassium from the body, whereas kidney excretion does.
Exam traps
- Increased kidney excretion of potassium does adjust total body potassium, but it starts after a small delay and takes around 6 to 12 hours, so it cannot explain a steady plasma potassium in the first minutes after a meal.
- Potassium lost in the faeces (stool) is not a fast, adjustable response to a single meal: in the steady state it is only about 10% of intake, with the other 90% excreted in the urine.
- Turning down the Na-K-ATPase pump would leave more potassium outside cells and make the rise in plasma potassium worse. After a meal, insulin increases pump activity instead, and reduced pump activity is a cause of hyperkalaemia.
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