Principal cells and potassium secretion in the late nephron
Long-term potassium balance is set by principal cells at the end of the nephron (the kidney's filtering and urine-making unit), which adjust how much potassium they secrete into the urine.
Remember
Principal cells secrete potassium in amounts that can be varied to match intake, while intercalated cells (the acid-base cells of the distal nephron) do the reverse and take potassium back into the blood, especially when the body is short of it.
Potassium is freely filtered at the glomerulus (the kidney's filter), and most of it is then reabsorbed, meaning taken back into the blood, before the fluid reaches the end of the nephron. About two-thirds is reabsorbed in the proximal convoluted tubule (the first coiled section after the glomerulus) and about 20% further along, in the thick ascending limb of the loop of Henle.
A small proportion therefore arrives at the principal cells of the late distal convoluted tubule and the cortical collecting duct (the part of the collecting duct that runs through the kidney's outer layer). Secretion here means potassium is moved from the blood into the tubular fluid, to be lost in urine. Secretion is substantial with a normal or high potassium level, and little when potassium is low.
A sodium-potassium pump (an energy-using pump) in the blood-facing membrane loads principal cells with potassium. Sodium entering the principal cell from the lumen (the space inside the tubule) leaves the lumen electrically negative, and potassium leaves through channels on the lumen-facing side. A high plasma potassium, aldosterone (a hormone made by the outer layer of the adrenal glands) and fast tubular flow each push secretion up.
Intercalated cells sit among the principal cells but do the opposite: they reabsorb potassium, especially when the body is short of it.
This slow adjustment is external balance, the long-term control of total body potassium. It takes around 6 to 12 hours, whereas internal balance moves potassium into or out of cells within minutes.
Key facts
- Site
- regulated potassium secretion takes place in the late distal convoluted tubule and cortical collecting duct, carried out by principal cells.
- Speed
- external balance is a slow adjustment, taking around 6 to 12 hours.
- Upstream
- the proximal tubule takes back about two-thirds of filtered potassium and the thick ascending limb about 20%.
- Regulators
- a high plasma potassium and aldosterone increase secretion.
- Low potassium
- there is little secretion if the potassium level is low.
- Intercalated cells
- they reabsorb potassium, especially when the body is short of it.
Exam traps
- Intercalated cells sit in the same late region as principal cells, but under normal conditions they reabsorb potassium rather than secrete it, especially when the body is short of potassium.
- The proximal convoluted tubule handles the most potassium, but that is reabsorption following sodium and water, so it does not make the adjustable secretion that sets long-term balance.
- Taking back more potassium would retain it in the body, the opposite of increasing excretion, so increasing reabsorption is not the way to lose potassium over hours.
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