aldosterone bio ocr

i dont understand the 4th point:
- sodium ions pumped out of collecting duct cells (into tissue fluid) (and potassium ions pumped in)
- lowers water potential in tissue fluid
- concentration gradient established
- sodium ions reabsorbed from the collecting duct lumen
- water diffuses into collecting duct cells / out of lumen via osmosis

Reply 1

0gg

as stated in the first bullet point, sodium ions are moved from the cd cells to the tissue fluid. that means that the concentration of sodium ions in the cd cells decreases, which then means that sodium ions diffuse from the lumen into the cells (through channel proteins) because there is a concentration gradient of sodium ions between them.

the key point is that concentration gradient was created by sodium ions being actively transported out of the cd cells into the lumen.

Reply 2

randomtest123

Original post by 0gg

as stated in the first bullet point, sodium ions are moved from the cd cells to the tissue fluid. that means that the concentration of sodium ions in the cd cells decreases, which then means that sodium ions diffuse from the lumen into the cells (through channel proteins) because there is a concentration gradient of sodium ions between them.
the key point is that concentration gradient was created by sodium ions being actively transported out of the cd cells into the lumen.

so its almost as if the Na+ ions that were pumped out are now being replaced with the ones from the collecting duct lumen, but how would this eventually cause greater reabsorption? Would the ions then move into the tissue fluid from the collecting duct cells, thus lowering water potential of interstitial fluid so more water reabsorbed?