17. Osmoregulation in the kidney, the countercurrent system Flashcards
Osmoregulation: what is it
The primary aim is to maintain osmotic homeostasis. This is so because a shift in the osmotic environment may destroy life important physiological processes.
Hyperosmosis: caused by (step 1)
E.g. mannit and glucose (cannot be reabsorbed after filtration)
▪ E.g. sodium (can be reabsorbed after filtration)
Hyperosmosis: step 2
The EC and IC getting osmotic equalized (minutes): hyperosmotic isovolaemia
Hyperosmosis: step 3
Hypothalamic osmoreceptor activity increase
Hyperosmosis: step 4
4) Blood ADH level increases
Hyperosmosis: step 5
5) Distal tubule: AQP-2 expression increases
Hyperosmosis: step 6
6) Free water clearance decreases, water retention
Hyperosmosis: step 7
7) Isosmotic = hypovolemia
Hyposmosis: when does it occur
In case of reduced salt intake or primary salt loss the osmotic concentration of EC decreases:
Hyposmosis step 1
The primary hyposmosis inhibits the ADH production and release.
Hyposmosis step 2
Free water and clearance increases, and then hypovolemic isosmosis develops.
Hyposmosis step 3
By long term regulatory mechanisms it is turned back to the direction of isosmotic isovolemia.
ADH mechanism: The role of ADH maintaining isosmosis: step 1
Experimentally perfused kidney (heart-lung preparation)produces hyposmotic urine. The reason is the lack of endocrine mechanisms: ADH promptly readjusts ISOSMOSIS!
ADH mechanism: The role of ADH maintaining isosmosis: step 2
Damage to hypothalamic ADH secreting locus: results in hyposmotic Urine
ADH mechanism: The role of ADH maintaining isosmosis: step 3
Increased diuresis following extra H2O load can be promptly blocked by ADH
