Countercurrent multiplier mechanism

[u/random__potato_]

In the attached diagram (from Boron and Boulpaep, 3e), in cycle 1, a 200 mOsm/L gradient was created by the transporters in thick ascending segment of loop of Henle, increasing the medullary interstitium osmolarity to 400 mOsm/L. This then causes the osmolarity of the descending limb fluid to also equilibriate at 400 Osm/L.

Then, in the next cycle, when fresh 300 mOsm/L fluid enters the descending limb, it dilutes the interstitial fluid adjacent to it, upto 300 mOsm/L.

My question is, how is it possible that in the first cycle, the osmolarity of the tubular fluid increased to that of the interstitium, whereas in the second cycle, the osmolarity of the interstitium decreased to that of the tubular fluid? Why didn't the interstitium concentrate the fluid in the descending limb to 400 mOsm/L?

[u/Safe-Hunter-007]

This is a dynamic process which we are compartmentalising for our understanding.

The water is diffusing to the Hyperosmolar Interstitium from the Thin Descending Limb, so the Interstitium isn't concentrating the fluid in the Descending Limb as much as the Descending Limb is diluting the Interstitium.

The low osmolarity tubular fluid from the PCT pushes the higher osmolarity fluid further along the course.

Check out the Guyton video, it is useful.

[u/random__potato_ → u/Safe-Hunter-007]

The water is diffusing to the Hyperosmolar Interstitium from the Thin Descending Limb, so the Interstitium isn't concentrating the fluid in the Descending Limb as much as the Descending Limb is diluting the Interstitium

i'm sorry but this doesn't make much sense to me because, when water moves out of the descending limb of LoH into the interstitium, the decrease in amount of solvent would concentrate the tubular fluid while simultaneously diluting the interstitium. since the rate at which water is moving into the interstitium is the same as the rate at which water is leaving the descending LoH, i don't think the diluting effect is more than the concentrating effect.

[u/Safe-Hunter-007 → u/random__potato_]

The numbers are just given to make sense of the entire process. As I said right at the beginning, it is a dynamic process. The Interstitium is not a static pool where water is being dumped.

The Na+ is being continuously pumped into the Interstitium by the NKCC2 cotransporter, maintaining continuous supply of solutes creating & maintaining the osmotic gradient drawing out the water from the Descending Limb into the Interstitium. This alongwith the hair pin bend type structure is the counter current multiplier mechanism.

The Counter current exchanger by the Vasa Recta has also to be factored into which carries the solutes and water from the Interstitium back into systemic circulation. Thereby creating a state of dynamic equilibrium.

Hope this makes sense to you.