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If we see nephrons, in the descending part of Loop of Henle (LoH), water movement is allowed but solute movement is not. On the contrary, in ascending LoH, solute movement is allowed but not water. What exactly restricts this movement?
I am particularly interested in understanding the movement of water across cell membranes and then answering the above query.
Water across cell membranes occurs due to osmosis( which is based on a concentration gradient). so water diffuses through the cell membrane. sometimes water channels called aquaporins can be found.
https://www.anaesthesiamcq.com/FluidBook/fl1_2.php
https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/aquaporin
In the loop of Henle, the water, and Na+ & Cl- exchange occurs due to the counter-current principle takes place. the interstitial fluid has a certain sodium chloride concentration. the filtrate that is entering the descending limb of the loop of Henle, has a particularly low NaCl concentration.so because of that water moves from the loop of Henle to the interstitial fluid.simultaneously, at the ascending limb of the loop of Henle just adjacent to that particular point, water concentration is high in the interstitial fluid. so because of that NaCl moves to the interstitial fluid (Na+ actively, and for ion balance, Cl- passively.) this happens along the length of the loop of Henle. what restricts water movement from the ascending limb of the loop of Henle is that that limb has no aquaporins and the descending limb has numerous (with relative to other places in the nephron). anyway, here's a link for you. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3468499/
As @Roshelle Perera points out, the textbook reason for differential solute and water movement in different parts of the LoH is osmolarity, which is nicely explained in this wikipedia article.
Coming to an interesting point, about the movement of water across cell membranes through aquaporins, this is helpful:{1}
The discovery of aquaporin membrane water channels by Agre and co-workers (6, 7, 191, 192) answered a long-standing biophysical question of how water crosses biological membranes specifically, and provided insight, at the molecular level, into the fundamental physiology of water balance and the pathophysiology of water balance disorders. Out of at least 10 aquaporin isoforms, at least 7 are known to be present in the kidney at distinct sites along the nephron and collecting duct.
Aquaporin-1 (AQP1) is extremely abundant in the proximal tubule and descending thin limb where it appears to be the main site for proximal nephron water reabsorption. It is also present in the descending vasa recta. AQP2 is abundant in the collecting duct principal cells and is the chief target for the regulation of collecting duct water reabsorption by vasopressin.
The journal further elaborates on these APQ's and their function in detail, do read it if you are interested, but the following diagram sums it up pretty well, so I am just going to expand on this:
A: schematic representation of the structural organization of aquaporin-1 (AQP1) monomers in the membrane (top and bottom).
Aquaporins have six membrane-spanning regions, both intracellular NH2 and COOH termini, and internal tandem repeats that, presumably, are due to an ancient gene duplication (top). The topology is consistent with an obverse symmetry for the two similar NH2- and COOH-terminal halves (bottom).
The tandem repeat structure with two asparagine-proline-alanine (NPA) sequences has been proposed to form tight turn structures that interact in the membrane to form the pathway for translocation of water across the plasma membrane.
Of the five loops in AQP1, the B and E loops dip into the lipid bilayer, and it has been proposed that they form “hemichannels” that connect between the leaflets to form a single aqueous pathway within a symmetric structure that resembles an “hourglass.”
B: AQP1 is a multisubunit oligomer that is organized as a tetrameric assembly of four identical polypeptide subunits with a large glycan attached to only one.
The direcrion of flow of water, again, depends on osmolarity.
...Only inward water flow (swelling) was examined, but it was predicted that the direction of water flow through AQP1 is determined by the orientation of the osmotic gradient. Consistent with this, it was later demonstrated that AQP1-expressing oocytes swell in hyposmolar buffers but shrink in hyperosmolar buffers.
Please note that the linked journal is brilliantly detailed, so there are several points that I have skimmed over. I recommend reading it if you have time.
{1}-https://journals.physiology.org/doi/full/10.1152/physrev.00024.2001#:~:text=Aquaporin%2D1%20(AQP1)%20is,for%20proximal%20nephron%20water%20reabsorption.&text=AQP2%20is%20abundant%20in%20the,duct%20water%20reabsorption%20by%20vasopressin.
