As <PERSON> 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 <PERSON> 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).
1. 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). 171
The topology is consistent with an obverse symmetry for the two similar NH2- and COOH-terminal halves (bottom).
2. 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.
3. 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. 171