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The loop of Henle has evolved because osmosis consumes and produces dioxygen, O2, and the looped thin segment allows the O2 produced within the thin descending limb during exosmosis, to recirculate out of the thin ascending limb and into the vasa recta, that flows in the opposite direction to the filtrate to return the O2 to the ascending vasa recta. The pump that generates osmotic transfer of water, is the phase that water forms as it is forced into a solid phase when it contacts surfaces. This surface phase is charge polarized, because the hydrogen ions that separate the lattice sheets in the ice phase of water have been squeezed out , to make it more dense than water (the only way a phase change from pressure against surfaces is favourable), and the excluded hydrogen ions protonate water on top of this solid phase. Traditional osmosis is a result of asymmetry in this phase on either side of a membrane, from that the solid phase of water is impaired by solutes. But that is not the only way this pump can generate osmosis. In a narrow tube, the positively charged surface of this phase will repel across the diameter of the tube, pushing protons across the tube wall, causing the negatively charged (H3O2-)n phase to break down into electrons, that transfer across the wall as well, and O2, while the protons and electrons combine with O2 on the outside of the tube to form water.
This osmosis effect in narrow tubes is the physiological basis for the kidney. It removes water from the filtrate (via the ascending vasa recta that drains into the interlobular artery), and dehydrates the medulla. The reason the narrow tube has formed a loop, is that a straight tube that drained into the papilla would also lose all the O2 that was produced in the osmotic pump mechanism. But a tube that loops, could return the O2 to be used again, by reversing the osmosis. The kidney evolved the loop of Henle to do exactly that, and it has oriented the vasa recta in the opposite flow direction specifically to recirculate the O2.
The recirculation of O2, is in a 1:2 relationship with another recirculation system, the H2O that is transferred osmotically to consume O2 at the thin ascending limb. This water recirculates from the collecting duct, and into the thin ascending limb. It dilutes the filtrate as it travels from the bend of the loop of Henle and into the collecting duct, but is then reabsorbed, leaving the filtrate isotonic with what it was at the bend of the loop of Henle. It is there only to maintain the recirculation of the O2. The result is the same as if a straight tube that drained into the papilla received infinite O2.
To support the endosmosis at the thin ascending limb that transfers the O2 in the loop of Henle back into the vasa recta, this segment has evolved a lumen diameter that is gradually increasing, all the way from the bend of the loop of Henle up to the thick ascending limb. Any surplus water, besides that necessary to recirculate the O2, is prevented from entering the medulla, by the low blood supply to the medulla that the kidney evolved to avoid adding any water after the desalination takes place at the thin descending limb.
The kidney is able to increase or decrease the desalination effect at the thin descending limb, by regulating filtration pressure. Less pressure means less water is forced into the surface phase , and a weaker repelling force across the lumen. The kidney has evolved so that the filtration pressure is strongest in the tubes that supply the largest surface area, the juxtamedullary nephrons with long loops that reach the inner medulla. It supplies these glomeruli with more proximal branches from the interlobular artery.
The recirculation of water is regulated by the resistance to flow in the collecting duct tree, and, the permeability to water in the collecting duct epithelia. The hormone vasopressin, secreted by the posterior lobe of the pituitary gland, regulates both of these. It contracts fibroblasts in the medulla, shrinking the diameter of the collecting duct, and, it upregulates aquaporin channels in the epithelial cells.
Kidney function has to be maintained even as systemic pressure, cardiac output, changes with different activities, such as at rest or during physically demanding activity. It has evolved a reflex to do exactly this, tubuloglomerular feedback at the juxtaglomerular apparatus. It is able to maintain filtration pressure even as systemic pressure changes. This ensures that the desalination mechanism at the thin descending limb, is proportional to the recirculation of water, i.e., that the kidney does what the hypothalamus tells it to do (via vasopressin, the antidiuretic hormone) regardless of if the body is at rest or not.