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# Reverse osmosis effect in narrow tubes concentrates urine in the kidney
Contrary to normal osmosis, reverse osmosis moves water against the osmolarity gradient, from the salt water side to the fresh water side. It allows the production of fresh water from salt water, and is the most widely used desalination method worldwide. Reverse osmosis is normally pressure-based, but, a different type of reverse osmosis takes place in narrow tubes (micrometer scale) that also moves water in the direction opposite to normal osmosis, and therefore allows the production of fresh water. Such an effect is the basis of how the kidney concentrates urine, and it takes place in the thin segment of the nephron (it is why the nephron has evolved a narrow segment. )
In the kidney, the narrow tube effect is amplified by pressure, and the kidney adjusts how much it wants to concentrate the filtrate by adjusting the glomerular filtration pressure. To do this, it has evolved the tuberoglomerular feedback system in the juxtaglomerular apparatus. The role of tuberoglomerular feedback is simply to regulate the strength of the reverse osmosis effect in the thin segment, and it acts both locally via renin and systemically via angiotensin to achieve sufficient filtration pressure. This allows the body to prevent dehydration.
The kidney thus concentrates urine by pressure, although the pressure itself is not the basis of the effect, it only amplifies the effect that is inherent to narrow tubes.
# The role of adsorbed water in the mechanism of osmosis
It was discovered in 2009 that the driving force for osmosis is the thin layer of adsorbed water that forms at hydrophilic surfaces. This adsorbed phase of water has the unusual (and unpredicted) property of being charge polarized, it releases protons into the surrounding water that gets positively charged, and itself gains a negative charge from the surplus of hydroxide ions it is left with. The force behind this charge polarization effect is analogous to the force in semiconductor junctions: diffusion, from thermal energy. Water in the adsorbed phase (that is a semi-solid phase that is halfway between liquid and solid phase) auto-ionizes just like liquid water, but contrary to water in the liquid phase the hydroxide ions are locked into the semi-solid mass that is anchored to the hydrophilic surface. The more mobile hydrogen ion will diffuse outwards to a larger extent than the hydroxide ion, causing the charge separation effect.
Osmosis happens when there is an asymmetry between the thickness of the adsorbate on either side of a membrane. The asymmetry will cause protons to "even out their concentration" around the combined negative mass of the adsorbates on either side, so that the charge distribution is balanced out. The relative surplus and deficit of protons that results, will cause hydroxide ions on the exosmotic side (where water is moving from) to break down into dioxide, water and electrons, and the electrons will transfer over to the endosmotic side where there is a surplus of protons, and combine with the protons and dioxide to form dihydrogen monoxide, water. Water itself does not physically "move", it is broken down and built up on either side of the membrane. Only electrons and protons move during osmosis. The effect is analogous to what takes place in an acid-base battery.
Salt impairs the formation of the adsorbed phase in the same way it impairs the solid phase (a colligative property), and an increase in osmolarity on one side will decrease the adsorbate at the membrane on that side, causing the effect of proton equilibration and the subsequent breakdown of hydroxide ions on the hypoosmotic side of the membrane. Pressure, acts in an opposite way to increase the thickness of the adsorbate, and therefore generates an osmotic transfer of water in the reverse direction to osmolarity-based osmosis.
In narrow tubes, the proximity of the adsorbate around the circumference of the tube causes an increase in the net positive charge of the lumen of the tube, and this effect will be yet another factor in how the protons even out their concentration (charge wise) across both sides of the membrane of the tube. This provides a method for reverse osmosis that can operate at very low pressures, such as that within the filtration system of the kidney.