Water moves out of the descending limb of the LOH because the solute concentration in the interstitial space is higher than the solute concentration in the filtrate. The ascending limb of the LOH is impermeable to water and permeable to NaCl. In this region, NaCl is actively transported out because the concentration of solutes in the interstitial space is greater than the concentration of solutes in the filtrate. When the solutes leave the ascending limb of the LOH, water cannot follow. Because of this, the concentration of the filtrate increases as it moves up the ascending limb.
The two types of cells present in the collecting duct are the principal cells and the intercalated cells, which secrete H+ into the lumen of the collecting ducts.
The principal cells, located in the distal portion of the ducts, are sensitive to the hormones aldosterone and ADH (antidiuretic hormone), meaning that they have receptors for these hormones.
In response to the hormone ADH, the principal cells decrease their water permeability. This means that more water is retained and less leaves the body as ADH is present when the individual is retaining water. In response to the hormone aldosterone, the principal cells increase Na+ reabsorption back into the blood and decrease K+ secretion into the urine.