text: what is the purpose of the portal system in the nephrons of the human kidney?   glomeruli capillaries reform into arterioles and then split off into capillaries again around the other parts of the nephron (renal portal system).   tubular reabsorption. a process where much of the water and all that is deemed 'good' is reabsorbed from the proximal convoluted tubule, loop of henle, and distal convoluted tubule.   the glomeruli capillaries had reformed into arterioles. now this blood passes into a second capillary bed that starts by surrounding the proximal convoluted tubule.   having gone through the glomeruli, this blood now has a   low hydrostatic pressure   high solute concentration from the proteins that were left behind in the filtered blood (i.e. high osmotic pressure.)    water readily reenter the proximal convoluted tubule because of the osmotic gradient caused by the proteins in the remaining blood (and reabsorbed solutes) and because there is little hydrostatic pressure to oppose the movement. some of this passive type of water reabsorption continues into the loop of henle. about 80% of the water that was filtered is reabsorbed passively in the proximal tubule. the other 20% now becomes more difficult to recover.   those items that are good for the body are reabsorbed by a combination of free diffusion, facilitated diffusion, and active transport. the proximal convoluted tubule is the primary structure that does this, but the loop of henle, distal convoluted tubule, and collecting duct get heavily involved too, depending upon what is being reabsorbed.   glucose and all monosaccharides are tenaciously recovered. no monosaccharides should appear in the urine. if they do, the active transport systems were probably overloaded which suggests that there was to much glucose in the blood to start with, which suggests diabetes.   amino acids are tenaciously reabsorbed with strong active transport mechanisms.   ions are reabsorbed as needed (homeostasis). hormones help decide what ions are too high or too low and the ions are reabsorbed accordingly (e.g h+, ca++, mg++, hpo4-- , cl-, hco3- , etc.) . note that the reabsorption of some of these ions (e.g h+, hpo4-- , hco3-, etc.) is involved in ph control too.   in other words...reabsorption