The glomerular filtrate contains many substances the body needs. Selective reabsorption is the process in which useful substances are taken back from the filtrate into the blood, while waste products such as urea are left behind or only partly reabsorbed. It is selective because different substances are reabsorbed by different mechanisms and in different amounts.Most reabsorption, about two thirds of the filtrate, happens in the proximal convoluted tubule. Further reabsorption of ions and water happens in the loop of Henle, distal convoluted tubule and collecting duct.
Substances are reabsorbed by several mechanisms:• active transport of sodium ions, using ATP, out of the tubule cells into the blood
• co-transport of glucose and amino acids with sodium ions into the tubule cells
• osmosis of water, following the movement of solutes
• diffusion of some ions and of urea
• endocytosis of any small proteins that were filteredTypical results in a healthy person:• glucose and amino acids: 100% reabsorbed, so none appears in urine
• sodium ions and water: over 99% reabsorbed
• urea: only about half reabsorbed, by diffusion, so it becomes concentrated in urineReabsorbed substances pass into the capillaries surrounding the tubule, which are supplied by the efferent arteriole. This blood has a low hydrostatic pressure and, having lost fluid in the glomerulus, a high concentration of plasma proteins and so a low water potential, which helps water return to it.If blood glucose is very high, as in untreated diabetes mellitus, the carrier proteins become saturated and not all the glucose can be reabsorbed, so glucose appears in the urine.
Substances are reabsorbed by several mechanisms:• active transport of sodium ions, using ATP, out of the tubule cells into the blood• co-transport of glucose and amino acids with sodium ions into the tubule cells
• osmosis of water, following the movement of solutes
• diffusion of some ions and of urea
• endocytosis of any small proteins that were filteredTypical results in a healthy person:• glucose and amino acids: 100% reabsorbed, so none appears in urine
• sodium ions and water: over 99% reabsorbed
• urea: only about half reabsorbed, by diffusion, so it becomes concentrated in urineReabsorbed substances pass into the capillaries surrounding the tubule, which are supplied by the efferent arteriole. This blood has a low hydrostatic pressure and, having lost fluid in the glomerulus, a high concentration of plasma proteins and so a low water potential, which helps water return to it.If blood glucose is very high, as in untreated diabetes mellitus, the carrier proteins become saturated and not all the glucose can be reabsorbed, so glucose appears in the urine.