Glucose is an important component of the blood that is usually completely reabso

The transport of glucose by the proximal tubules is a perfect example of transport maximum. This is the point at which all of the transporters of a substrate are occupied (100% saturated) and thus, further increases in substrate concentration do not affect their rate of transport. The reabsorption of glucose involves its transport across the apical membranes of the proximal tubule epithelial cells by sodium-linked cotransporters. As sodium moves into the cell, glucose follows. This causes an increase in intracellular glucose concentration, which drives glucose into the peritubular space across the epithelial cell's basolateral membrane through a carrier protein by facilitated diffusion. As the concentration of glucose in the blood increases, as occurs in a diabetic, all of the transporters are saturated and glucose begins to remain in the filtrate. The plasma concentration at which substrate is observed in the urine is termed the renal threshold. In most individuals, little glucose ever reaches the urine. However, diabetics are characterized by the overflow of glucose into the urine, which leads to the sweet smell of the urine. Interestingly, the renal threshold occurs before the transport maximum is reached. This occurs because, even though all of the transporters are not occupied, some of the glucose molecules make it through the proximal tubule without coming into contact with an unoccupied transporter.