Final answer:
In the kidney's renal tubules, glucose reabsorption utilizes secondary active transport through Na+/glucose symport proteins, facilitated by Na+/K+ ATPases that create an electrochemical gradient. Glucose exits the cells into the capillaries by facilitated diffusion. Excessive blood glucose can lead to glycosuria if the transport maximum of the renal tubules is surpassed.
Step-by-step explanation:
Glucose Reabsorption in the Renal Tubules
In the kidney, the process of glucose reabsorption takes place using secondary active transport. Specifically, the Na+/glucose symport protein performs this function within the renal tubules. This happens in the proximal convoluted tubule (PCT) where transporter proteins simultaneously move sodium (Na+) and glucose from the tubular lumen into the cell. Na+/K+ ATPases on the basal membrane create an electrochemical gradient, allowing for the active transport of sodium and the cotransport of glucose against its concentration gradient. Finally, glucose exits the cell towards the peritubular capillaries by facilitated diffusion, completing the reabsorption process.
If the blood glucose level is exceptionally high, as in diabetes mellitus, it may exceed the capacity of transporter proteins (transport maximum, or Tm), resulting in the spilling of glucose into the urine, a condition known as glycosuria. Typically, almost 100 percent of glucose filtered by the kidneys is reabsorbed unless these transport capacities are exceeded.