By John Brennan | Updated Mar 24, 2022
Blood enters the kidney through the renal artery, which branches into a network of capillaries that supply the nephrons—the kidney’s functional units. Each adult kidney contains roughly one million nephrons, each composed of a glomerulus and a tubular system where filtration and reabsorption occur.
Within the glomerulus, high blood pressure forces water, salts, amino acids, and small molecules—including glucose—through the capillary walls into Bowman's capsule. This ultrafiltration removes waste while preventing loss of large proteins and cells.
After filtration, the tubular portion of the nephron—specifically the proximal tubule—reclaims valuable solutes. Unlike the distal tubule, which secretes waste into the urine, the proximal tubule’s epithelial cells actively transport glucose back into the bloodstream.
On the apical membrane of proximal‑tubule cells sit SGLT2 proteins that couple sodium reabsorption to glucose uptake. The sodium pump maintains a high luminal sodium concentration; as sodium ions move back into the cell, they drag glucose molecules along, allowing glucose to enter the cell without direct energy expenditure.
Once inside the cell, glucose exits into the bloodstream through GLUT2 transporters on the basolateral membrane. GLUT2 operates passively, acting like a revolving door that permits glucose to move from a higher intracellular concentration to the lower plasma level.
In conditions of hyperglycemia, the proximal tubule’s capacity to reabsorb glucose is exceeded, and excess glucose is secreted by the distal tubule and appears in the urine.
