Glucose is an essential energy source for the body, and its complete reabsorption in the kidneys is vital to avoid energy wastage and maintain metabolic balance. Despite being freely filtered from blood in the kidney’s glomerulus, glucose is scarcely detected in normal urine because almost all of it is reabsorbed efficiently within the nephron.

Glucose Filtration and Reabsorption in the Kidney

• Blood is filtered through the glomerulus into Bowman’s capsule, allowing glucose and other small molecules to pass into the filtrate.

• Once filtered, glucose enters the tubular system of the nephron, where reabsorption mechanisms ensure its return to the bloodstream.

• Failure to reabsorb glucose completely leads to glycosuria—presence of glucose in urine—a marker of pathological conditions like diabetes mellitus.

Primary Site of Glucose Reabsorption: Proximal Convoluted Tubule

• The proximal convoluted tubule (PCT) is the main site where glucose reabsorption occurs.

• Epithelial cells lining this section possess specialized sodium-glucose cotransporters (SGLTs):

  • SGLT2: High-capacity, low-affinity transporter in the early PCT segment reabsorbs ~90% of glucose.

  • SGLT1: High-affinity, low-capacity transporter in the later PCT segment cleans up the remaining glucose.

• After uptake into tubular cells, glucose exits into blood capillaries by facilitated diffusion through GLUT transporters.doubtnut+3

Why Other Nephron Segments Are Not Responsible

• Collecting duct: Primarily involved in water and electrolyte reabsorption regulated by hormones; does not reabsorb glucose.

• Ascending loop of Henle: Impermeable to water, deals mainly with ion transport, no glucose reabsorption.

• Bowman’s capsule: Filtration site; substances like glucose enter here but are not reabsorbed here.

Clinical Correlation: Glycosuria

• When plasma glucose exceeds renal threshold (~180 mg/dL), SGLTs become saturated, and glucose appears in urine.

• This happens in uncontrolled diabetes mellitus and certain kidney diseases, highlighting the importance of PCT in glucose conservation.

Summary Table

Conclusion

The reason glucose is never seen in normal urine is because it is almost entirely reabsorbed by the proximal convoluted tubule. This highly efficient reabsorption system ensures valuable energy resources are conserved for body functions.

Correct Answer: (1) Proximal convoluted tubule