1. Filtration: Glucose, being a small molecule, freely passes from the blood in the glomerulus into the Bowman's capsule, becoming part of the filtrate.
2. Reabsorption: As the filtrate flows through the proximal convoluted tubule (PCT), nearly all of the glucose is reabsorbed back into the blood. This occurs through a sodium-glucose cotransporter (SGLT) protein. This protein uses the energy from sodium moving down its concentration gradient (from the filtrate into the PCT cells) to transport glucose against its concentration gradient (from the filtrate into the PCT cells).
3. Return to blood: Once inside the PCT cells, glucose moves into the interstitial fluid surrounding the tubules and finally back into the blood capillaries.
Important Considerations:
* Renal threshold: There's a limit to how much glucose the SGLT can reabsorb. This limit is called the renal threshold, and it's usually around 180 mg/dL. If blood glucose levels rise above this threshold, glucose will spill into the urine (glucosuria).
* Diabetes: People with diabetes often experience hyperglycemia (high blood glucose levels). In these cases, the renal threshold is exceeded, leading to glucosuria.
In summary: Glucose entering the nephron is usually fully reabsorbed, but this process can be overwhelmed in conditions like diabetes.
