How does a nephron form urine from blood?

A nephron produces urine through three primary processes: glomerular filtration, tubular reabsorption, and tubular secretion.12

Glomerular Filtration

Urine formation begins in the renal corpuscle, where blood pressure forces fluid and small solutes out of the glomerulus and into Bowman’s capsule. This ultrafiltration process retains blood cells and large proteins while allowing water, ions, and waste products to enter the nephron tubule as filtrate.134

Tubular Reabsorption and Secretion

As the filtrate travels through the nephron tubule, its composition is significantly modified:12
* Tubular Reabsorption: The body recovers essential substances—such as water, glucose, and various salts—by moving them from the filtrate back into the surrounding peritubular capillaries. This ensures that valuable nutrients are retained while waste products remain in the tubule.42
* Tubular Secretion: The nephron actively or passively adds specific substances, such as hydrogen ions, potassium, and certain drugs, from the blood directly into the filtrate.52

The final product, now concentrated as urine, is conveyed through collecting tubules to the renal pelvis for excretion.1