How do nephrons form urine from blood?

Nephrons produce urine through a three-step process that cleanses the blood and maintains internal balance:

• Glomerular Filtration: Blood enters the glomerulus, a cluster of capillaries enclosed by Bowman’s capsule. Under pressure, water and small solutes (crystalloids) are filtered out of the blood plasma into the nephron tubule, while larger molecules like proteins and blood cells are retained.12
• Selective Reabsorption: As the filtrate travels through the long nephron tubule, the body reclaims essential substances. The majority of water, along with necessary salts and nutrients like glucose, is reabsorbed by the cells lining the tubules and returned to the bloodstream.13
• Tubular Secretion: The nephron actively or passively adds specific substances—such as hydrogen ions, potassium, and certain drugs—from the peritubular capillaries into the filtrate. This final adjustment helps regulate the body's acid-base balance and electrolyte levels.43

The resulting fluid, now concentrated as urine, is conveyed through collecting tubules to the renal pelvis for eventual excretion. This complex filtration and regulation system is vital for preventing the accumulation of toxic waste products in the blood.13