Kidney Filtration: How Nephrons Produce Urine Step by Step

Each kidney contains ~1 million nephrons—the functional units that filter blood and form urine via four processes.

1. Glomerular Filtration

• Blood enters glomerulus via afferent arteriole.
• Hydrostatic pressure forces plasma (water, electrolytes, glucose, amino acids, waste) into Bowman's capsule.
• Cells/proteins retained (size/charge barrier).
• GFR ~125 mL/min (180 L/day).

2. Tubular Reabsorption

• Proximal tubule: 65% Na⁺, H₂O, glucose, amino acids, HCO₃⁻ (active + passive).
• Loop of Henle: Countercurrent multiplication → medullary gradient; reabsorbs 25% Na⁺/Cl⁻, 15% H₂O.
• Distal tubule: Fine-tuning Na⁺, Ca²⁺, pH (aldosterone, PTH).
• Collecting duct: ADH-regulated water reabsorption (concentrating urine).

3. Tubular Secretion

• Active transport from peritubular capillaries → tubule lumen.
• Substances: H⁺ (acid-base), K⁺, creatinine, drugs (penicillin), toxins.
• Regulates pH, K⁺, clears non-filtered compounds.

4. Concentration & Excretion

Final urine: 1–2 L/day (0.5–20 mOsm/kg to 1200 mOsm/kg). ADH maximizes concentration; absence → dilute urine.

Regulation

• RAAS, ADH, ANP, sympathetic tone.
• Autoregulation (myogenic, tubuloglomerular feedback) maintains GFR.