The Urinary System: How Urine Is Formed—From Blood Filtration to Excretion

Urine isn't "turned into" anything—it's the end product of a continuous filtration, reabsorption, and secretion process that maintains homeostasis.

Step-by-Step Urine Formation

1. Glomerular Filtration (Renal Corpuscle):
   • Blood enters via afferent arteriole → glomerular capillaries
   • Hydrostatic pressure forces plasma (minus proteins/cells) into Bowman's capsule
   • Filtration rate: ~180 L/day (entire plasma volume filtered ~60×/day)
2. Tubular Reabsorption (Proximal Tubule → Loop of Henle → Distal Tubule → Collecting Duct):
   • ~99% of filtrate reclaimed: water, glucose, amino acids, ions, bicarbonate
   • Active transport (Na⁺/K⁺-ATPase drives), co-transport, paracellular
   • Regulated by hormones: ADH (water), aldosterone (Na⁺/K⁺), PTH (Ca²⁺), ANP
3. Tubular Secretion:
   • Active transport from peritubular capillaries → tubule lumen
   • H⁺ (acid-base), K⁺, drugs, toxins, creatinine, organic acids/bases
   • Fine-tunes blood composition
4. Excretion:
   • Final urine: ~1–2 L/day (0.5 mL/min)
   • Flows: Collecting duct → papillary duct → minor calyx → major calyx → renal pelvis → ureter → bladder → urethra

Key Hormonal Regulation

• ADH (vasopressin): ↑ water permeability in collecting duct (aquaporin-2)
• RAAS: Angiotensin II + aldosterone → ↑ Na⁺ reabsorption → ↑ water retention
• ANP/BNP: Atrial stretch → ↓ Na⁺ reabsorption, ↑ GFR → natriuresis
• PTH: ↑ Ca²⁺ reabsorption (distal tubule), ↓ phosphate reabsorption

Clinical Relevance

• GFR estimation (creatinine, cystatin C) assesses kidney function
• Urinalysis detects protein, glucose, blood, casts → disease markers
• Diuretics target specific transporters (loop, thiazide, K⁺-sparing)

The kidney filters 180 L to excrete 1.5 L of waste—remarkable precision.