Ph of urine is,And why?
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Here is a thorough answer drawn directly from multiple authoritative medical textbooks:
pH of Urine
Normal Range
4.5 to 8.0, with a typical value of ~5 to 6 (mildly acidic).
Why is Urine Normally Acidic?
The kidneys are responsible for maintaining the body's acid-base balance. Every day, normal metabolism (protein catabolism, cellular respiration) generates a net acid load - roughly 40-80 mEq of H+ per day. The kidneys excrete this excess acid by:
- Secreting H+ ions into the tubular lumen
- Reabsorbing bicarbonate (HCO3-) filtered at the glomerulus
- Generating new bicarbonate to replenish the body's buffer stores
This continuous net acid excretion keeps urine pH around 5, making it typically acidic.
- Goldman-Cecil Medicine, p. 1170
Factors That Shift Urine pH
| Condition | Urine pH | Why |
|---|---|---|
| Normal (average) | ~5-6 | Daily net acid excretion by kidneys |
| High protein diet | More acidic | Protein metabolism produces more H+ |
| Vegetarian diet | More alkaline | Plant foods are base-producing |
| After meals ("alkaline tide") | Rises temporarily | HCO3- secreted into blood to balance gastric acid secretion transiently raises urine pH |
| Metabolic/respiratory acidosis | <6 (acidic) | Kidneys compensate by excreting more H+ |
| Metabolic/respiratory alkalosis | >6 (alkaline) | Kidneys retain H+ and excrete more HCO3- |
| Urease-producing infection (Proteus, Klebsiella) | >7.5 (strongly alkaline) | Bacteria split urea into ammonia, markedly raising pH - this can cause struvite (staghorn) calculi |
| Renal Tubular Acidosis (RTA) Type 1 (distal) | Always >5.5 (paradoxically alkaline despite acidosis) | Inability to create a proton gradient in distal tubule; urine cannot be acidified below pH 5.5 |
| RTA Type 2 (proximal) | High until HCO3- threshold is exceeded, then drops to ~5 | Impaired HCO3- reabsorption in proximal tubule |
| RTA Type 4 | ~5 (acidic, with positive urine net charge) | Hypoaldosteronism reduces H+ and K+ secretion |
| Uric acid / cystine stones | Acidic | These stones form in acidic urine; alkalinization is used as treatment |
- Quick Compendium of Clinical Pathology, p. 4154-4158
- Campbell Walsh Wein Urology, p. 39
- Goldman-Cecil Medicine, p. 1170
Key Clinical Pearls
-
Urine pH >5 in systemic metabolic acidosis should raise suspicion for RTA - the kidney is failing to appropriately acidify urine.
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Urine pH >7.5 with clinical features of UTI strongly suggests Proteus or Klebsiella infection (urease-splitters), which can lead to struvite stones.
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Standing urine becomes more alkaline over time (CO2 loss, bacterial growth), so pH should be tested fresh.
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Urine pH is used therapeutically: alkalinizing urine treats uric acid stones and cystinuria; acidifying urine speeds elimination of certain drugs (e.g., amphetamines, whose half-life doubles in alkaline urine).
-
Brenner and Rector's The Kidney, p. 1002
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