Your body’s chemical reactions constantly produce acids — from metabolism, CO₂, and even your diet. Yet your blood pH stays between 7.35–7.45 — a very narrow and essential range for life. How? Because the lungs and kidneys work together to keep it that way: Lungs remove volatile acid (CO₂). Kidneys handle fixed acids (H⁺, lactic acid, etc.) and generate new bicarbonate (HCO₃⁻)
🧩 1️⃣ Overview of Acid–Base Regulation
| Organ/System | Response Speed | Mechanism |
|---|---|---|
| Buffer systems | Immediate | Bind/release H⁺ (e.g. HCO₃⁻, phosphate, proteins) |
| Lungs | Minutes | Adjust CO₂ (acid) by altering ventilation |
| Kidneys | Hours–Days | Reabsorb HCO₃⁻, secrete H⁺, generate new HCO₃⁻ |
🧠 2️⃣ How the Kidneys Maintain Acid–Base Balance
The kidneys act as the final line of defense — they don’t just filter blood; they chemically adjust it.
A. Reabsorption of Filtered Bicarbonate (HCO₃⁻)
- Occurs mainly in the Proximal Convoluted Tubule (PCT)
- For every H⁺ secreted into the lumen, one HCO₃⁻ is reabsorbed into the blood.
Simplified process:
- H⁺ combines with filtered HCO₃⁻ → forms H₂CO₃ (carbonic acid)
- H₂CO₃ → CO₂ + H₂O (via carbonic anhydrase)
- CO₂ diffuses into tubular cell → reforms HCO₃⁻
- HCO₃⁻ enters blood → maintains pH
B. Excretion of H⁺ Ions
- When blood is acidic, kidneys secrete more H⁺.
- This happens mainly in: Distal convoluted tubule. Collecting duct.
Two main ways to remove H⁺ safely:
- Buffered by phosphate → forms H₂PO₄⁻ (titratable acid)
- Buffered by ammonia (NH₃) → forms ammonium (NH₄⁺)
C. Generation of New Bicarbonate
Every time an H⁺ is excreted (as NH₄⁺ or H₂PO₄⁻), the kidney creates a new bicarbonate molecule that enters the bloodstream. This is how the kidneys “replace” the bicarbonate that was used up buffering acids.
⚗️ 3️⃣ Acid–Base Disorders (High-Yield Clinical Section)
| Disorder | Primary Problem | Compensation | Examples |
|---|---|---|---|
| Metabolic acidosis | ↓ HCO₃⁻ | ↓ CO₂ (hyperventilation) | DKA, renal failure, diarrhea |
| Metabolic alkalosis | ↑ HCO₃⁻ | ↑ CO₂ (hypoventilation) | Vomiting, diuretics |
| Respiratory acidosis | ↑ CO₂ | ↑ HCO₃⁻ (renal retention) | COPD, hypoventilation |
| Respiratory alkalosis | ↓ CO₂ | ↓ HCO₃⁻ (renal excretion) | Hyperventilation, anxiety |
🚰 4️⃣ Urine Formation — The Big Picture
Urine is the final product of filtration, reabsorption, and secretion — representing the body’s way of excreting waste while conserving essentials.
Steps of Urine Formation:
- Glomerular Filtration – blood → Bowman’s capsule
- Tubular Reabsorption – reclaim water and solutes
- Tubular Secretion – remove unwanted ions & toxins
- Excretion – final urine passes through collecting duct → ureter → bladder → urethra
Composition of Normal Urine
| Component | Presence | Notes |
|---|---|---|
| Water | ~95% | Main solvent |
| Urea | Present | From protein breakdown |
| Creatinine | Present | From muscle metabolism |
| Na⁺, K⁺, Cl⁻ | Variable | Electrolyte balance |
| No glucose / protein / blood | Abnormal if present | Indicates renal pathology |
🧠 Clinical correlations:
- Proteinuria: glomerular damage (nephrotic syndrome)
- Glycosuria: diabetes mellitus
- Hematuria: infection, stones, or trauma
Urine pH and Color
- Normal pH: 4.5–8 (acidic urine helps excrete acids)
- Color: pale yellow (urochrome pigment)
- Odor: slightly aromatic (ammonia-like after standing)
🧩 5️⃣ Summary Table
| Aspect | Main Site | Key Action | Clinical Note |
|---|---|---|---|
| HCO₃⁻ reabsorption | PCT | Prevents loss of base | Carbonic anhydrase inhibitors ↓ this |
| H⁺ secretion | DCT/CD | Removes excess acid | Buffers: phosphate, ammonia |
| New HCO₃⁻ | DCT/CD | Generates base | Net from H⁺ excretion |