Clearance tells us how efficiently the kidneys remove a substance from blood — a window into how the nephron handles everything from water to waste.
🧪 1️⃣ What is Renal Clearance?
The volume of plasma completely cleared of a substance per unit time.
Formula:
C = \frac{U \times V}{P}
Where:
- C = Clearance (mL/min)
- U = Urine concentration
- V = Urine volume per min
- P = Plasma concentration
🩸 2️⃣ Types of Clearance Based on Renal Handling
| Substance Type | Example | Renal Handling | Clearance Relation |
|---|---|---|---|
| Completely reabsorbed | Glucose, amino acids | Filtered, fully reabsorbed | C = 0 |
| Filtered only | Inulin, creatinine | Not reabsorbed or secreted | C = GFR |
| Filtered + secreted | Para-aminohippuric acid (PAH), urea | Actively secreted | C > GFR |
🩸 3️⃣ Glomerular Filtration Rate (GFR)
Definition: The rate at which plasma is filtered through all glomeruli per minute. It’s the gold standard measure of kidney function.
Normal GFR: ~125 mL/min in men, ~110 mL/min in women
Equation: GFR = K_f \times (P_{GC} - P_{BS} - \pi_{GC})
Where:
- K_f = Filtration coefficient
- P_{GC} = Glomerular capillary pressure
- P_{BS} = Bowman’s space pressure
- \pi_{GC} = Glomerular oncotic pressure
B. Measurement of GFR
1. Inulin Clearance — Gold Standard
GFR = \frac{U_{inulin} \times V}{P_{inulin}}
2. Creatinine Clearance — Practical Clinical Method
GFR \approx \frac{U_{cr} \times V}{P_{cr}}
C. Estimated GFR (eGFR) — Simplified for Clinical Use
Normal eGFR: >90 mL/min/1.73m²
🚰 4️⃣ Renal Plasma Flow (RPF) and PAH Clearance
Concept: To understand blood flow through kidneys, we use para-aminohippuric acid (PAH) — it’s filtered and secreted completely.
RPF = \frac{U_{PAH} \times V}{P_{PAH}}
Normal RPF: ~600 mL/min
Renal blood flow (RBF): RBF = \frac{RPF}{1 - hematocrit} ≈ 1.2 L/min (~25% of cardiac output)
⚖️ 5️⃣ Filtration Fraction (FF)
Represents how much plasma entering the kidney is actually filtered.
FF = \frac{GFR}{RPF}
Normal FF: ~20%
🧩 6️⃣ Clinical Correlations
| Parameter | Change | Possible Cause |
|---|---|---|
| ↓ GFR | ↑ Creatinine, ↓ Urine output | Renal failure, hypovolemia |
| ↑ GFR | (rare) | Pregnancy, high cardiac output |
| ↑ BUN & Creatinine | ↓ Clearance | Kidney dysfunction (azotemia) |
| ↓ RPF | Renal artery stenosis | ↓ Perfusion pressure |
| ↑ FF | Efferent constriction | Angiotensin II effect |
🧠 7️⃣ Quick Summary Table
| Test / Concept | Purpose | Normal Range / Meaning |
|---|---|---|
| Clearance formula | Determines rate of removal from plasma | C = (U×V)/P |
| GFR (inulin / creatinine) | Measures filtration capacity | ~125 mL/min |
| RPF (PAH) | Measures plasma flow to kidney | ~600 mL/min |
| Filtration fraction (FF) | % of plasma filtered | ~20% |
| Serum creatinine | Inverse to GFR | ↑ = renal impairment |