2.1 Renal Physiology, Glomerular Filtration & Acid-Base Balance

2.1 Renal Physiology, Glomerular Filtration & Acid-Base Balance

Mastery of renal physiology, glomerular filtration hemodynamics, neurohormonal regulation, acid-base calculations, and acute kidney injury (AKI) classification is essential for the certified registered nurse anesthetist (CRNA). General and regional anesthesia profoundly influence renal blood flow (RBF) and tubular handling of electrolytes and water. Furthermore, pre-existing renal disease alters the pharmacokinetics and pharmacodynamics of induction hypnotics, muscle relaxants, and analgesics.

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1. Functional Nephron Anatomy & Segmental Tubular Physiology

The kidneys receive approximately 20% to 25% of resting cardiac output (~1,200 mL/min), representing the highest tissue perfusion rate per gram of tissue in the body. The functional unit of the kidney is the nephron, comprising approximately 1 to 1.2 million units per kidney. Nephrons are divided into cortical nephrons (~85%, located in the outer cortex with short loops of Henle) and juxtamedullary nephrons (~15%, located at the corticomedullary junction with long loops of Henle extending deep into the hypertonic renal medulla, essential for generating concentrated urine via countercurrent multiplication).

Glomerulus / Bowman's Space (Filtration: 125 mL/min)
   │
   ▼
Proximal Convoluted Tubule (PCT)
• 65-70% Na+, H2O, K+, Cl- reabsorption
• 100% glucose (threshold 180-200 mg/dL) & amino acids
• 85-90% HCO3- reabsorption (carbonic anhydrase dependent)
• Iso-osmotic reabsorption (~300 mOsm/kg)
   │
   ▼
Loop of Henle
• Descending Thin Limb: Highly permeable to H2O (aquaporin-1), impermeable to solutes (luminal fluid concentrates to ~1200 mOsm/kg at hairpin)
• Ascending Thin Limb: Impermeable to H2O, permeable to NaCl
• Thick Ascending Limb (TAL): Impermeable to H2O; active Na+/K+/2Cl- cotransporter (NKCC2; blocked by loop diuretics); generates medullary hypertonicity
   │
   ▼
Distal Convoluted Tubule (DCT)
• Early DCT: Impermeable to H2O (cortical diluting segment); Na+/Cl- cotransporter (NCCT; blocked by thiazides); PTH-stimulated active Ca2+ reabsorption
• Hypo-osmotic luminal fluid (~100 mOsm/kg)
   │
   ▼
Collecting Duct (Late DCT & Medullary Collecting Duct)
• Principal Cells: ENaC channels (Na+ reabsorption), ROMK (K+ secretion), regulated by Aldosterone; Aquaporin-2 water channels inserted by ADH (V2 receptor / cAMP)
• Type A Intercalated Cells: Active H+-ATPase & H+/K+-ATPase (acid secretion, HCO3- reabsorption in acidosis)
• Type B Intercalated Cells: Pendrin (Cl-/HCO3- exchanger; HCO3- secretion, H+ reabsorption in alkalosis)

Segmental Transport Mechanisms & Diuretic Targets

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2. Glomerular Filtration Rate & Renal Autoregulation

Determinants of Glomerular Filtration

Glomerular Filtration Rate (GFR) averages 125 mL/min in healthy young adults (~180 L/day). Filtration is governed by Starling forces across the glomerular capillary wall:

$$\text{GFR} = K_f \times [ (P_{\text{GC}} - P_{\text{BS}}) - (\Pi_{\text{GC}} - \Pi_{\text{BS}}) ]$$

• $K_f$ = Glomerular ultrafiltration coefficient (surface area $\times$ hydraulic permeability).
• $P_{\text{GC}}$ = Glomerular capillary hydrostatic pressure (~50-60 mmHg; favors filtration; regulated by afferent and efferent arteriolar tone).
• $P_{\text{BS}}$ = Bowman's space hydrostatic pressure (~15 mmHg; opposes filtration; elevated in ureteral obstruction).
• $\Pi_{\text{GC}}$ = Glomerular capillary oncotic pressure (~25 mmHg at afferent end, rising to ~35 mmHg at efferent end; opposes filtration).
• $\Pi_{\text{BS}}$ = Bowman's space oncotic pressure (~0 mmHg in healthy glomeruli).
• Filtration Fraction (FF): $\text{FF} = \frac{\text{GFR}}{\text{RPF}} \approx \frac{125\text{ mL/min}}{625\text{ mL/min}} \approx 20\%$.

Renal Autoregulation: Mechanisms & Limits

Renal autoregulation maintains a relatively constant Renal Blood Flow (RBF) and GFR across a Mean Arterial Pressure (MAP) range of 80 to 180 mmHg (or perfusion pressures of 75-160 mmHg).

Renal Perfusion Pressure (MAP 80 - 180 mmHg)
   ├── 1. Myogenic Response (Fast, 1-3 seconds):
   │     Elevated transmural stretch on afferent arteriolar smooth muscle
   │     → Opens stretch-activated non-selective cation channels
   │     → Membrane depolarization & voltage-gated Ca2+ entry (L-type)
   │     → Immediate afferent arteriolar constriction (prevents hyperfiltration)
   │
   └── 2. Tubuloglomerular Feedback (TGF; 10-30 seconds):
         Elevated GFR / RBF → Increased NaCl delivery to Macula Densa (TAL)
         → Increased NKCC2 transport of Na+/K+/2Cl- into macula densa cells
         → Basolateral ATP and Adenosine release
         → Binds Adenosine A1 receptors on afferent arteriolar smooth muscle
         → Afferent arteriolar vasoconstriction & inhibition of renin release

                    AUTOREGULATION WINDOW
   RBF / GFR  │                  ____________________
   (mL/min)   │                 /                    \
              │                /                      \
              │               /                        \
              │              /                          \
              │             /                            \
              └────────────┴──────────────────────────────┴──────────
               0          80                             180      MAP (mmHg)
               [Linear Drop] [Glomerular Protection]   [Pressure Diuresis]

NCE Clinical Pearl — The MAP Thresholds:

• MAP 80–180 mmHg: Autoregulation active; GFR and RBF are held constant.
• MAP < 80 mmHg: Autoregulation is exhausted; RBF and GFR decline linearly with further reductions in perfusion pressure.
• MAP < 50 mmHg: Glomerular capillary hydrostatic pressure ($P_{\text{GC}}$) drops below opposing oncotic and Bowman's pressures; glomerular ultrafiltration ceases entirely, resulting in acute anuria.

Afferent vs. Efferent Arteriolar Control & Pharmacologic Traps

Glomerular capillary pressure ($P_{\text{GC}}$) and GFR are precisely tuned by modulating pre-glomerular (afferent) versus post-glomerular (efferent) arteriolar resistance:

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3. Neurohormonal Control: RAAS Cascade, Aldosterone & ADH

                             THE RAAS CASCADE

   Renal Hypoperfusion (MAP <80 mmHg)  │  Decreased NaCl to Macula Densa  │  Beta-1 Adrenergic Surge
   ────────────────────────────────────┼──────────────────────────────────┼─────────────────────────
                                       ▼
                       Juxtaglomerular (JG) Granular Cells
                                       │
                                       ▼ Secretes
                                     RENIN (Enzyme)
                                       │
            Angiotensinogen (Liver) ────┴────► ANGIOTENSIN I (Decapeptide)
                                                    │
                       Pulmonary Endothelium (ACE) ──┴──► ANGIOTENSIN II (Octapeptide)
                                                                │
       ┌───────────────────────┬────────────────────────┬───────┴────────────────────────┐
       ▼                       ▼                        ▼                                ▼
Potent Systemic         Preferential Efferent    Zona Glomerulosa (Adrenal)       Hypothalamus / Pituitary
Vasoconstriction (AT1)  Arteriolar Constriction  • Secretes ALDOSTERONE           • Stimulates ADH release
(Increases SVR & MAP)   (Preserves GFR)          • Na+ reabsorption / K+ excretion • Stimulates Thirst drive

ADH (Vasopressin) vs. Aldosterone: Key Distinctions

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4. Acid-Base Physiology & Clinical Calculations

Henderson-Hasselbalch Formulation

The fundamental equation describing the bicarbonate buffer system:

$$\text{pH} = \text{p}K_a + \log \left( \frac{[\text{HCO}_3^-]}{0.0307 \times \text{PaCO}_2} \right) = 6.10 + \log \left( \frac{24}{0.0307 \times 40} \right) = 6.10 + \log(19.54) = 6.10 + 1.29 = 7.39$$

• The ratio of $[\text{HCO}_3^-]$ to $(0.03 \times \text{PaCO}_2)$ is normally 20:1. Any process altering this ratio shifts blood pH away from 7.40.

Primary Acid-Base Disturbances & Expected Compensation Formulas

Anion Gap & Delta Calculations

Serum Anion Gap (AG) = [Na+] - ([Cl-] + [HCO3-])
Normal Range: 8 to 12 mEq/L (mean ~10 mEq/L)

• Correction for Hypoalbuminemia: Serum albumin is the major unmeasured anion (~2.5 mEq/L per 1 g/dL albumin). For every 1.0 g/dL drop in serum albumin below 4.0 g/dL, the normal expected baseline Anion Gap decreases by 2.5 mEq/L:

$$\text{Corrected Anion Gap} = \text{Observed AG} + 2.5 \times (4.0 - [\text{Serum Albumin in g/dL}])$$

• Differential Diagnosis of Metabolic Acidosis:

  • High Anion Gap Metabolic Acidosis (HAGMA - "GOLD MARK"): Glycols (ethylene/propylene), Oxoproline (chronic acetaminophen), L-Lactate (sepsis, shock, hypoperfusion), D-Lactate (short bowel syndrome), Methanol, Aspirin (salicylates), Renal failure (uremia, sulfate/phosphate retention), Ketoacidosis (diabetic, alcoholic, starvation).
  • Normal Anion Gap / Hyperchloremic Metabolic Acidosis (NAGMA - "USEDCARP"): Ureterosigmoidostomy, Saline (0.9% NaCl resuscitation), Enteric loss (diarrhea), Drugs (Acetazolamide), Carbonic anhydrase inhibitors, Addison's disease, Renal Tubular Acidosis (Types 1, 2, 4), Pancreatic fistula.

• Delta-Delta ($\Delta-\Delta$) Ratio: Evaluates for mixed acid-base disorders in the presence of a HAGMA:

$$\text{Delta Ratio} = \frac{\Delta \text{AG}}{\Delta \text{HCO}_3^-} = \frac{\text{Observed AG} - 10}{24 - [\text{Measured } \text{HCO}_3^-]}$$

• $\text{Delta Ratio} = 1.0\text{ to }2.0$: Pure High Anion Gap Metabolic Acidosis.
• $\text{Delta Ratio} < 1.0$ (or $<0.8$): Mixed HAGMA and Normal Anion Gap (Hyperchloremic) Metabolic Acidosis (e.g., DKA treated with massive 0.9% normal saline, or lactic acidosis plus severe diarrhea).
• $\text{Delta Ratio} > 2.0$: Mixed HAGMA and Metabolic Alkalosis (e.g., DKA or lactic acidosis with concomitant nasogastric suctioning or vomiting) or pre-existing compensated chronic respiratory acidosis.

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5. Acute Kidney Injury: RIFLE & KDIGO Criteria

Acute Kidney Injury (AKI) is an abrupt decline in renal function occurring over hours to days, resulting in nitrogenous waste retention (azotemia) and dysregulation of extracellular volume and electrolytes.

KDIGO Staging System for AKI

Prerenal Azotemia vs. Intrinsic Acute Tubular Necrosis (ATN)

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6. Anesthetic Effects on Renal Hemodynamics & Pharmacology

Impact of Anesthetic Agents & Positive Pressure Ventilation

1. Inhalation Anesthetics & IV Hypnotics:
   • Volatile agents (Sevoflurane, Desflurane, Isoflurane) and IV induction agents (Propofol) cause dose-dependent myocardial depression and systemic vasodilation $\rightarrow$ decrease MAP and cardiac output $\rightarrow$ indirect reduction in RBF, GFR, and urine output.
   • Autoregulation curve shifts rightward during deep anesthesia, narrowing the protective autoregulatory window.
2. Mechanical Positive Pressure Ventilation & PEEP:
   • Increased intrathoracic pressure $\rightarrow$ decreases venous return (preload) and right ventricular stroke volume $\rightarrow$ decreases cardiac output.
   • Stimulates arterial baroreceptors and atrial stretch receptors $\rightarrow$ triggers sympathetic activation, RAAS stimulation, and non-osmotic ADH release $\rightarrow$ leads to oliguria and renal vasoconstriction.
3. Sevoflurane, Compound A & Fluoride Nephrotoxicity:
   • Compound A (Fluoromethyl-2,2-difluoro-1-[trifluoromethyl]vinyl ether): Formed when Sevoflurane reacts with strong base carbon dioxide absorbents (potassium hydroxide $\text{KOH}$ or sodium hydroxide $\text{NaOH}$) found in dry soda lime or Baralyme. Produces proximal tubular necrosis in Wistar rats.
   • FDA Recommendation: Maintain fresh gas flow (FGF) $\ge 1\text{ L/min}$ for procedures lasting up to 2 hours, and $\ge 2\text{ L/min}$ for procedures $>2\text{ hours}$ to prevent Compound A accumulation. Modern absorbents (Amsorb, calcium hydroxide without strong bases) eliminate Compound A formation.
   • Inorganic Fluoride ($F^-$): Sevoflurane undergoes ~2-5% hepatic CYP2E1 metabolism releasing fluoride ions. Unlike Methoxyflurane (which produced vasopressin-resistant polyuric renal failure at serum $F^- > 50\ \mu\text{mol/L}$ due to extensive intra-renal metabolism), Sevoflurane does not cause nephrogenic diabetes insipidus because it is rapidly cleared via the lungs and lacks intra-renal bioactivation.

Anesthetic Drug Clearance in Renal Impairment