Renal System Study Guide

Renal Anatomy & Physiology

Kidney Functions

• Fluid and electrolyte balance
• Acid-base regulation (excrete H+, reabsorb/generate HCO3)
• Waste product removal (urea, creatinine)
• Blood pressure regulation (RAAS, fluid balance)
• Erythropoietin production (stimulates RBC production)
• Vitamin D activation (calcium metabolism)

Nephron Structure

• Glomerulus: Filtration of blood
• Proximal tubule: Reabsorbs ~65% of filtered Na, water, glucose, amino acids
• Loop of Henle: Concentrating mechanism, creates medullary gradient
• Distal tubule: Fine-tuning of Na, K, regulated by aldosterone
• Collecting duct: Water reabsorption under ADH control

Renal Assessment Parameters

• GFR (Glomerular Filtration Rate): Best overall measure of kidney function
• Normal GFR: >90 mL/min/1.73m²
• Creatinine: Muscle metabolism byproduct, inversely related to GFR
• BUN (Blood Urea Nitrogen): Protein metabolism byproduct
• BUN:Creatinine ratio: Normal 10-20:1
• BUN:Cr >20:1 suggests prerenal cause

Acute Kidney Injury

AKI Definition (KDIGO)

• Increase in serum creatinine ≥0.3 mg/dL within 48 hours, OR
• Increase in creatinine ≥1.5x baseline within 7 days, OR
• Urine output <0.5 mL/kg/hr for 6 hours
• AKI is common in ICU (30-50% of patients)

Pre-Renal AKI

• Cause: Decreased renal perfusion (hypovolemia, heart failure, sepsis)
• BUN:Cr ratio >20:1
• FENa <1% (kidneys retaining sodium)
• Urine sodium <20 mEq/L
• Urine osmolality >500 mOsm/kg (concentrated urine)
• Treatment: Restore volume and perfusion

Intrinsic Renal AKI (ATN)

• Acute Tubular Necrosis: Most common intrinsic cause
• Causes: Prolonged ischemia, nephrotoxins (contrast, aminoglycosides, NSAIDs)
• BUN:Cr ratio 10-15:1
• FENa >2% (tubular damage impairs sodium reabsorption)
• Urine sodium >40 mEq/L
• Urine osmolality <350 mOsm/kg (unable to concentrate)
• Muddy brown casts on urinalysis
• Treatment: Remove offending agent, supportive care, may need dialysis

Post-Renal AKI

• Cause: Urinary tract obstruction (stones, BPH, tumor, clot)
• Must be bilateral or obstruct single functioning kidney
• Diagnosis: Hydronephrosis on ultrasound
• Treatment: Relieve obstruction (Foley, nephrostomy, stent)
• Watch for post-obstructive diuresis after relief

Chronic Kidney Disease

CKD Staging by GFR (mL/min/1.73m²)

• Stage 1: GFR ≥90 - Normal or high GFR with evidence of kidney damage
• Stage 2: GFR 60-89 - Mildly decreased GFR with kidney damage markers
• Stage 3a: GFR 45-59 - Mild to moderately decreased
• Stage 3b: GFR 30-44 - Moderate to severely decreased
• Stage 4: GFR 15-29 - Severely decreased (pre-dialysis planning stage)
• Stage 5: GFR <15 - Kidney failure (ESRD, dialysis or transplant needed)
• GFR calculation uses creatinine, age, sex, and race

Albuminuria Staging (mg/g creatinine)

• A1: <30 mg/g - Normal to mildly increased (low risk)
• A2: 30-300 mg/g - Moderately increased (microalbuminuria)
• A3: >300 mg/g - Severely increased (macroalbuminuria/proteinuria)
• Albuminuria indicates glomerular damage and predicts progression
• Both GFR stage AND albuminuria category determine overall risk
• Higher albuminuria = faster progression and higher cardiovascular risk

CKD Risk Stratification

• Green (low risk): G1-G2 with A1 - annual monitoring
• Yellow (moderate risk): G1-G2 with A2, or G3a with A1 - monitor every 6-12 months
• Orange (high risk): G3a with A2, G3b with A1-A2 - monitor every 3-6 months
• Red (very high risk): G4-G5 or any stage with A3 - monitor at least every 3 months
• Risk increases exponentially with lower GFR and higher albuminuria

CKD Complications

• Fluid overload: Edema, hypertension, pulmonary congestion
• Electrolyte imbalances: Hyperkalemia, hyperphosphatemia, hypocalcemia
• Metabolic acidosis: Impaired H+ excretion, reduced HCO3 generation
• Anemia: Decreased erythropoietin production (typically starts Stage 3)
• Renal osteodystrophy: Secondary hyperparathyroidism, bone disease
• Uremic syndrome: Encephalopathy, pericarditis, bleeding tendency, pruritus
• Cardiovascular disease: Leading cause of death in CKD patients

CKD Management Goals

• Blood pressure control: Target <130/80 mmHg (ACE-I or ARB preferred)
• Diabetes control: Target A1c <7% (individualize in advanced CKD)
• Protein restriction: May slow progression in advanced stages
• Avoid nephrotoxins: NSAIDs, aminoglycosides, IV contrast (with precautions)
• Anemia management: Erythropoiesis-stimulating agents when Hgb <10 g/dL
• Bone-mineral disease: Phosphate binders, vitamin D supplementation
• Prepare for RRT: Fistula planning when GFR <30 (Stage 4)

KDIGO AKI Staging Criteria

KDIGO Definition of AKI

• Increase in serum creatinine ≥0.3 mg/dL within 48 hours, OR
• Increase in creatinine ≥1.5x baseline within 7 days, OR
• Urine output <0.5 mL/kg/hr for 6 hours
• KDIGO = Kidney Disease: Improving Global Outcomes
• Standardized criteria for consistent AKI diagnosis

AKI Stage 1 (Mild)

• Creatinine: Increase ≥0.3 mg/dL OR 1.5-1.9x baseline
• Urine Output: <0.5 mL/kg/hr for 6-12 hours
• May be reversible with prompt treatment
• Focus on removing nephrotoxins and optimizing perfusion

AKI Stage 2 (Moderate)

• Creatinine: 2.0-2.9x baseline
• Urine Output: <0.5 mL/kg/hr for 12-24 hours
• Higher risk for progression to Stage 3
• Close monitoring of fluid balance and electrolytes

AKI Stage 3 (Severe)

• Creatinine: ≥3.0x baseline OR increase to ≥4.0 mg/dL
• Urine Output: <0.3 mL/kg/hr for ≥24 hours OR anuria for ≥12 hours
• OR: Initiation of Renal Replacement Therapy (RRT)
• High mortality, often requires dialysis
• May progress to ESRD

Clinical Application

• Always establish baseline creatinine for accurate staging
• Monitor BOTH creatinine AND urine output
• Early recognition allows for intervention before severe AKI
• Staging helps guide treatment intensity and prognosis
• Higher stages associated with increased mortality and length of stay

Dialysis

Indications for Dialysis (AEIOU)

• A - Acidosis (refractory metabolic acidosis)
• E - Electrolytes (hyperkalemia unresponsive to treatment)
• I - Intoxication (dialyzable toxins: methanol, ethylene glycol, lithium)
• O - Overload (fluid overload refractory to diuretics)
• U - Uremia (symptomatic: encephalopathy, pericarditis, bleeding)

Hemodialysis (HD)

• Intermittent: 3-4 hours, 3x/week typically
• CRRT (Continuous): For hemodynamically unstable ICU patients
• CRRT modes: CVVH (hemofiltration), CVVHD (diffusion), CVVHDF (both)
• Access: AV fistula (best long-term), AV graft, tunneled catheter
• Complications: Hypotension, arrhythmias, air embolism, bleeding

Peritoneal Dialysis

• Uses peritoneal membrane as dialysis membrane
• Dialysate instilled into peritoneal cavity, dwells, then drained
• Types: CAPD (manual exchanges), APD (cycler overnight)
• Advantages: Can do at home, preserves residual renal function
• Complications: Peritonitis (cloudy effluent, abdominal pain), catheter issues

Electrolyte Imbalances

Sodium Disorders

• Hyponatremia (<135 mEq/L): SIADH, heart failure, cirrhosis, thiazides
• Symptoms: Confusion, seizures, coma (especially if acute or severe)
• Treatment: Fluid restriction (SIADH), NS or hypertonic saline
• Correct slowly: Max 8-10 mEq/L per 24 hours to avoid osmotic demyelination
• Hypernatremia (>145 mEq/L): Water deficit, diabetes insipidus
• Treatment: Free water replacement, correct slowly

Potassium Disorders

• Normal: 3.5-5.0 mEq/L
• Hyperkalemia (>5.0): Renal failure, acidosis, cell lysis, ACE-I/spironolactone
• ECG changes: Peaked T waves → widened QRS → sine wave → VF
• Treatment: Calcium gluconate (stabilize), insulin/glucose, albuterol, kayexalate, dialysis
• Hypokalemia (<3.5): Diuretics, vomiting, diarrhea
• ECG changes: Flattened T waves, U waves, prolonged QT
• Treatment: IV or oral potassium replacement, correct magnesium

Calcium & Phosphorus

• Inverse relationship: High phosphorus = Low calcium
• Hypercalcemia: Malignancy, hyperparathyroidism
• Symptoms: "Stones, bones, groans, psychiatric moans"
• Treatment: IV fluids, loop diuretics, bisphosphonates, calcitonin
• Hypocalcemia: CKD, hypoparathyroidism, low vitamin D
• Symptoms: Tetany, Chvostek sign, Trousseau sign, prolonged QT
• Treatment: IV calcium gluconate, treat underlying cause

Magnesium

• Normal: 1.5-2.5 mEq/L
• Hypomagnesemia: Alcoholism, diuretics, diarrhea, malnutrition
• Symptoms: Similar to hypocalcemia, arrhythmias, refractory hypokalemia
• Treatment: IV magnesium sulfate
• Hypermagnesemia: Renal failure, iatrogenic (MgSO4 for preeclampsia)
• Symptoms: Loss of DTRs, respiratory depression, cardiac arrest
• Treatment: IV calcium, dialysis if severe

AKI Lab Comparison

Pre-Renal vs ATN vs Post-Renal

• Pre-Renal: BUN/Cr >20:1, FENa <1%, Urine Na <20, Urine Osm >500
• ATN: BUN/Cr 10-15:1, FENa >2%, Urine Na >40, Urine Osm <350
• Post-Renal: BUN/Cr 10:1, FENa variable, variable urine output

Key Points

• FENa = (Urine Na × Plasma Cr) / (Plasma Na × Urine Cr) × 100
• FENa not reliable if patient on diuretics
• Urine microscopy helpful: Muddy brown casts = ATN
• Ultrasound: Look for hydronephrosis in post-renal