Gross Anatomy
Paired retroperitoneal organs (~10-12 cm, 150 g). Right kidney slightly lower (liver). T12-L3 vertebral levels.
External Anatomy
- Renal capsule — fibrous outer covering
- Gerota fascia — perirenal fat cushions kidney
- Hilum (medial) — contains Renal Vein (anterior), Artery (middle), Ureter (posterior) — VAU mnemonic
Internal Anatomy
- Cortex — outer; glomeruli, PCT, DCT, cortical collecting ducts
- Medulla — inner; loops of Henle, vasa recta, medullary collecting ducts; renal pyramids → papillae → minor calyces → major calyces → renal pelvis → ureter
| Vessel | Notes |
|---|---|
| Renal artery → segmental → interlobar → arcuate → interlobular | End arteries; no anastomoses → infarcts are wedge-shaped |
| Afferent → glomerulus → efferent arteriole | Unique portal-like arrangement |
| Vasa recta | Long loops into medulla; countercurrent exchange; concentrate urine |
Nephron
~1 million nephrons per kidney. Cortical nephrons (85%) = short loops. Juxtamedullary (15%) = long loops; key for concentration.
| Segment | Key Function | Reabsorbs |
|---|---|---|
| PCT | Bulk iso-osmotic reabsorption | 67% Na, H2O; ALL glucose, AA, HCO3 |
| Thin descending loop | H2O permeable only (concentrates) | H2O passively |
| Thick ascending loop | NKCC2; H2O impermeable; dilutes fluid | Na, K, Cl (furosemide target) |
| DCT | Ca2+ (PTH); NCC (thiazide target) | Na, Cl, Ca2+ |
| Collecting duct | Aldosterone → Na/K; ADH → H2O (AQP2) | Na (aldosterone); H2O (ADH) |
Diuretic targets: Loop (furosemide) → NKCC2 in thick ascending limb. Thiazide → NCC in DCT. Amiloride/triamterene → ENaC in collecting duct. Acetazolamide → carbonic anhydrase in PCT.
Glomerular Filtration Barrier
Three layers filter ~180 L/day by size AND charge:
- Fenestrated endothelium — 70-100 nm pores; bars cells; negatively charged glycocalyx
- GBM — type IV collagen + heparan sulfate (negative charge repels albumin)
- Podocytes — foot processes + nephrin-bridged filtration slits; main size barrier
Loss of GBM charge (minimal change disease) → albumin leaks → nephrotic syndrome (proteinuria >3.5 g/day, hypoalbuminemia, edema, hyperlipidemia). GFR normal (no GBM destruction). Responds to steroids.
GFR ~125 mL/min normal. Estimated by CKD-EPI using creatinine. Inulin clearance = gold standard. Cystatin C more accurate in some populations.
Juxtaglomerular Complex
- JG cells (granular cells) — modified smooth muscle in afferent arteriole; secrete renin in response to: ↓BP, ↑sympathetics, ↓NaCl at macula densa
- Macula densa — specialized DCT cells sensing NaCl; ↓NaCl → signal JG cells to release renin
- Lacis cells — between macula densa and JG cells; signaling role
RAAS
Renin → Angiotensinogen → Ang I → (ACE in lung) → Ang II → ↑Aldosterone + ↑vasoconstriction + ↑ADH + ↑thirst
ACE inhibitors ↓ efferent tone → ↓GFR → creatinine rise expected. Contraindicated in bilateral renal artery stenosis (precipitates acute renal failure) and pregnancy (teratogenic).
Embryology
Kidney develops from intermediate mesoderm through 3 stages: Pronephros (week 4; regresses) → Mesonephros (weeks 4-8; male genital ducts) → Metanephros (permanent kidney).
- Ureteric bud (from mesonephric duct) → ureter, pelvis, calyces, collecting ducts
- Metanephric mesenchyme → glomerulus, PCT, loop of Henle, DCT
Horseshoe kidney = lower poles fuse; caught on IMA → stays at L3; most common renal fusion anomaly; associated with Turner syndrome. Potter sequence = bilateral renal agenesis → oligohydramnios → lung hypoplasia + limb deformities + facial anomalies.
Clinical Pearls
| Condition | Key Features | Urinalysis |
|---|---|---|
| Nephrotic syndrome | >3.5 g/day protein; edema; hyperlipidemia; hypercoagulable | Fatty casts; oval fat bodies |
| Nephritic syndrome | Hematuria; HTN; oliguria; mild proteinuria | RBC casts (pathognomonic) |
| Minimal change disease | Most common nephrotic in kids; podocyte effacement | Massive proteinuria; no RBC |
| IgA nephropathy | Most common GN worldwide; hematuria 1-3d after URI | Hematuria (gross) |
| CKD | GFR <60 >3 months; anemia, hyperK, acidosis, renal osteodystrophy | Waxy/broad casts |
Dialysis indications — AEIOU: Acidosis (refractory), Electrolytes (hyperkalemia), Intoxication, Overload (fluid refractory to diuretics), Uremia (pericarditis, encephalopathy).