Urinary System

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Urinary System — Key Facts for NEET PG

• Nephron: Functional unit — 1 million per kidney; Parts: Glomerulus → Bowman’s capsule → PCT → Loop of Henle → DCT → Collecting duct
• GFR: ~125 mL/min (180 L/day); Filtration fraction ~20%
• Reabsorption: Most occurs in PCT (65% Na⁺, all glucose, amino acids); Loop concentrates via countercurrent multiplication
• JGA: Juxtaglomerular apparatus — macula densa + JG cells + extraglomerular mesangium; Renin release → Angiotensin II
• ⚡ Exam tip: Loop of Henle creates medullary concentration gradient; Countercurrent multiplier preserves gradient; vasa recta (peritubular capillaries) maintain it

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Urinary System — NEET PG Study Guide

Kidney Anatomy

Location: T12-L3 retroperitoneal Right kidney: Slightly lower (displaced by liver) Coverings: Renal fascia → Perirenal fat → Fibrous capsule

Regions:

• Cortex: Outer region — contains glomeruli, PCT, DCT
• Medulla: Inner — contains loops of Henle, collecting ducts; pyramids (8-18), papillae
• Pelvis: Funnel-shaped; Major and minor calyces → renal pelvis → ureter

Nephron Structure

Types:

Cortical Nephrons (85%):

• Glomerulus in outer cortex
• Short loop of Henle (thin descending, thin ascending)
• Peritubular capillaries

Juxtamedullary Nephrons (15%):

• Glomerulus near corticomedullary junction
• Long Loop of Henle (penetrates deep into medulla)
• Create medullary concentration gradient
• Vasa recta important

Glomerulus

Structure:

• Fenestrated endothelium (70-100 nm pores)
• Basement membrane (GBM) — fused epithelial and endothelial basement membranes
• Podocytes with filtration slits (25-60 nm)
• Mesangial cells (support, phagocytosis)

Filtration Barrier:

• Size selective: Albumin retained (7 nm)
• Charge selective: Negatively charged (heparan sulfate) repels proteins
• GFR = 125 mL/min

NCE Exam Pattern

Common question types:

1. Nephron structure and function
2. Glomerular filtration
3. Tubular reabsorption and secretion
4. Countercurrent mechanism
5. Renal regulation of fluid and electrolytes

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Urinary System — Comprehensive NEET PG Notes

Detailed Theory

1. Renal Blood Supply

Arteries:

• Renal artery → Segmental → Interlobar → Arcuate → Interlobular → Afferent arteriole → Glomerulus

Veins:

• Efferent arteriole → Peritubular capillaries/vasa recta → Interlobular → Arcuate → Interlobar → Renal vein

Special Features:

• Two capillary beds in series: Glomerulus (high pressure ~55 mmHg) then peritubular (low pressure)
• Afferent > Efferent arteriolar resistance maintains glomerular capillary pressure

Cortical vs. Juxtamedullary Nephron Circulation:

• Cortical: Peritubular capillaries
• Juxtamedullary: Vasa recta (long, hairpin capillaries) — important for countercurrent exchange

2. Glomerular Filtration

Forces Driving Filtration (Starling Forces):

Forces favoring filtration:

• PGC (Glomerular capillary hydrostatic pressure): ~55 mmHg

Forces opposing filtration:

• PBS (Bowman’s space hydrostatic pressure): ~15 mmHg
• πGC (Glomerular capillary oncotic pressure): ~30 mmHg

Net Filtration Pressure:

NFP = PGC - PBS - πGC
NFP = 55 - 15 - 30 = 10 mmHg

GFR Regulation:

Autoregulation (kidney maintains GFR despite BP changes 80-180 mmHg):

• Myogenic mechanism: Afferent arteriole stretch → vasoconstriction
• Tubuloglomerular feedback: Macula densa detects NaCl → adjusts afferent arteriole diameter

Hormonal/Neural:

• Sympathetic: ↑ α1 → Afferent constriction → ↓ GFR
• Angiotensin II: Preferentially constricts efferent → maintains GFR (at cost of ↑ peritubular pressure)
• NSAIDs: Inhibit prostaglandins → ↓ GFR

3. Tubular Reabsorption

Proximal Convoluted Tubule (PCT):

• Reabsorbs 65% of filtered Na⁺
• Na⁺/H⁺ exchanger (NHE3) — drives reabsorption
• Glucose, amino acids: Na⁺-coupled secondary active transport (SGLT2 in PCT)
• Water follows solutes (isosmotic)
• Complete glucose reabsorption (max transport Tm ~375 mg/min)
• HCO₃⁻ reabsorption (carbonic anhydrase)
• NH₄⁺ secretion (acid excretion)
• Organic anion and cation secretion

Loop of Henle:

• Descending limb: Water permeable, NaCl impermeable → water leaves → tubular fluid concentrates
• Thick ascending limb: Water impermeable, NaCl reabsorbed via NKCC2 (furosemide-sensitive) → dilutes tubular fluid
• Thin ascending limb: NaCl impermeable (passive paracellular reabsorption in inner medulla)
• Creates medullary concentration gradient (300 → 1200 mOsm/kg)

Distal Convoluted Tubule (DCT):

• Na⁺ reabsorbed via NCC (thiazide-sensitive)
• Ca²⁺ reabsorption via TRPV5 channels (PTH regulated)
• Na⁺/Cl⁺ exchange

Collecting Duct:

• Principal cells: ADH-responsive water reabsorption (AQP2 channels)
• α-intercalated cells: H⁺ secretion (type A), K⁺ secretion
• β-intercalated cells: HCO₃⁻ secretion (claw cell)
• ADH: Increases water permeability → dilute urine becomes concentrated

4. Countercurrent Multiplication

Principles:

• Two parallel tubes with flow in opposite directions
• Multiplied concentration difference along length
• Loop of Henle: Hairpin arrangement

Mechanism:

1. Thick ascending limb actively pumps NaCl into medullary interstitium (Na⁺/K⁺/2Cl⁻)
2. This dilutes tubular fluid in ascending limb
3. Descending limb passively allows water out (high medullary osmolarity)
4. Concentrated fluid enters ascending limb
5. More NaCl pumped out
6. Loop continues, gradient maintained

Vasa Recta:

• Countercurrent exchange system for capillaries
• Low pressure, slow flow
• Carries away water and solutes without disrupting gradient
• Maintains medullary hyperosmolarity

5. Regulation of Urine Concentration

When Dehydrated:

• ↑ Plasma osmolality → ↑ ADH release
• ADH → ↑ Aquaporin-2 insertion in collecting duct
• Water reabsorbed → Concentrated urine (up to 1200 mOsm/kg)
• Urine volume decreases

When Hydrated:

• ↓ Plasma osmolality → ↓ ADH
• No AQP2 insertion
• Water remains in collecting duct
• Dilute urine produced (50-100 mOsm/kg)

6. Regulation of Sodium and Volume

Aldosterone (from adrenal zona glomerulosa):

• Stimulates Na⁺ reabsorption in DCT and collecting duct
• Secretion triggered by: ↑ K⁺, ↓ Na⁺, ↓ blood volume (via angiotensin II)
• Acts on ENaC (epithelial Na⁺ channels)
• K⁺ or H⁺ secreted in exchange

ANP/BNP (atrial/brain natriuretic peptides):

• Released with ↑ atrial stretch (volume expansion)
• ↑ Na⁺ excretion (natriuresis)
• ↓ Renin, ↓ aldosterone
• ↑ GFR

Renin-Angiotensin-Aldosterone System (RAAS):

1. ↓ Renal perfusion pressure → JG cells release renin
2. Renin converts angiotensinogen → angiotensin I
3. ACE (lung endothelium) converts Ang I → Angiotensin II
4. Ang II: Vasoconstriction → ↑ BP; Stimulates aldosterone; Stimulates ADH; Stimulates thirst
5. Aldosterone → ↑ Na⁺ reabsorption → water follows → ↑ blood volume

7. Acid-Base Balance

Sources of Daily H⁺:

• CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (15,000 mmol/day)
• Sulfur and phosphorus containing amino acids (metabolic)

Renal H⁺ Handling:

In PCT:

• H⁺ secretion (NHE3, H⁺ ATPase)
• HCO₃⁻ reabsorption
• NH₄⁺ secretion (primary mechanism of acid excretion)

In Collecting Duct (α-intercalated cells):

• Type A intercalated cells: H⁺ ATPase secretes H⁺, HCO₃⁻ transported out
• Generates new HCO₃⁻ (metabolic alkalosis correction, acidosis correction)

Phosphate Buffer:

• HPO₄²⁻ + H⁺ → H₂PO₄⁻ (excreted)
• Each H⁺ excreted generates new HCO₃⁻

8. Kidney Function Tests

Glomerular Function:

• GFR estimation: Cockcroft-Gault, CKD-EPI, MDRD formulas
• Creatinine clearance: Approximates GFR
• Serum creatinine: Less reliable alone

Tubular Function:

• Urine concentrating ability: Water deprivation test
• Urine dilution: Water loading test
• Fractional excretion of Na⁺ (FENa): <1% in prerenal azotemia

Proteinuria:

• Albuminuria: Microalbuminuria (30-300 mg/day), Clinical albuminuria (>300 mg/day)
• Bence Jones protein: Multiple myeloma (light chains)

9. Micturition

Ureters:

• Transport urine from kidney to bladder
• Peristaltic contractions (pacemaker in minor calyx)
• 3 constrictions: UPJ, pelvic brim, entry to bladder

Bladder:

• Detrusor muscle (smooth muscle)
• Trigone (smooth triangle between ureteric orifices and internal urethral orifice)
• Internal urethral sphincter (involuntary, smooth muscle)
• External urethral sphincter (voluntary, skeletal muscle)

Micturition Reflex:

1. Bladder distension → Stretch receptors → Afferent signals → Spinal cord
2. Parasympathetic (S2-S4) → Detrusor contraction
3. Internal sphincter relaxes (involuntary)
4. With voluntary relaxation of external sphincter → urination
5. Frontal lobe controls voluntary urination (inhibits micturition center)

10. Clinical Correlations

Acute Kidney Injury (AKI):

• Prerenal: ↓ Renal perfusion (70% of cases)
• Intrinsic: ATN (ischemic, nephrotoxic), Acute interstitial nephritis, Glomerulonephritis
• Postrenal: Obstruction

Chronic Kidney Disease (CKD):

• Progressive loss of nephrons
• Causes: DM, HTN, GN, PKD
• Stages: GFR <60 for >3 months
• ESRD: Dialysis or transplant

Glomerulonephritis:

• Nephritic syndrome: Hematuria, proteinuria, HTN, edema, oliguria (RBC casts)
• Nephrotic syndrome: Heavy proteinuria (>3.5 g/day), hypoalbuminemia, edema, hyperlipidemia

Renal Stone Disease:

• Calcium oxalate (most common)
• Struvite (infection stones)
• Uric acid (acidic urine, gout)
• Cystine (genetic)

Practice Questions for NEET PG

1. Describe the structure of a nephron and the function of each segment.
2. Explain the countercurrent mechanism of urine concentration.
3. Discuss the regulation of GFR.
4. Explain the renin-angiotensin-aldosterone system.
5. How do the kidneys regulate acid-base balance?

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