Structure & Function of the Upper Urinary Tract

Comprehensive coverage of kidney and ureter anatomy, nephron physiology, renal vasculature, and detailed clinical correlations.

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1. OVERVIEW

The upper urinary tract comprises the kidneys and ureters — the organs responsible for filtering blood, forming urine, and conveying it to the bladder. The kidneys are paired retroperitoneal organs that perform excretion, electrolyte regulation, acid-base balance, blood pressure control, and endocrine functions. The ureters are muscular conduits that propel urine from the renal pelvis to the urinary bladder via peristalsis.

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2. THE KIDNEYS — GROSS ANATOMY

Location & Position

• Position: Paired retroperitoneal organs located between the T12 and L3 vertebrae.
• Orientation: The left kidney sits slightly more superior than the right (due to the liver).
• Shape: Bean-shaped (reniform); medial concavity = hilum; lateral convexity.
• Size: ~4–5 inches (10–12 cm) long; ~150 g in adults (size of a fist).
• Covering: Enclosed by renal fascia (Gerota's fascia) — a dense elastic connective tissue sheath; surrounded by perirenal fat.

External Features

Renal Hilum Contents

• Anterior to posterior: Renal vein → Renal artery → Ureter.

The renal sinus is the cavity within the hilum containing fat, vessels, and the collecting system.

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3. THE KIDNEYS — INTERNAL STRUCTURE

The kidney is divided into two primary regions: the renal parenchyma and the collecting system (pyelocalyceal system).

A. Renal Parenchyma

Renal Cortex (Outer Layer)

• Contains most nephron components: renal corpuscles, proximal & distal convoluted tubules, cortical collecting ducts.
• Renal Mantle: Peripheral cortical tissue covering the base of each renal pyramid.
• Renal Columns (of Bertin): Extensions of cortical tissue projecting inward between pyramids toward the renal sinus.

Renal Medulla (Inner Layer)

• Organized into renal pyramids (~9 per kidney).
• Contains medullary rays (descending/ascending limbs of Loop of Henle + medullary collecting ducts).
• Each pyramid + overlying cortex = renal lobe (anatomical unit).
• Renal Papilla: Apex of each pyramid; urine drains through the area cribrosa into minor calyces.

B. Renal Collecting System

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4. THE NEPHRON — FUNCTIONAL UNIT

The nephron is the structural and functional unit of the kidney. Each adult kidney contains 1 to 1.5 million nephrons (occasionally >2.5 million).

Types of Nephrons

A. Renal Corpuscle (Filtration Unit)

The renal corpuscle consists of the glomerulus + Bowman's capsule.

The Glomerulus

• A tuft of fenestrated capillaries supplied by the afferent arteriole and drained by the efferent arteriole.
• Filtration Barrier (3 layers):
  1. Fenestrated Endothelium: Pores 70–100 nm; provides size selectivity.
  2. Glomerular Basement Membrane (GBM): Composed of type IV collagen, laminin, fibronectin, proteoglycans; provides charge selectivity (repels negatively charged proteins).
  3. Podocytes (Visceral Epithelium): Specialized cells with interdigitating foot processes forming slit diaphragms — the final filtration barrier.

Bowman's Capsule

• Double-walled cup surrounding the glomerulus.
• Parietal layer: Simple squamous epithelium.
• Visceral layer: Podocytes.
• Bowman's (Capsular) Space: Collects the ultrafiltrate.
• Vascular pole: Where afferent/efferent arterioles enter/exit.
• Urinary pole: Where filtrate exits into the PCT.

B. Proximal Convoluted Tubule (PCT)

• Location: Renal cortex; adjacent to glomerulus.
• Epithelium: Simple cuboidal with brush border (dense microvilli) → massively increases surface area.
• Key Features: Abundant mitochondria for active transport; numerous vesicles for transcellular transport.
• Function: Reabsorbs 60–80% of the ultrafiltrate:
  — ~65% of Na⁺, ~60% of water
  — Nearly 100% of glucose and amino acids
  — ~90% of bicarbonate
  — Secretes organic acids and bases

C. Loop of Henle

A U-shaped structure that penetrates the renal medulla, creating the countercurrent multiplier system essential for urine concentration.

D. Distal Convoluted Tubule (DCT)

• Location: Renal cortex; near its glomerulus of origin.
• Epithelium: Simple cuboidal; fewer microvilli than PCT.
• Key Features: Highest mitochondrial density in the nephron; extensive basolateral amplification.
• Function:
  — Fine-tuning of Na⁺, K⁺, Ca²⁺, and pH.
  — Aldosterone-sensitive Na⁺ reabsorption (ENaC channels).
  — PTH-sensitive Ca²⁺ reabsorption.
  — Variable water permeability (regulated by ADH indirectly).

E. Collecting Duct System

• Connecting Tubule (CNT): Transition zone; contains intercalated cells (H⁺/HCO₃⁻ regulation).
• Collecting Duct (CD): Lined by principal cells (Na⁺/water reabsorption) and intercalated cells (acid-base).
• Function: Final urine modification; ADH-dependent water reabsorption via aquaporin-2 channels.
• Multiple nephrons drain into a single collecting duct → medullary collecting duct → papillary duct → area cribrosa → calyx.

F. Juxtaglomerular Apparatus (JGA)

The JGA regulates glomerular filtration rate (GFR) via tubuloglomerular feedback.

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5. BLOOD SUPPLY & VASCULATURE

Renal Arterial Tree

Abdominal Aorta → Renal Artery → Segmental Arteries → Interlobar Arteries → Arcuate Arteries → Interlobular Arteries → Afferent Arterioles → Glomerulus → Efferent Arterioles → Peritubular Capillaries / Vasa Recta → Interlobular Veins → Arcuate Veins → Interlobar Veins → Renal Vein → IVC.

Unique Features

• Efferent arterioles form a second capillary bed (unusual in the body):
  — Peritubular capillaries: Surround PCT and DCT in cortex.
  — Vasa recta: Long, hairpin capillaries accompanying juxtamedullary nephrons' loops of Henle; maintain medullary osmotic gradient (countercurrent exchange).

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6. KIDNEY FUNCTIONS SUMMARY

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7. THE URETERS — GROSS ANATOMY

General Characteristics

• Length: ~25–30 cm (adult).
• Diameter: Widest at renal pelvis (~10 mm); narrowest at ureterovesical junction (~1–3 mm).
• Shape: S-shaped, collapsible muscular tubes.
• Course: Retroperitoneal; descend on psoas major, cross the pelvic brim, enter bladder posterolaterally.

Three Anatomical Constrictions (Narrowings)

These are common sites for kidney stone impaction:

1. Ureteropelvic Junction (UPJ) — where renal pelvis narrows into ureter.
2. Pelvic Brim — where ureter crosses the iliac vessels.
3. Ureterovesical Junction (UVJ) — where ureter enters the bladder wall (intramural portion).

Relations

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8. THE URETERS — HISTOLOGY

The ureter wall consists of three layers (from lumen outward):

1. Mucosa

• Urothelium (Transitional Epithelium): 4–6 cell layers; dome-shaped superficial cells that flatten when stretched. Accommodates changes in urine volume.
• Lamina Propria: Loose connective tissue; contains collagen, elastic fibers, and lymphocytes.

2. Muscularis Propria

• Upper 2/3: Inner circular + outer longitudinal smooth muscle layers.
• Lower 1/3: Additional inner longitudinal layer (three layers total) near the bladder.
• Function: Generates peristaltic waves to propel urine.

3. Adventitia

• Outer connective tissue layer with blood vessels, nerves, and lymphatics; anchors ureter to surrounding structures.

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9. URETER FUNCTION & PERISTALSIS

Urine Transport

• Urine is propelled by peristaltic contractions initiated by pacemaker cells in the proximal renal pelvis.
• Contractions occur every 10–15 seconds, moving urine in small boluses.
• The ureterovesical junction acts as a physiological one-way valve:
  — The intramural ureter courses obliquely through the bladder wall.
  — Bladder contraction compresses the intramural ureter, preventing vesicoureteral reflux (VUR).

Neurovascular Supply of the Ureter

Arterial Supply:
— Upper: Renal, gonadal, aortic branches.
— Middle: Common iliac artery branches.
— Lower: Superior/inferior vesical arteries.

Lymphatic Drainage:
— Upper: Lumbar (para-aortic) nodes.
— Lower: Common iliac, external iliac, and internal iliac nodes.

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10. CLINICAL CORRELATIONS