Structure of kidney - cortex, medulla, nephron, envelopes

Structure of the Kidney

General Characteristics

• Shape and Size:
  • Bean-shaped with a smooth convex lateral surface and concave medial surface.
  • Dimensions: 10–12 cm long, 5–7 cm wide, and 3–4 cm thick.
  • Weight: 120–170 g (varies by sex and body size).
• Orientation:
  • The right kidney is slightly lower than the left, due to the position of the liver.
  • Positioned retroperitoneally, along the T12 to L3 vertebrae.
• Color:
  • Reddish-brown due to rich vascularization.

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External Anatomy and Envelopes

External Features

• Renal Hilum:
  • Located on the concave medial margin.
  • Entry/exit point for:
    • Renal artery (posterior to the vein).
    • Renal vein (anteriorly positioned).
    • Renal pelvis (continuous with the ureter).
• Poles:
  • Superior pole: Topped by the adrenal gland and protected by ribs 11–12.
  • Inferior pole: Near the iliac crest, more palpable on deep inspiration.

Envelopes (Support Structures)

1. Renal Capsule:
   • Dense, fibrous connective tissue covering the kidney surface.
   • Protects against trauma and infection.
2. Perirenal Fat:
   • Surrounds the renal capsule.
   • Acts as a cushion and thermal insulator.
3. Renal Fascia (Gerota’s Fascia):
   • Encloses the kidney and adrenal gland.
   • Divided into anterior and posterior layers.
4. Pararenal Fat:
   • Lies external to the renal fascia, providing additional protection and support.

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Internal Anatomy

1. Cortex

• Location: Outer layer beneath the renal capsule.
• Color: Brownish-red due to dense capillary networks.
• Structures:
  • Renal corpuscles: Glomerulus and Bowman’s capsule, the site of blood filtration.
  • Proximal and distal tubules: Involved in reabsorption and secretion.
  • Medullary rays: Extensions of the medulla into the cortex.
  • Renal columns: Extend into the medulla, separating the pyramids.

2. Medulla

• 
  Location: Inner portion of the kidney.
• Color: Lighter than the cortex.
• Structures:
  • Renal pyramids: Triangular structures (8–18 per kidney).
    • Base faces the cortex; apex (renal papilla) points toward the renal sinus.
  • Renal papillae: Deliver urine to the minor calyces.
  • Loops of Henle and collecting ducts: Responsible for urine concentration.

3. Renal Sinus

• Central cavity containing:
  • Renal pelvis: Funnel-shaped structure continuous with the ureter.
  • Major and minor calyces: Collect urine from the papillae.
  • Fat, vessels, and nerves: Fill the remaining space for structural support.

4. Nephron (Functional Unit)

• Approximately 1–1.5 million nephrons per kidney.
• Functions: Blood filtration, reabsorption, secretion, and urine formation.
• Components:
  1. Renal Corpuscle:
     • Glomerulus: Filters blood plasma.
     • Bowman’s capsule: Collects filtrate for further processing.
  2. Renal Tubule:
     • Proximal Convoluted Tubule (PCT): Reabsorbs water, ions, and nutrients.
     • Loop of Henle: Creates a concentration gradient for urine.
     • Distal Convoluted Tubule (DCT): Adjusts electrolyte and pH balance.
  3. Collecting Duct:
     • Drains multiple nephrons and transports urine to the renal papillae.

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Blood Supply and Venous Drainage

Arterial Supply

• Renal Arteries: Branch from the abdominal aorta at L1–L2.
• Branches:
  1. Segmental Arteries: Enter the hilum.
  2. Interlobar Arteries: Run between the renal pyramids.
  3. Arcuate Arteries: Arch along the base of the pyramids.
  4. Cortical Radiate Arteries: Extend into the cortex.

Venous Drainage

• Renal Veins: Drain into the inferior vena cava.
• Left Renal Vein receives:
  • Left gonadal vein.
  • Left suprarenal vein.

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Lymphatic Drainage

• Lymphatic vessels follow the renal veins.
• Drain into lumbar (aortic and caval) lymph nodes.

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General Features

• Tubular structures (25–30 cm) transporting urine to the bladder.
• Retroperitoneal throughout their course.
• Constrictions:
  1. Ureteropelvic junction.
  2. At the pelvic brim (crossing iliac vessels).
  3. Ureterovesical junction (entry to bladder).

Blood Supply

• Abdominal Segment: Supplied by renal and gonadal arteries.
• Pelvic Segment: Supplied by superior vesical and inferior vesical arteries.

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Syntopy

Right Kidney

• Anterior: Liver, duodenum, right colic flexure.
• Posterior: Diaphragm, 12th rib, psoas major, quadratus lumborum.

Left Kidney

• Anterior: Stomach, pancreas, spleen, left colic flexure.
• Posterior: Diaphragm, 11th and 12th ribs, psoas major.

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Clinical Relevance

Renal Pathologies

1. Kidney Stones (Nephrolithiasis):
   • Mineral deposits form in the renal pelvis or ureters, causing severe pain.
   • Common sites: Ureteropelvic junction, pelvic brim, and ureterovesical junction.
2. Polycystic Kidney Disease:
   • Genetic disorder causing fluid-filled cysts that enlarge the kidney.
3. Renal Failure:
   • Acute or chronic failure to filter blood adequately.
   • May require dialysis or transplantation.
4. Pyelonephritis:
   • Infection of the kidney tissue, often secondary to urinary tract infections.

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Additional Facts

• Development: Kidneys develop from the metanephros during the 5th–12th weeks of embryogenesis.
• Variation: Horseshoe kidney and renal agenesis are congenital anomalies.
• Physiology: Filters ~50 gallons of blood daily, excreting ~1.5 liters of urine.