Anatomy of the Kidney

Introduction

• Kidneys are major excretory organs located on the posterior abdominal wall, one on each side of the vertebral column.
• Extend from T12 vertebra to center of L3 vertebra.
• Right kidney is slightly lower than the left due to the liver.

Location and Orientation

• Upper ends (poles) of kidneys are closer together than lower poles.
• Upper poles are 2.5 cm away from the midline, hilum is 5 cm away, and lower poles are 7.5 cm away.
• Transpyloric plane passes through the upper part of the right kidney's hilum and lower part of the left kidney's hilum.

Shape and Measurements

• Shape: Bean-shaped.
• Measurements:
  • Length: 11 cm
  • Width: 6 cm
  • Thickness: 3 cm
  • Weight: 150 g (males), 135 g (females)

External Features

• Each kidney has:
  • Poles: Superior and inferior.
  • Surfaces: Anterior (convex) and posterior (flat).
  • Borders: Medial (concave and slopes downward) and lateral (convex).
• Hilum: Medial border with deep vertical slit transmitting:
  • Anterior to posterior: Renal vein, renal artery, renal pelvis, subsidiary branches of renal artery, lymphatics, nerves.

Anterior Relations of the Kidneys

• Right Kidney:
  • Right suprarenal gland, right lobe of liver, second part of duodenum, hepatic flexure of colon.
• Left Kidney:
  • Left suprarenal gland, spleen, stomach, pancreas, splenic vessels, splenic flexure of colon.

Posterior Relations

• Same for both kidneys except for:
  • Ribs: Right kidney associated with 12th rib; left kidney with 11th and 12th ribs.
• Muscles: Diaphragm, quadratus lumborum, psoas major, transversus abdominis.
• Nerves: Subcostal (T12), iliohypogastric (L1), ilioinguinal (L1).

Capsules of the Kidney

1. Fibrous Capsule: True capsule, thin membrane closely covering the kidney.
2. Perirenal Fat: Adipose tissue between fibrous capsule and renal fascia.
3. Renal Fascia: Fibroelastic sheath surrounding the kidney.
4. Pararenal Fat: Between renal fascia and anterior layer of thoracolumbar fascia.

Macroscopic Structure

• Cortex: Outer layer below the renal capsule, granular appearance.
  • Divided into renal columns and cortical arches.
• Medulla: Inner area containing 5-11 renal pyramids.
  • Each pyramid has a base directed to cortex and an apex projecting into minor calyx.
• Renal Sinus: Cavity within kidney communicating outside through hilum, containing vessels, nerves, fat, and renal pelvis.

Microscopic Structure

• Composed of 1 to 3 million urine-forming tubules (nephrons).
• Nephron: Functional unit, consists of renal corpuscle and renal tubule.
  • Segments: Proximal convoluted tubule, thin descending limb, thick ascending limb, distal convoluted tubule, collecting ducts.
  • Two types: Cortical Nephrons (85%) and Juxtamedullary Nephrons (15%).

Blood Supply

• Renal Artery: Supplies kidneys, arises from abdominal aorta.
  • Accessory renal arteries present in 30% of individuals.
  • Divides into anterior and posterior divisions at the hilum.
• Arterial Flow: Renal artery → Segmental arteries → Interlobar arteries → Arcuate arteries → Interlobular arteries → Efferent arterioles.

Venous Drainage

• Renal Veins: Drain blood from kidneys, exit at hilum, empty into inferior vena cava.
  • Left renal vein is longer than the right and drains the left suprarenal and gonadal veins.

Lymphatic Drainage

• Lymphatics drain into para-aortic lymph nodes at the level of renal arteries.

Nerve Supply

• From the renal plexus (sympathetic and parasympathetic fibers).
• Sympathetic fibers: T10 to L1.
• Parasympathetic fibers: Vagus nerve.