urine

Learning Objectives

  • Describe the structure and functions of the kidney.
  • Describe the microscopic structures of the kidney.
  • Explain glomerular filtration.
  • Discuss the production of urine.
  • List the chemical compositions of urine.
  • Describe the structure and functions of the ureters, urinary bladder, and urethra.

Components of the Urinary System

  • Kidneys: Major excretory organs.
  • Ureters: Transport urine from kidneys to urinary bladder.
  • Urinary Bladder: Temporary storage for urine.
  • Urethra: Tube that transports urine out of the body.

Functions of the Kidney

  1. Excretion of Wastes: Removal of metabolic wastes (urea, ammonia, creatinine, uric acid).
  2. Ion Regulation: Balances Na⁺, K⁺, Ca²⁺, Cl⁻, phosphate hormones.
  3. Blood pH Regulation: Excretes H⁺, conserves HCO₃⁻.
  4. Blood Volume Regulation: Controls water via ADH and ANP.
  5. Blood Pressure Regulation: Secretes renin in low BP conditions.
  6. Blood Osmolarity Maintenance: Keeps solute concentration near 300 mOsm/L.
  7. Hormone Production: EPO for RBC formation; Calcitriol for calcium absorption.
  8. Blood Glucose Regulation: Gluconeogenesis (forming glucose from non-carbohydrate sources).

Internal Anatomy of Kidneys

  • Weight: Each kidney weighs approximately 150g.
  • Dimensions: About 11cm long, 6cm wide, 3cm thick.
  • Cortex: Contains nephrons (functional units).
  • Medulla: Composed of renal pyramids and columns.

Nephron Structure

  • Renal Corpuscle: Glomerulus + Bowman's capsule.
  • Tubules: Proximal tubule, Loop of Henle, Distal convoluted tubule.
  • Collecting Ducts: Drain filtrate into renal pelvis.

Filtration Membrane Layers

  1. Endothelium: Fenestrated; allows small solutes to pass.
  2. Basement Membrane: Blocks large proteins.
  3. Podocyte Layer: Contains filtration slits for water and small molecules.

Urine Formation Processes

  1. Glomerular Filtration: Blood pressure forces fluid into Bowman’s capsule.
  2. Tubular Reabsorption: Reclaims useful materials (e.g., water, glucose) into the blood.
  3. Tubular Secretion: Adds wastes and ions from blood into filtrate.

Glomerular Filtration Regulation

  • Intrinsic Mechanisms: Stable GFR at MAP 80-180 mmHg.
  • Extrinsic Neural Regulation: Sympathetic response reduces GFR during stress.
  • Extrinsic Hormonal Regulation: Involves RAA system and ANP.

Chemical Composition of Urine

  • Urine is about 95% water; 5% solutes (urea, creatinine, uric acid).

Ureters and Urethra

  • Ureters: Conveys urine; walls made of mucosa, muscularis, and adventitia.
  • Urethra: Drains urine; shorter in females (3-4 cm), longer in males (up to 20 cm); has internal and external sphincters.

Micturition

  • Act of emptying the bladder; stretch receptors trigger spinal reflexes that coordinate urine storage and release.