L23 - Urine

• What is one main function of the kidneys?

  Maintain body’s fluid osmotic concentration
• What happens during dehydration in terms of urine production?

  Small volume of concentrated urine is produced
• What occurs during overhydration regarding urine volume?

  Large volume of dilute urine is produced
• What are the countercurrent mechanisms in the kidneys?

  • Relies on juxtamedullary nephrons & vasa recta
  • Fluid flows in opposite directions in adjacent segments
  • Establishes medullary osmotic gradient
  • Allows kidneys to vary urine concentration
• What are the two mechanisms of countercurrent mechanisms?

  Countercurrent multiplier and countercurrent exchanger
• How do the ascending and descending limbs of the nephron loop differ in permeability?

  Descending limb allows H2O, ascending limb allows NaCl
• What does the countercurrent multiplier do in the nephron loop?

  Creates an osmotic gradient for H2O reabsorption
• How does the countercurrent exchanger maintain the osmotic gradient?

  By allowing blood in vasa recta to remain isosmotic
• What is the summary of countercurrent mechanisms for urine concentration?

  • Juxtamedullary nephron loop creates medullary osmotic gradient
  • Vasa recta maintains the medullary osmotic gradient
• What is the typical composition of urine?

  About 95% water and 5% solutes
• What are the main nitrogenous wastes found in urine?

  Urea, uric acid, and creatinine
• What is the typical pH range of urine?

  About 4.5 to 8.0
• What causes urine to develop an ammonia odor over time?

  Bacterial metabolism of urea
• What pigment gives urine its yellow color?

  Urobilin (urochrome)
• What is the function of the ureters?

  Convey urine from kidneys to bladder
• What are the structural features of the ureters?

  • Inner mucosa continuous with kidney pelvis & bladder
  • Double-layered muscularis for peristalsis
• What is the capacity of the urinary bladder?

  About 500-800 ml of urine
• How does the bladder accommodate urine without increasing pressure significantly?

  Detrusor muscle and folded mucosa allow distention
• What is the length of the urethra in females?

  3-4 cm long
• What is the length of the urethra in males?

  About 20 cm long
• What are the two sphincter muscles associated with the urethra?

  • Internal urethral sphincter: involuntary, smooth muscle
  • External urethral sphincter: voluntary, skeletal muscle
• What is micturition?

  Act of emptying the bladder
• What is required for micturition to occur?

  Contraction of detrusor muscle and relaxation of sphincters
• How does the nervous system regulate micturition?

  Parasympathetic stimulates, sympathetic inhibits micturition
• What are the key points regarding the control of urine concentration?

  • Medullary osmotic gradient regulates urine osmolarity
  • ADH increases water reabsorption during dehydration
  • ADH decreases water reabsorption during overhydration
• What is the effect of diuretics on urine output?

  They increase urine output
• How do osmotic diuretics work?

  They carry water with them, reducing reabsorption
• What condition can act as an osmotic diuretic?

  Diabetes mellitus with high glucose concentration
• What are the physical and chemical properties of urine?

  • Composition: 95% water, 5% solutes
  • Slightly acidic (pH 6)
  • Develops ammonia odor over time
  • Yellow color from urobilin
• What are the true or false statements regarding the urinary system?

  • Medullary gradient established by countercurrent multiplier: True
  • Water reabsorbed only in descending limb: True
  • Cortical nephrons important for concentrated urine: False
  • ADH levels rise during overhydration: False
  • Diuretic increases urine output: True
  • Bilirubin gives urine yellow color: False
  • Transitional epithelium withstands pressure: True
  • Relaxation of sphincters required for micturition: True
  • Inhibition of micturition during stress: True