06: Regulation of Osmolality

Question 1

Excretion of water pathway

Answer 1

1. Water ingestion
2. Dilution of plasma, ECV, ICV
3. Swelling of hypothalamic osmoreceptor cells
4. Inhibition of vasopressin release + inhibition of thirst
5. Restoration of water content

Question 2

Dilute urine

Answer 2

• Uosm < Posm (nl ~285)
• Max dilution = 50 mOsm
• Specific gravity < 1.010

Question 3

Mechanisms of body water depletion

Answer 3

• Lung (H2O exhaled): 0.5-1.0 L/day
• Skin (sweat; dilute NaCl sol): up to 5-10 L/day during heavy exercise
• GI tract (secretions; dilute NaCl sol): <0.5 L/day; diarrhea up to 20 L/day
• Kidney (urine): 0.5 L/day nl; potential for 20 L/day

Question 4

Formation of a concentrated urine

Answer 4

• Hyperosmotic medulla
• Draws water out of collecting duct
  
  • Water conserved
  • Urine concentrated
• Progressively more concentrated moving from cortex to tip of medulla
• Maximum concentration = 1200 mOsm

Question 5

Minimal urine volume

Answer 5

• AKA maximal concentration
  
  • Uosm > Posm
  • Specific gravity > 1.010
• Urine volume normal if:
  
  • contains osmotic waste products produced during the day (filtration nl)
  • Excrete ~600 mOsm waste/day
    
    • Excreted in 500 mL of urine if urine maximally concentrated (1200 mOsm/L)
    • Thus, 500mL/24 hrs ~20 mL/hr (nl)

Question 6

Nephrogenic Diabets Insipidus

Answer 6

• Congenital X-linked recessive form: mutation in vasopressin receptor, AVPR2
• Autosomal (recessive and dominant) forms: mutation in AQP2 gene that codes for water channel protein, aquaporin

NB: **Central DI **= vasopressin not present