Metabolism

Master answer

Urea

• Amino acids -> ammonia -> urea
• 30g produced per 100g of dietary protein per day
• Normal serum concentration: 3-6mmol/L

• 60% of renally filtered urea is eliminated

Important in maintaining the counter-current multiplier in the renal loop of Henle and collecting duct

• Ascending limb – active transport of sodium and chloride out of lumen
  • Increases osmolality of interstitial fluid
• Descending limb – permeable to water
• Filtrate constantly refreshed
  • Leads to concentration gradient, with osmolality increasing toward the medullary interstitial fluid and tubular lumen
• In absence of vasopressin, dilute urine is excreted
• In presence of vasopressin
  • Aquaporin 2 channels open in the collecting duct, allowing movement of water out of the lumen and into the interstitial fluid, particularly in medulla where osmolality is high
  • Collecting duct becomes permeable to urea
    • Urea increases osmolality of interstitum
      • Allowing resorption of Na⁺ and Cl^–, while maintaining a high interstitial osmolality

Mooney 2016

Answer by Mooney