Sodium and Water Homeostasis


General (NEJM 2015;372:55 & 373:1350)

  • Disorders of serum sodium are generally due to Δs in total body water, not sodium
  • Hyper- or hypo-osmolality → rapid water shifts → Δs in brain cell volume → Δ MS, seizures

Key hormones

  • Antidiuretic hormone (ADH): primary hormone that regulates sodium concentration
    • Stimuli: hyperosmolality (290–295 mOsm), ⇊ effective arterial volume, angiotensin II
    • Action: insertion of aquaporin-2 channels in principal cells → passive water reabsorption
    • urine osmolality is an indirect functional assay of the ADH-renal axis
    • Uosm range: 50 mOsm/L (no ADH) 200 mOsm/L (maximal ADH)