Sodium and Water balance

Normal osmolality 275-290 mmol/l

• intracellular volume regulated by plasma osmolality - maintained by adjusting water balance (losses vs intake)
• plasma volume regulated by sodium balance

Water balance

• normal plasma osmolality 275-290 mmol/l
• alterations affect hypothalamus
  • thirst
  • ADH
• normal kidney can cope with 10-20l/day above requirements - hyponatraemia only if defective renal water excretion

Sodium balance

• receptors sense decrease in effective circulating volume
  • kidney, heart
• release
  • aldosterone - stimulates sodium re-absorption in distal nephron
  • renin-angiotensin system

Disease States

• Hyponatraemia
  • including Syndrome of Inappropriate ADH secretion (SIADH)
• Hypernatraemia

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