Ageing Physiology

Maintaining normal fluid status, or euvolaemia, is a basic requirement for health involving thirst, kidney function, hormone systems, and fluid shifts between body compartments. Normal fluid balance is regulated by the renin-angiotensin-aldosterone system and anti-diuretic hormone, which control electrolyte and fluid retention, excretion, vasoconstriction, blood pressure, and thirst. Ageing weakens the ability to maintain fluid homeostasis, causing a blunted thirst response, altered anti-diuretic hormone activity, reduced creatinine clearance, and reduced ability to concentrate urine. Fluid is also lost through the gut, lungs, and skin in less regulated ways, adding complexity to fluid balance. Age-related physiological changes increase the likelihood of both dehydration and fluid overload.