Fluid Compartmentalization: Total Body Water (TBW) constitutes $\sim 60\%$ of adult body mass. Of this, two-thirds ($40\%$ mass) resides within the Intracellular Fluid (ICF) and one-third ($20\%$ mass) in the Extracellular Fluid (ECF: $15\%$ interstitial, $5\%$ vascular plasma).
Hypothalamic Osmoregulation: Osmoreceptors in the supraoptic nucleus detect changes in serum osmolality ($285\text{--}295\text{ mOsm/kg}$). A $1\text{--}2\%$ elevation triggers posterior pituitary secretion of Antidiuretic Hormone (ADH / Vasopressin), stimulating Aquaporin-2 channels in collecting ducts.
Exercise Sweat Losses: Thermoregulatory eccrine sweating accounts for $0.5\text{ to }2.0\text{ L/hr}$ of hypotonic fluid loss during sustained exertion, requiring proportional water and sodium replenishment.