Gastrointestinal electrolyte losses during emesis:

• Gastric juice contains substantial concentrations of hydrochloric acid ($\text{HCl}$) and sodium chloride ($\text{NaCl}$). During prolonged vomition, the massive ejection of gastric fluids causes extensive depletion of both sodium ($\text{Na}^+$) and chloride ($\text{Cl}^-$) ions (leading to hypochloremia, hyponatremia, and metabolic alkalosis).
• Potassium is lost to a lesser degree compared to the bulk sodium and chloride losses.
• While potassium and chloride are lost, sodium is quantitatively the predominant extracellular cation lost alongside chloride.
• Bicarbonate is retained, not lost, in upper gastric vomiting.

Chloride distribution in physiological fluid compartments:

• Cerebrospinal Fluid (CSF) contains the highest physiological chloride concentration among all body fluids, typically measuring $120\text{--}130\text{ mEq/L}$ (mmol/L) due to active epithelial secretion by choroid plexus cells and Donnan equilibrium balancing low CSF protein content.
• Blood plasma has a lower chloride concentration of approximately $98\text{--}106\text{ mEq/L}$.
• Urine chloride concentration varies with dietary intake and tubular handling, typically lower than CSF under resting conditions.
• Milk contains low chloride concentrations ($25\text{--}35\text{ mEq/L}$) due to high lactose osmolality.