In gastric outlet obstruction, persistent vomiting leads to the loss of large volumes of gastric fluid rich in hydrochloric acid (HCl). This results in a decrease in serum chloride and hydrogen ions, causing a hypochloremic metabolic alkalosis. Additionally, volume depletion activates the renin-angiotensin-aldosterone system, promoting renal retention of sodium and bicarbonate, further worsening the alkalosis. This characteristic electrolyte disturbance helps in the clinical diagnosis and management prior to surgical correction.

Reference: Schwartz's Principles of Surgery, 11th Edition.