Mr. Dаniels, а 73-yeаr-оld male with a histоry оf hypertension and mild chronic kidney disease, arrives in the Emergency Department after a 3-day episode of vomiting and diarrhea from presumed gastroenteritis. In the last 24 hours, he has barely eaten or drunk anything and feels extremely weak and dizzy when standing. Initial Assessment: Vital signs: T 36.2°C, HR 114, BP 86/54 (lying), 68/48 (sitting), RR 24, SpO₂ 97% (RA) Physical exam: Poor skin turgor, dry oral mucosa, cool extremities, decreased capillary refill Neuro: Alert but fatigued, periodic confusion Urine Output: 150 mL over the last 12 hours Labs: Na+ = 150 mmol/L (135-145) K+ = 3.0 mmol/L (3.5-5.0) Cl- = 112 mmol/L (98-106) HCO₃⁻ = 17 mmol/L (22-28) BUN = 42 mg/dL (7-20) Creatinine = 2.0 mg/dL (baseline 1.3 two months ago) Glucose = 98 mg/dL ABG: pH 7.30, PaCO₂ 32 mmHg, HCO₃⁻ 17 mmol/L, PaO₂ 96 mmHg Current Orders: Two large-bore IV lines placed Isotonic saline at 150 mL/hr Continuous cardiac monitoring Strict intake and output STAT repeat labs and telemetry updates ordered Mr. Daniels’ ABG reveals pH 7.30 and HCO₃⁻ 17 mmol/L. Which system is providing fast-acting compensation for his acid-base disturbance?