Final answer:
The secondary defense against changes in pH that activates within minutes is respiratory compensation. It alters the respiratory rate to adjust the blood's CO2 levels, impacting the pH balance.
Step-by-step explanation:
The secondary defense against changes in pH that requires minutes to be activated is respiratory compensation. This mechanism involves adjusting the blood pH by altering the rate of respiration. When the body needs to correct metabolic acidosis, the respiratory rate increases to expel more CO₂ and thereby reduce acidity by adjusting the bicarbonate to carbonic acid ratio more favorably. Conversely, the respiratory system slows down to retain CO₂ in an attempt to compensate for metabolic alkalosis, though this mechanism has its limits.