Final answer:
The most rapid defense against changes in blood pH is buffering of hydrogen ions. Respiratory compensation and renal compensation mechanisms also play a role, but they take varying amounts of time.
Step-by-step explanation:
The most rapid defense against changes in blood pH is buffering of hydrogen ions. Buffer systems in the body, such as proteins, HCO3-, and ammonia, have a very large capacity to absorb or release hydrogen ions as needed to resist a change in pH. These buffers can act within fractions of a second to maintain blood pH within a narrow range.
Respiratory compensation and renal compensation mechanisms also play a role in maintaining blood pH, but they take varying amounts of time. The respiratory tract can adjust blood pH upward or downward in minutes by changing the respiratory rate and exhaling CO2 or conserving CO2. The renal system can also adjust blood pH through the excretion of hydrogen ions and the conservation of bicarbonate, but this process takes hours to days to have an effect.