Which other system in the body works closely with the kidneys to regulate arterial blood ph?

The correct answer is the respiratory system.

The pH of the extracellular fluids (blood) can be controlled by separately regulating the partial pressure of CO2, which determines the carbonic acid concentration via respiration and the bicarbonate ion concentration in the extracellular fluids via renal system. Control of the carbonic acid concentration in the blood is achieved by changes in the rate and depth of breathing. Breathing releases or retains CO2 (and thus carbonic acid) in the blood plasma and thus, changes its pH.