pH = 7.42

CO2 = 28

HCO3 = 18

This blood gas presents a new problem: the pH is normal, but the CO2 and the HCO3 are both abnormal. This could be a completely compensated respiratory alkalosis or it might be a completely compensated metabolic acidosis. How do we choose between these two possibilities?
Keep in mind that most physiological compensatory systems are strongest when the value they control is far from normal and weakest when the value is close to normal. For example, if you have a steep metabolic acidosis your respiratory control center will stimulate rapid, deep breaths. On the other hand, a very mild acidosis will barely change breathing patterns, if at all. The compensatory mechanisms that control acid - base balance become weaker as the pH approaches the normal value of 7.40. In fact, the compensation will usually stop several hundredths of a pH unit short of 7.40. Because of that, this blood gas probably represents a compensated respiratory alkalosis. The pH of 7.42, although technically normal, is slightly to the alkalotic side of 7.40. Renal excretion of HCO3 lowered the pH, but stopped before it got all the way down to 7.40 :
A FULLY COMPENSATED RESPIRATORY ALKALOSIS