Acid Base Balance

pH<7.35 => acidosis
pH>7.45 => alkalosis

major extracellular buffering system of body HCO3-/H2CO3
H2O + CO2 << H2CO3 >> H+ + HCO3-

le Chatelier's principle usually followed.

1. PaCO2 controlled by changes in ventilation (normal value 35-45 mm Hg) 40 is usual value.
acidosis compensated by hyperventilation (lt shift), alkalosis by hypoventilation (rt shift).
2.HCO3- (22-30 meq/L) 24 is usual value
acidosis compensated by renal reabsorption of bicarbonates by tubular cells from tubular fluid,collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to increased formation of the NH3 buffer, alkalosis compensated by excretion of more bicarbonate from kidney, by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering rates of glutamine metabolism and ammonia excretion

major extracellular buffer- bicarbonate & ammonia
major intracellular buffer- proteins & phosphates

For usual calculation, look for the primary defect in terms of acidosis or alkalosis by pH value.
Next is identifying the primary defect in terms of metabolic or respiratory acidosis or alkalosis, both the reason and effect being in the same direction in terms of change in hydrogen ion concentration.
Third is to identify the compensatory mechanism, if any