The substance concentration of base in extracellular fluid determined at a pH of 7.40 and Pco2 of 40 mm Hg. Because this quantity cannot be determined directly, a model of extracellular fluid is used as a basis. The model consists of one volume of blood plus two volumes of plasma. As with the base excess of blood, this quantity helps one assess the respiratory versus metabolic components in acid-base balance. In contrast to the base excess of blood, the base excess of extracellular fluid is said to be more representative of the acid base status of the accessible fluid compartment and thus more appropriate for deciding on and evaluating therapy.