The concepts of buffer base and base excess form part of the Astrup acid-base system. The term buffer base refers to all substances in the buffering system of whole blood that are able to bind excess H+. Bicarbonate forms slightly more than one half of the total buffer base; hemoglobin makes up about one third of the total buffer base, consisting of three fourths of the nonbicarbonate buffer system. Normal buffer base values for any patient are therefore calculated on the basis of the actual hemoglobin concentration as well as normal values for pH and HCO–3. The term base excess refers to any difference in the measured total quantity of blood buffer base from the patient’s calculated normal value. Thus, an increase in total buffer base (e.g., an increase in HCO–3) is considered a positive base excess; a decrease in total buffer base from calculated normal (e.g., a decrease in HCO–3) is considered a negative base excess (some prefer to use the terms “base excess” and “base deficit” rather than positive or negative base excess). Venous blood has a base excess value about 2.0-2.5 mEq/L higher than arterial blood.