Stewart in 1983 (Can ./ Physiol Pharmacol 1983: 61: 1444) reintroduced plasma buffer base under the name "strong ion difference" (SID). Buffer base was originally introduced by Singer and Hastings in 1948 (Medicine (Balrimore) 1948: 27: 223). Plasma buffer base. which is practically equal to the sum of bicarbonate and albuminate anions, may be increased due to an excess of base or due to an increased albumin concentration. Singer and Hastings did not consider changes in albumin as acid-base disorders and therefore used the base excess, i.e., the actual buffer base minus the buffer base at normal pH and pCOz, as measure of a non-respiratory acid-base disturbance. Stewart and followers, however, consider changes in albumin concentration to be acid-base disturbances: a patient with normal pH, pCO,, and base excess but with increased plasma buffer base due to increased plasma albumin concentration get the diagnoses metabolic (strong ion) alkalosis (because plasma buffer base is increased) combined with metabolic hyperalbuminaemic acidosis. Extrapolating to whole blood, anaemia and polycytaemia should represent types of metabolic alkalosis and acidosis, respectively. This reveals that the Stewart approach is absurd and anachronistic in the sense that an increase or decrease in any anion is interpreted as indicating an excess or deficit of a specific acid. In other words, a return to the archaic definitions of acids and bases as being the same as anions and cations.We conclude that the acid-base status (the hydrogen ion status) of blood and extracellular fluid is described in terms of the arterial pH, the arterial pCO,, and the extracellular base excess. It is measured with a modem pHblood gas analyser. The electrolyte status of the plasma is a description of the most important electrolytes, usually measured in venous blood with a dedicated electrolyte analyser, i.e., Na', Cl-, HCO,-, and K'. Albumin anions contribute significantly to the anions, but calculation requires measurement of pH in addition to albumin and is usually irrelevant. The bicarbonate concentration may be used as a screening parameter of a nonrespiratory acid-base disturbance when respiratory disturbances are taken into account. A disturbance in the hydrogen ion status automatically involves a disturbance in the electrolyte status, whereas the opposite need not be the case.

In several previous experiments to determine the composition of interstitial fluid, the results varied depending on the collecting technique, and the electrolyte concentrations differed from those of a hypothetical ultrafiltrate of plasma. In our approach, since a change of position from standing to supine is accompanied by hemodilution with interstitial fluid, we used the changes in hematocrit and composition of plasma in 20 subjects before and after lying down to calculate the composition of added interstitial fluid. The estimated protein concentration was 20.6 g/L, and the concentrations of total calcium and magnesium were low, in accord with a lower concentration of protein-bound calcium and magnesium. The activity of free cations was also lower, in agreement with a Donnan equilibrium potential of 1 mV across the endothelium. The concentration of leukocytes and platelets decreased according to the hemodilution, implying no escape or mobilization of these elements.

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