Bicarbonate and the Renal Regulation of Acid Base Balance1

Under the usual circumstance of ingestion of an acid ash diet the normal individual is faced with a deficit of available base. Those stores of circulating bicarbonate which enter the glomerular filtrate along with other crystalloids of the plasma are carefully conserved; only minute amounts are lost in the urine. But when an alkaline ash diet is ingested or sodium bicarbonate is administered, supplies of available base exceed the needs of the body, and the excess is excreted in the urine as bicarbonate. Reabsorption and excretion of bicarbonate constitute, therefore, the primary renal means of defending the body against depletion and expansion of its alkali reserve. The quantitative importance of the reabsorptive processes in the economy of the human body can be illustrated by a simple calculation. Approximately 190 liters of plasma, containing on an average 25 milhmols of bicarbonate per liter, are filtered through the glomeruli in 24 hours (20). Thus, 4,750 millimols of bicarbonate, or 400 grams when expressed as the sodium salt, are tentatively excreted into the tubular urine each day, roughly 5 times the total body store of available base. A little over 2 millimols are excreted per day in 1.5 liters of urine of pH 6.0. Thus the reabsorptive mechanism is 99.95 per cent efficient under ordinary conditions. In contrast, the excretory processes are somewhat less impressive, although more than 100 grams of sodium bicarbonate have been ingested and excreted per day on certain therapeutic and experimental regimes. The present paper is concerned with two aspects of the renal regulation of acid base balance: 1, a quantitation of the relationship between the plasma concentration and the rates of tubular reabsorption and urinary excretion of bicarbonate; 2, an elucidation of the relationship between the mechanism for the reabsorption of bicarbonate and the mechanism for acidifying the urine. METHODS. Our experiments have been performed on normal female mongrel dogs trained to lie quietly with loose restraint on a comfortable animal board. The dogs were routinely fasted for 16 to 20 hours before use, and were well hydrated at the start of each experiment by the administration of approximately 40 cc. of water per kilo of body weight by stomach tube. In several experiments plasma bicarbonate was reduced by the oral administration of 200 to 300 cc. of 3 per cent ammonium chloride per day for several days prior to the experiment. In all experimentIs plasma bicarbonate was increased by the intravenous