Observations on phosphates in blood and on the urinary excretion of phosphates

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Abstract
Examinations were made of ultrafiltrates and dialysates of plasma to determine the state of the acid soluble phosphate, which was found to be present in 2 forms: (1) as a simple salt of ortho-phosphoric acid, of which at least 80% is ultrafilter-able and dialysable and therefore neither colloidal nor bound to the plasma colloids; (2) as organically combined (ester) phosphoric acid. The amount of (2) is usually about 10% of the amount of (1) both in normal plasma and in that from patients with nephritis. Further experiments were made to determine the distribution of inorganic phosphate between plasma and corpuscles, using specimens of blood (a) without anticoagulant, (b) using oxalate, (c) using heparin, (d) using plasma from umbilical cords. Free phosphate was invariably found in the corpuscles but in distinctly lower concentration than in the plasma. The amount of inorganic phosphate in the plasma, as determined by the usual methods, is less than the amount to be expected from the Donnan equilibrium between corpuscles and plasma. The very low concentration of ester P in the plasma cannot be raised by taking phosphoric esters by mouth. It can be raised temporarily by intravenous injection of phosphoric esters. In the intact rabbit or in man, there is no evidence in favor of the view that the ester P of the plasma controls the rate of excretion of inorganic phosphate. On the contrary, the evidence is all in favor of the control of such excretion by the plasma level of inorganic phosphate. There appears to be a renal threshold for phosphate excretion. Within wide limits there is no relationship between the amount of urinary phosphate and the urinary volume.

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