THE RENAL EXCRETION OF HEMOGLOBIN: REGULATORY MECHANISMS AND THE DIFFERENTIAL EXCRETION OF FREE AND PROTEIN-BOUND HEMOGLOBIN*†

Abstract

A study of urinary hemoglobin excretion and renal hemoglobin transport was made in 10 normal human subjects. Under conditions of a progressively increasing plasma hemoglobin concentration, hemoglobinuria did not occur until the plasma level exceeded the capacity of plasma proteins to bind hemoglobin. When this capacity, which averaged 128 mg%, was exceeded, free, unbound hemoglobin appeared in plasma and was excreted in the urine. A linear relationship was demonstrated between the excretory rate and plasma level of free hemoglobin as the plasma concentration increased. From this relationship calculations were made of the glomerular clearance and tubular reabsorption of free hemoglobin. The glomerular clearance averaged 5 ml/minute/1.73 m2 of body surface area and averaged 5% of the inulin clearance. The estimated maximal reabsorptive rate averaged 1.0 ml/minute/1.73 m2 of body surface area. This value was too small to be considered as conclusive evidence of tubular reabsorptive activity. Neither protein-bound hemoglobin nor methemalbumin was excreted in the urine when these constituents were present in plasma. This was attributed to a failure of these large heme-proteins to traverse the glomerular membrane.