1. The elimination of neutral red by the renal epithelium is a composite process, consisting of a direct and an indirect secretion.
2. The mechanism controlling direct secretion is concerned with the permeability of the two cell membranes. These two membranes may be affected independently in the direction of either an increased or decreased permeability, with a corresponding increase or decrease in the elimination of the dye.
3. The mechanism controlling indirect secretion is concerned with the mitochondrial apparatus of the cell. By means of change in the form and constituent substance of its structures, the dye is concentrated within the cell and slowly eliminated.
4. Direct secretion, depending on the condition of sensitive membranes, is easily disturbed. Such disturbances account for the wide variations in the elimination of dye observed in the functioning of abnormal kidneys. Indirect secretion, depending on the simpler factor of the solubility of the dye in the protoplasmic constituents, continues even when the cells are severely damaged.