1. Urea is present in the cells of the proximal convoluted tubule in a concentration higher than that of the blood or than that of the cells of any of the other kidney tubules.
2. Such a condition can only be reconciled to an assumption of an active secretion (excretion) on the part of these cells.
3. Urea also passes through the glomerular filter with the other crystalloids of the blood plasma.
4. The final concentration of urea is due to the above mentioned secretion by the proximal convoluted tubule, and to the absorption of water in other parts of the tubule.