  Various models have been proposed to explain the urine concentrating
mechanism in mammals, however uncertainty remains regarding the origin of the
energy required for the production of concentrated urine. We propose a novel
mechanism for concentrating urine. We postulate that the energy for the
concentrating process is derived from the osmotic potentials generated by the
separation of afferent blood into protein-rich efferent blood and
protein-deplete filtrate. These two streams run in mutual juxtaposition along
the length of the nephron and are thus suitably arranged to provide the osmotic
potential to concentrate the urine. The proposed model is able to qualitatively
explain the production of various urine concentrations under different clinical
conditions. An approach to testing the feasibility of the hypothesis is
proposed.
