This rete is responsible for the osmotic isolation of the inner medulla from the rest of the kidney and so permits the excretion of a hypertonic urine when circumstances require. Since the rete also isolates the inner medulla from gaseous exchange, any metabolism in this area is anaerobic, and red cells, which would serve no purpose there, are ordinarily shunted from the arteriolae recti by an unknown mechanism into the capillary plexus surrounding the tubules of the outer zone of the medulla.