Comparative aspects of blood-brain exchange

A comparative study of the exchange of material between blood, brain tissue, and cerebrospinal fluid (CSF) is presented, mainly utilizing data from the dogfish and the rat. Cerebral blood flow is much lower in the dogfish than in mammalian species and, as a result, may be more limiting in the distribution of material between blood and central nervous tissue in this animal. The blood-brain barrier (BBB) of the dogfish seems to be nearly as tight to extracellular nonelectrolytes, for example, sucrose, as is the BBB of the rat. In both species a major portion of the transport of hexoses across the BBB seems to occur by a carrier-like process. The tissue distribution spaces for materials ranging in size from water to inulin are virtually identical in the rat and the dogfish. Finally, the dogfish appears to possess a special urea transfer system which facilitates the flow of this compound from blood to CSF to brain transfer tissue.