ABSTRACT
Water homeostasis in the central nervous system (CNS) is vital for optimal brain function. Diffusional fluxes of water between blood and CNS are rapid and bidirectional. In a seminal investigation of humans by Sweet and colleagues more than six decades ago, heavy water (deuterium oxide) was used as a marker to demonstrate the rapidity and regionality of water distribution between blood and CSF (Sweet et al. 1950). This pioneering study at Harvard importantly addressed rates and routes of water permeation. Later investigations of rapid tracer uptake by adult rat CSF and brain revealed that radiolabeled water and antipyrine (lipid-soluble) penetrated large cavity CSF and cortical regions to achieve distribution equilibrium in seconds to minutes (Johanson and Woodbury 1977, 1978); thus, neither permeability nor blood fl ow were rate-limiting factors in the uptake of these extensively-distributed tracers across the blood-CSF barrier (BCSFB) and blood-brain barrier (BBB).
