Microcirculation and Capillary Exchange

A newly developed technique for determination of the electrical resistance of the capillary wall was applied to microvessels at the surface of the frog brain. Current was injected into a capillary or venule via a microelectrode and the ensuing intravascular potential profile away from the current source was determined with a second microelectrode placed at various positions along the capillary. The membrane resistance was calculated according to the theory for leaky cables used in determinations of axon membrane resistance. The average resistance was 1870 omega . cm2. Since the surface vessels of the frog brain are devoid of glial investment but otherwise similar to brain parenchymal vessels, the results prove that the endothelium is the site of the blood-brain barrier. The electrical resistance is similar to that of a 'tight' epithelium.