Vascular Mechanisms Controlling a Constant Blood Supply to the Brain ("Autoregulation")

The segmental resistance in the major arteries of the brain and the respective smaller cerebral arteries carrying blood from the circle of Willis was computed (with a mathematical method developed recently) using the inlet and outlet pressures of the internal carotid arteries, as well as the venous pressure in the brain sinuses of dogs. Under the conditions of stepwise changes of the perfusion pressure the following localization of "autoregulatory" responses of the cerebral arteries has been found: changes in the inlet pressure of the internal carotid artery produced corresponding changes of its vascular resistance resulting in a relative constancy of the outlet pressure of the artery, i.e., pressure in the circle of Willis; resistance changes in the smaller brain arteries were evident only when the alterations of the perfusion pressure were too big and the major arteries were unable to eliminate the disturbance. The responses of the internal carotid arteries were eliminated when their muscular layer was maintained normal, but deprived of the nervous control (when the arteries were continuously perfused with blood or oxygenated Ringer-Krebs bicarbonate solution shortly after the death of the animal). Thus, evidence was obtained that the vascular responses were brought about by a nervous and not by a purely muscular mechanism, as is usually assumed.