Reaction of the vasa nervorum of the facial nerve during stimulation with neurotransmitters.

The local blood flow of a tissue depends on the pressure difference in the respective vessel system. In approximate terms, the flow of fluid in an arterial vascular system can be described by the Hagen-Poiseuille law based on the laminar flow of a homogeneous fluid in a rigid tube with wettable walls. The flow-time volume prevailing in a vessel is accordingly proportional to the fourth power of the radius. Regulation of local blood flow is therefore very much more effective when increase of the pressure difference.