Starling Resistors in cerebral blood flow:
The Starling Resistor (named for its inventor) can be generalized to any compressible tubular vessel, for example a vein, carrying a flow of fluid (venous blood) under pressure from one end to the other, and passing through a segment where it is surrounded by another fluid, in this case spinal fluid, which is also under pressure.
The effect of the external pressure on the vessel will, if it exceeds the internal pressure of the flowing fluid, cause the vein to collapse. If there is sufficient pressure to threaten the blood supply this way from spinal fluid (as in hydrocephalus) one would expect the venous pressure would respond by increasing so as to prevent the collapse of veins. Maybe that is part of the reason for the (spontaneous) ballooning of cerebral veins seen on MRIs, and the increased venous pressure.
If there is less than this collapsing pressure, the result is that the external spinal fluid exerts a force on the compressible wall of the vein which acts as a source of friction which would not otherwise be there. The effect is to increase the apparent viscosity of the blood in the vein. There has been no change in the diameter of the vein, but there has been a concomitant change in the resistance to blood flow due to the increased viscosity. The effect on flow is not the same magnitude as it would have been if the vein had really narrowed, but, in the same direction in all respects.
The blood viscosity would not go from thick inside the brain, to thin immediately outside. There would be a gradual transition, or perhaps a turbulence caused by the transiton from an area of external pressure to an area of none. In any case the increased viscosity might be felt in neck veins. That may be why the ultrasound guy in Montreal said the blood in my neck veins was like peanut butter.
On a personal note, perhaps what runs in my family is an inability to let off excess spinal fluid pressure. My brother got it at birth with his hydrocephalus. Maybe I got it gradually in my forties with my CCSVI.
you will be very interested in the talks of Dr Beggs at the symposium. he will talk a lot about the relationship of csf and venous blood flow