Adrenergenic tone

[Rhino1000]

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Adrenergenic tone is the constant sympathetic nervous input of all the precapillary sphincters. I wrote in my notes when I read TPR's chapter on the cardiovascular system, that the adrenergenic tone "ensures that a level of resistance is always present so perfusion is everywhere possible."

Can anyone explain the mechanism by which adrenergenic tone is beneficial to perfusion? Specifically, I have a few questions:
Does this not lower the blood pressure in the blood capillaries? After all, I believe that arteriole/precapillary sphincter constriction is the principle determinant for blood resistance. If it does lower the blood pressure for the local capillaries, then doesn't that decease the amount of perfusion through those capillaries? If this is the case, then is the purpose of the adrenergenic tone to decrease the blood pressure to the sites that don't need blood (and are subject to a heavy adrenergenic tone), while then necessarily increasing the blood pressure and therefore perfusion of capillaries receiving less of an adrenergenic tone? Or does the adrenergenic tone simply slow the rate of blood flow, making blood flow through capillaries for a longer period of time, thus allowing more materials to diffuse across the membrane in this fashion?

Explication regarding the mechanism of adrenergenic tone's effect on perfusion is appreciated!

 

[Rhino1000]

Also, if you happen to know, I believe nutrients and wastes pass through capillary intercellular clefts, or gaps in between cells, rather than literal diffusion across the thin epithelial cells. Can anyone verify this?