A beautifully illustrated report on the cerebral microcirculation of mice provides many answers to the role of precapillary sphincters in cerebral perfusion autoregulation. Grubb, S., Cai, C., Hald, B.O. et al. Precapillary sphincters maintain perfusion in the cerebral cortex. Nat Commun 11, 395 (2020). In previous blogs we considered the possibility of a Bernoulli effect in capillaries, created by a pre-capillary choke of some sort and a high enough plasma flow to severely reduce the post-choke hydrostatic pressure. The first problem with this hypothesis applied to the circulation as a whole is the substantial number of capillaries that don’t have a choke section. The problem is so serious that Tatsuo Sakai and Yasue Hosoyamada have claimed that;
… explanations and illustrations of the microcirculation with metarterioles and precapillary sphincters can be regarded as inappropriate and misleading in physiology textbooks about the organs and tissues that aim to teach in general and not specific terms.
The second problem is that blood flow velocity necessary to create a significant Bernoulli effect is far too low in the microcirculation. The cerebral cortex is of course a high-flow vascular loop, but even there the capillary red cell velocity was typically only about 4mm/s, approaching 10 mm/s within the precapillary sphincter where one existed; in the mouse cerebral circulation around 30% of capillaries had no choke section.
I have taken the liberty of pasting here a detail from Grubb, Cai and Hald et al. (2020), cited above. Notice that, without constriction, there is a steady pressure decline along the capillary in line with the concept of resistance that emerges from the Pouiseille formula relating pressure and flow in a pipe. Notice too that sphincter constriction sharply reduces the capillary pressure. The Bernoulli effect can be disregarded.