Read the linked article.

The diameter of the average RBC is ~7.7μ, about 1μ larger than the diameter of the average brain capillary. In order for RBCs to pass through capillaries it is necessary for them to deform (and in so doing place the maximum amount of surface area in contact with the vascular endothelium to facilitate gas exchange). RBC deformability is critically dependent upon RBC intracellular adenosine triphosphate (ATP) concentration being adequate. With periods of ischemia of ~ 7 minutes, RBCs become depleted of ATP and become rigid, making passage through brain capillaries more difficult (higher arterial pressure required) or impossible.

In-house research conducted by the author has demonstrated that the cerebral microcirculation remains profoundly compromised for 30-60 minutes following reperfusion, even when circulation is restored using cardiopulmonary bypass. Brain parencymal and endothelial cell swelling, as well as changes in the zeta potential of the red blood cells, may all be contributing to the extensive blood sludging and microvascular stasis observed after reperfusion following 10 minutes of global normothermic ischemia in the laboratory.