Electrical signal length constants and resolution of blood flow control. Concepts are depicted for brain vasculature. Similar principles likely apply in other organs, modified by specific angioarchitectural features and molecular configurations. (A) Brain vasculature. Top: Top-down view showing overlapping pial (surface) arterial and veinous networks. Bottom: Side-view showing a pial artery transitioning to a penetrating arteriole as it dives into the brain and gives way to the capillary network. Left inset: Neurovascular unit at the initial capillaries surrounded by contractile pericytes. Right inset: Neurovascular unit of the deep capillary bed with thin strand pericytes. EC, endothelial cell; GJ, gap junction; PSJ, peg-socket junction; RBC, red blood cell. (B) Diameter–voltage relationship for a pial artery showing that maximal working range is controlled across ∼30 mV of membrane potential. Adapted with permission from Knot and Nelson (1998). (C) Regenerative and passive signal transmission modes. Top: Theoretical signal transmission–distance relationships for regenerative and passive signals. Regenerative mechanisms will conduct over greater lengths of vessel due to the signal being actively renewed. Bottom: Graphical depiction of regenerative versus passive transmission modes. (D) The length constant of transmission, dictated by input amplitude and the electrical properties of the local vasculature, will determine the resolution of blood flow control. Top: Activation of large areas of tissue will initiate large signals in the vasculature that conduct over longer distances and induce more contractile cells to relax and produce a low-resolution blood flow increase that encompasses many cells. Middle: Smaller metabolically active fields will evoke electrical signals with a shorter length constant, recruiting fewer vessels and evoking more localized blood flow increases that perfuse a smaller tissue volume. Bottom: The smallest active regions may evoke a higher resolution blood flow increase still.