Figure 2. Schematic diagram of O₂ signalling in the microcirculation .

Oxygen in the environment of arterioles can act directly on the arteriolar wall or on cells in the lumen to produce a vasomotor effect (dilatation in the case of reduced $P_{{\mathrm{O}}_2}$, or constriction in the case of elevated $P_{{\mathrm{O}}_2}$). Alternatively, changes in $P_{{\mathrm{O}}_2}$ may be sensed by extravascular cells (parenchymal cells, mast cells, nerves, etc.), a mediator produced, which then acts on the vessel wall to produce the appropriate vasomotor effect. The $P_{{\mathrm{O}}_2}$ values shown below the cross section of the arterioles refer to tissue $P_{{\mathrm{O}}_2}$ in a superfused, intravital preparation measured at the midpoint between the venous ends of two capillaries as reference values, only.