Figure 1.

Schematic diagram represents the principle of negative feedback regulation in a steam engine using the centrifugal governor. Reducing the steam flow through the valve reduces the speed of the flyballs and causes a proportional downward shift of the gate in the valve, thereby increasing the flow of steam and speed of rotation. The principle of proportional regulation via negative feedback has been transferred to physiology as a model of local blood flow regulation via metabolic vasodilators. In this metaphor, a parenchymal cell is the sensor of the oxygen delivery rate, acting as the flyballs unit, which produces the metabolic vasodilators in response to hypoxia, and the smooth muscle cells in the arteriolar wall act as the gate valve. In this model active regulation is initiated by the lack of oxygen delivery and realized by reducing the arteriolar tone. The disadvantage of this model of regulation is the permanent state of tissue hypoxia required to produce metabolites and the failure to find an elusive set of metabolic vasodilators to account for the observed vasodilation.