Figure 2. Estimation of changes in vessel radius following a step increase in pressure.

In this experiment, a marker red cell was followed before and after a step increase in pressure from 57 to 71 cmH₂O. The logarithm of the distance of the red cell from the occlusion site, l, decreases linearly with time until the step increase in pressure (indicated by the vertical line at t = 2.34 s), when l drops from one linear relation of slope S₁ (-0.021) to a second linear relation of slope S₂ (-0.031). Extrapolation of S₁ to the time at which pressure was increased yields a value l₁ and backward extrapolation of the second slope to the same time yields the value l_2. If r₁, the vessel radius prior to the increase in pressure is known, r₂, the new equilibrium value of vessel radius after pressure has been increased, can be calculated from l₁ and l₂ (equation (4)). Also illustrated is the estimation of an intermediate length (using S₂) from which an intermediate value of r can be calculated (horizontal arrow).