Figure 10. Separation of the effects of microvascular pressure and flow velocity on P_S.

A shows measurements of P_S plotted against U in a single frog capillary before (⋄) and after (♦) partial occlusion of the vessel outflow. The corresponding balancing pressures (P_bal) applied to the micropipette are plotted against U in B. The vertical arrow in B indicates that after partial occlusion, P_bal had to be raised 16 cmH₂O to maintain a perfusion velocity of 1300 μm s⁻¹. This rise in P_bal was accompanied by an increase in P_S of 0.137 μm s⁻¹ corresponding to an increment in P_S of 0.0085 μm s⁻¹ for each increment of 1 cmH₂O in P_bal. Thus, under conditions of free flow, where an increase in U from 1300 to 3300 μm s⁻¹ is accompanied by an increase in P_bal of 3.5 cmH₂O, the associated increase in P_S should be 0.0298 μm s⁻¹. The measured increase in P_S is seen to be 0.20–0.25 μm s⁻¹.