Fig. 4.
Representation of interstitial fluid pressure (Pi) regulation in terms of both fluid compartments and an electrical analogy under transient (A) and steady-state conditions (B). With the electrical analogy, inlet resistance is equivalent to the inverse of the microvascular filtration coefficient (1/Kf), outlet resistance is equivalent to the effective lymphatic resistance (RL), and capacitance is equivalent to the interstitial compliance (ΔV/ΔPi). The effective inlet voltage is equivalent to [Pc − ασ(Cc − Ci)], which depends on capillary pressure (Pc), the reflection coefficient (σ), and the difference in capillary and interstitial protein concentrations (Cc − Ci). The effective outlet voltage is equivalent to (Pout − Pp), which depends on the lymphatic outlet pressure (Pout) and the effective lymphatic pump pressure (Pp). Under transient conditions, when interstitial inflow is greater than outflow, interstitial fluid volume increases. In this case, interstitial compliance affects transient changes in Pi. At steady state, inflow equals outflow, and interstitial compliance ceases to affect Pi. In this case, the ratio of 1/Kf and RL determines whether Pi approaches microvascular driving pressure [Pc − ασ(Cc − Ci)] or effective lymphatic pumping pressure (Pout − Pp).