U01.21.025 Capillary Fluid Exchange (Jv)

Written by

Learning Objective:
Understand and apply the Starling equation for capillary fluid exchange (Jv), which determines the movement of fluid across capillary membranes.

Formula:

$J_v = \text{net fluid flow} = K_f \Big[ (P_c - P_i) - \sigma (\pi_c - \pi_i) \Big]$

Explanation:

$J_v$ = Net fluid movement (positive = filtration, negative = absorption)
$K_f$ = Filtration coefficient (capillary permeability × surface area)
$P_c$ = Capillary hydrostatic pressure (pushes fluid out of capillary)
$P_i$ = Interstitial hydrostatic pressure (pushes fluid into capillary)
$\pi_c$ = Capillary oncotic (plasma) pressure (pulls fluid into capillary)
$\pi_i$ = Interstitial oncotic pressure (pulls fluid out of capillary)
$\sigma$ = Reflection coefficient (measure of capillary wall impermeability to proteins; ranges 0–1)