U01.07.025 Capillary fluid exchange

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Learning Objectives

Identify the four Starling forces that govern fluid movement across capillary walls. Master the Starling Equation to calculate net fluid flow ($J_v$). Analyze the pathophysiological mechanisms leading to edema. Distinguish between hydrostatic and oncotic pressure gradients.

The Starling Forces

Fluid movement is determined by the balance of hydrostatic and oncotic pressures between the capillary and the interstitium.

$P_c$ (Capillary Hydrostatic Pressure): Pushes fluid out of the capillary.
$P_i$ (Interstitial Hydrostatic Pressure): Pushes fluid into the capillary.
$\pi_c$ (Plasma Oncotic Pressure): Pulls fluid into the capillary (mediated by proteins like albumin).
$\pi_i$ (Interstitial Oncotic Pressure): Pulls fluid out of the capillary.

The Starling Equation

The net fluid flow ($J_v$) is calculated as follows:

$J_v = K_f \cdot [(P_c - P_i) - \sigma(\pi_c - \pi_i)]$

$K_f$ : Capillary permeability to fluid (Filtration coefficient).
$\sigma$ : Reflection coefficient (a measure of capillary impermeability to protein; $1.0$ means totally impermeable).

Pathophysiology of Edema

Edema is excess fluid outflow into the interstitium, occurring when Starling forces are imbalanced:

Mechanism	Primary Change	Clinical Examples
↑ Hydrostatic Pressure	$\uparrow P_c$	Heart Failure (HF), Venous obstruction.
↓ Plasma Proteins	$\downarrow \pi_c$	Nephrotic syndrome, Liver failure, Malnutrition.
↑ Capillary Permeability	$\uparrow K_f$	Toxins, infections, burns, and inflammation.
↑ Interstitial Oncotic	$\uparrow \pi_i$	Lymphatic blockage (e.g., lymphedema).