U01.07.030 Capillary fluid exchange

Capillary fluid exchange is essential for maintaining fluid balance between the capillaries and the surrounding tissues. The movement of fluid across capillary membranes is governed by Starling forces, which include various pressures that determine whether fluid will flow into or out of the capillary.

Starling Forces and Their Role in Fluid Movement

The following forces control the movement of fluid through the capillary walls:

Pc: Capillary Hydrostatic Pressure
- Pushes fluid out of the capillary into the interstitial space.
Pi: Interstitial Hydrostatic Pressure
- Pushes fluid into the capillary from the interstitial space.
πc: Plasma Colloid Osmotic (Oncotic) Pressure
- Pulls fluid into the capillary from the interstitial space.
πi: Interstitial Fluid Colloid Osmotic Pressure
- Pulls fluid out of the capillary into the interstitial space.

Net Fluid Flow Equation

The net fluid flow (Jv) across the capillary is determined by the following equation: Jv=Kf[(Pc−Pi)−σ(πc−πi)]

Where:

Kf: Capillary permeability to fluid
σ: Reflection coefficient (measure of capillary permeability to protein)

Factors Influencing Edema Formation

Edema refers to the accumulation of excess fluid in the interstitial space. It can result from changes in the Starling forces. The common causes of edema include:

Increased capillary pressure (Pc): Caused by conditions like heart failure.
Increased capillary permeability (Kf): Seen in toxins, infections, or burns.
Decreased interstitial fluid colloid osmotic pressure (πi): Due to lymphatic blockage.
Decreased plasma proteins (πc): Associated with nephrotic syndrome, liver failure, or protein malnutrition.

Table: Causes of Edema and Associated Starling Force Changes

Cause	Affected Starling Force	Effect
Heart Failure	Increased Capillary Pressure (Pc)	Fluid pushed out of the capillaries
Toxins, Infections, Burns	Increased Capillary Permeability (Kf)	More fluid escapes from capillaries
Lymphatic Blockage	Decreased Interstitial Osmotic Pressure (πi)	Less fluid returned to capillaries
Nephrotic Syndrome, Liver Failure, Protein Malnutrition	Decreased Plasma Colloid Osmotic Pressure (πc)	Fluid remains in the interstitial space

Fluid Movement Based on Pressure Differences

Fluid movement can be categorized into two types based on the relative pressures:

Net Fluid Flow Out of Capillary: When (Pc−Pi)>(πc−πi)
Net Fluid Flow Into Capillary: When (Pc−Pi)<(πc−πi)

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