Learning Objective: By the end of this lesson, the learner will be able to describe the Starling forces that govern capillary fluid exchange, apply the Starling equation to calculate net filtration or absorption, and understand how capillary permeability affects fluid flux.

Starling Equation — Key Concepts

Starling Forces

• Favoring filtration (out of the capillary):
  • Capillary hydrostatic pressure (PCP_CPC​)
  • Interstitial oncotic pressure (πIF\pi_{IF}πIF​)
• Opposing filtration (favoring absorption into the capillary):
  • Interstitial hydrostatic pressure (PIFP_{IF}PIF​)
  • Plasma oncotic pressure (πC\pi_CπC​)

Activity

Equation for Fluid Movement

Where:

• QfQ_fQf​ = net fluid movement (positive → filtration; negative → absorption)
• $k$ = filtration coefficient (depends mainly on capillary permeability and surface area)

Activity

Clinical Considerations

• Increased permeability (↑ k) → more fluid moves into interstitium → edema
• Tissue-specific variations:
  • Renal glomerulus → filtration along the whole capillary
  • Intestinal mucosa → absorption predominates
  • Muscle/skin → filtration at arteriolar end, absorption at venular end

Activity