Capillary Fluid Exchange

Introduction

• Microcirculation
  • capillaries, the smallest blood vessels, are the site of exchange of nutrients, waste products, and fluids
  • capillaries are thin-walled (a single layer of endothelial cells) and allow for the exchange of solutes and gases
    • O₂ and CO₂
      • simple diffusion through endothelial cells
    • water-soluble substances
      • e.g., water, glucose, and amino acids
• diffusion through aqueous clefts between cells

Exchange of Fluids

• Osmosis is driven by Starling forces (hydrostatic and osmotic pressures) 
  • protein contributes to osmotic pressure (noted as oncotic pressure)
  • filtration
    • net fluid out of capillaries into the interstitium
  • absorption
    • net fluid into capillaries from the interstitium
•  Starling equation
  • J_v = K_f [(P_c – P_i) – (π_c – π_i)]
  • fluid movement is determined by sum of hydrostatic and oncotic pressures
  • variables
    • Kf
      • “hydraulic conductance” or water permeability of the capillary wall
      • determines the magnitude of fluid movement
    • P_c
      • capillary hydrostatic pressure favors filtration out of the capillary
    • Pi
      • interstitial hydrostatic pressure opposes filtration
    • π_c
      • capillary oncotic pressure opposes filtration
      • osmotic pressure of capillary blood due to plasma proteins
    • π_i
      • interstitial oncotic pressure favors filtration
• the magnitude of fluid movement for a given pressure difference
• determined by hydraulic conductance (Kf, or water permeability) of the capillary wall

Edema

• Edema, or swelling
  • ↑ interstitial fluid volume
  • occurs when interstitial volume > ability of lymphatics to drain it back into circulation
    • from ↑ filtration
  • causes of edema
    • ↑P_c 
      • heart failure
    • ↓ π_c (↓ plasma proteins)
      • severe liver failure 
      • failure to synthesize proteins
      • nephrotic syndrome
      • loss of proteins in urine 
    • ↑ K_f (↑ capillary permeability)
      • burn, infection, and toxins
      • a release of histamine and cytokines
    • ↑ π_i
      • lymphatic blockage
      • filtration out of capillaries exceed the ability of lymphatics to return fluid to the circulation