Updated: 4/14/2018

Capillary Fluid Exchange

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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
    • Jv = Kf [(Pc - Pi) - (πc - πi)]
    • 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
      • Pc
        • capillary hydrostatic pressure favors filtration out of the capillary
      • Pi
        • interstitial hydrostatic pressure opposes filtration
      • πc
        • capillary oncotic pressure opposes filtration
        • osmotic pressure of capillary blood due to plasma proteins
      • πi
        • 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
      • ↑Pc
        • heart failure
      • ↓ πc ( plasma proteins)
        • severe liver failure 
        • failure to synthesize proteins
        • nephrotic syndrome
        • loss of proteins in urine 
      • ↑ Kf (↑ capillary permeability)
        • burn, infection, and toxins
        • a release of histamine and cytokines
      • πi
        • lymphatic blockage
        • filtration out of capillaries exceed the ability of lymphatics to return fluid to the circulation

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