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Overview

 
  • Starling equation 
    • fluid movement across a capillary wall is driven by Starling pressures across wall  
    • Jv = KF [(PC - PI) - σ(πC - πI)]
  • Variables
    • KF ("hydraulic conductance") is the permeability of the capillary wall
      • KF determines magnitude of fluid movement
    • (PC - PI) is the net hydrostatic force moving fluid out of the capillary and into the interstitial fluid
      • aka filtration
    • C - πI) is the net osmotic force moving fluid out of the interstitial fluid and into the capillary
      • aka absorption
      • in capillary, only plasma proteins contribute to osmotic pressure
        • aka oncotic pressure (colloid osmotic pressure)
      • σ is the reflection coefficient
  • Capillary exchange
    • the net driving pressure at the capillary arteriolar end is out of the capillary and into the interstitial fluid
      • aka filtration
    • the net driving pressure at the capillary venous end is out of the interstitial fluid and into the capillary
      • aka absorption
    • the change in net driving pressure is caused by a decrease in the capillary hydrostatic pressure (PC) along the length of the capillary
  • Edema
    • increase interstitial fluid volume (swelling) caused by excess filtration out of capillaries
    • causes of edema
      • capillary hydrostatic pressure (↑ Pc)
        • heart failure
      • ↓ capillary Oncotic Pressure (↓ πc)
        • aka plasma proteins
        • severe liver failure 
        • failure to synthesize proteins
        • nephrotic syndrome
        • loss of proteins in urine 
      • ↑ hydraulic conductance (↑ Kf)
        • aka ↑ capillary permeability
        • burn, infection, toxins
        • release of histamine, cytokines
      • ↑ colloid osmotic pressure ( πi)
        • lymphatic blockage
        • filtration out of capillaries exceeds ability of lymphatics to return fluid to circulation
 

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