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Capillary Fluid Exchange

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Topic updated on 03/09/17 6:34am

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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Qbank (1 Questions)

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(M1.CV.54) A 66-year-old male with a history of myocardial infarction presents to your primary care office with complaints of dyspnea on exertion and swollen feet and ankles. On exam, you note an elevated JVP and 2+ pitting edema of bilateral lower extremities. What is the most likely explanation for this patient's lower extremity edema? Topic Review Topic

1. Increase in capillary pressure
2. Decrease in plasma proteins
3. Increase in capillary permeability
4. Increase in colloid osmotic pressure
5. Increase in interstitial fluid pressure

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