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
          • e.g., water, glucose, and amino acids
          • diffusion through aqueous clefts between cells
        • e.g., water, glucose, and amino acids
        • diffusion through aqueous clefts between cells
      • e.g., water, glucose, and amino acids
      • diffusion through aqueous clefts between cells
    • 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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(M1.CV.13.54) 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?

QID: 100570

Increase in capillary pressure

62%

(90/146)

Decrease in plasma proteins

12%

(18/146)

Increase in capillary permeability

8%

(11/146)

Increase in colloid osmotic pressure

7%

(10/146)

Increase in interstitial fluid pressure

10%

(14/146)

M 1 D

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