Fluid movement in capillaries

Fluid movement in capillaries

  • The plasma flows through the capillary but its amount and direction in/out is determined by
    • Capillary hydrostatic pressure (out the capillaries)
    • Plasma oncotic pressure (into the capillaries)
    • Interstitial hydrostatic pressure (into the capillaries)
    • Interstitial oncotic pressure (out the capillaries)
  • Fluid shifts
    • Plasma-to-interstitial fluid shift results in edema
      • Elevation of venous hydrostatic pressure
      • Decrease in plasma oncotic pressure
      • Elevation of interstitial oncotic pressure
    • Interstitial fluid drawn into plasma decreases edema

Fluid spacing

  • Used to describe the distribution of body water
    • First spacing- Normal distribution
    • Second spacing- Abnormal (edema)
    • Third spacing- Fluid is trapped where it is difficult or impossible for it to move back into cells or blood vessels
  • Normal fluid balance



1200 mL

Solid food L

1000 m

Water from oxidation

300 mL


2500 mL


Insensible loss (skin and lungs)

900 mL

In feces

100 mL


1500 mL


2500 mL

Extracellular fluid volume imbalances

  • ECF volume deficit (hypovolemia)
    • Abnormal loss of body fluids, inadequate fluid intake, or plasma to interstitial fluid shift
    • Clinical manifestations related to loss of vascular volume as well as CNS effects
    • Correct the underlying cause and replace water and electrolytes (Oral, Blood products, and IV Fluids)
  • Fluid volume excess (hypervolemia)
    • Excess intake of fluids, abnormal retention of fluids, or interstitial-to-plasma fluid shift
    • Clinical manifestations related to excess volume
    • Remove fluid without changing electrolyte composition or osmolality of ECF (diuretics, fluid restriction, sodium restriction