I1ii: Define osmosis, colloid osmotic pressure and reflection coefficients and explain the factors that determine them


  • Movement of fluid across a semipermeable membrane from any area of high [H2O] to an area of lower [H2O]
  • Determined by:
    • Membrane permeability
    • Solute [ ] on either side of the membrane

Colloid Osmotic Pressure


  • The osmotic pressure exerted by protein in the plasma (notably albumin)

→ Determined by [protein]

Reflection Coefficient

  • Starling’s hypothesis: fluid movement across capillary wall depends on a balance of:

→ Hydrostatic P gradient

→ Oncotic P gradient

DRIVING PRESSURE = [(Pc – Pi) – (πc – πi)] pressure gradient

  • PC = hydrostatic P cap
  • Pi = hydrostatic P interstitium
  • ΠC = onc P cap
  • Πi = onc P interstitial

→ 2 additional factors considered for fluid flux:

    1. Filtration Coefficient (Kf)
    2. Reflection Coefficient

NB; Reflection co-efficient is a “correction factor” of Oncotic P

                → It is due to the leakage of small proteins

                → Anywhere between 0 → 1

e.g. GFR = very low [Pr] = Reflection Coefficient ∆

       Hepatic sinusoids = leak Pr +++ = Reflection coefficient close to 0