Mass transport

• Urinary system

• respiratory system

• cardiovascular system

• Diffusion

• Convection

• Transport by binding interactions

  
  

Diffusion describes the net movement from a region of high concetration to a region of low concentration. Molecules are in constant movement due to thermal energy.

• random molecular motion

• deltaC as driving force

Molecular motion arising from random molecule collisions

J = -D *(dc/dx)

Valid for dilute solutions in a steady state

Predicts how diffusion causes the concentration to change in time.

dc/dt = D (d^2c/dx^2)

Diffusion depends on the concentration gradient across the membrane deltaC_mem, not on the concentration gradient of the fluids.

Partition coefficient: K = C_mem/C

For K<1, the surface concetration of lower than the fluid’s concetration

• Movements of molecules resulting from the bulk movement of a fluid

• Fluid movement resulting from the application of forces ( gravity, pressure, shear)

• Free convection: Gradients inside the fluid: Temperature, density, concentration differences

• Forced convection: Movement of fluid under the influence of external forces (pressure difference)• Laminar or turbulent

Description of the flux through a porous membrane

• Convection = Movement of large mass of particles in the same direction through the fluid

• Diffusion = Movement of single particles and transfer of particle’s momentum and energy toother particles in the fluid

JMem = D_mem * (K/L) *(c1 - c2)

dc1/dt * V1 = Jmem * A_mem

The significance of diffusion or convection can be evaluated by the Peclet number 𝑃𝑒, whichrepresents the ratio of transport by convection to mass transport by diffusion