Membrane transport 3
Osmosis and filtration. Dialysis and hemodialysis. Water exchange through membranes and between blood and tissues.
Osmosis: The diffusion of a solvent from a dilute solution through a semipermeable membrane to a more concentrated one.
I.e. osmosis is a diffusion of solvent from its higher to lower concentration.
Semipermeable membranes:
- Only allow solvent molecules to go through
- Cell membranes are semipermeable
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
Van't Hoff's law (for osmotic pressure):
R=gas constant, T=temperature, c=concentration.
Hypotonic: lower concentration of solutes, water moves into the cell from out the solution. Isotonic: Equal concentrations, no movement of water. Hypertonic: Higher concentration of solutes, water moves out of the cell into the solution.
Filtration: Movement of fluid through a selectively permeable membrane from an area of higher hydrostatic pressure to an area of lower hydrostatic pressure Arterial end of capillary has hydrostatic pressure > than osmotic pressure so fluid & diffusible solutes move out of capillary
The filtration law is the Poiseuille's law applied to the membrane channels:
The osmosis could be formally considered as a filtration of solvent through the membrane caused by the osmotic pressure. For ideal semipermeable membrane the solvent flux will be:
Theoretically, water can permeate through membranes following three pathways. These pathways can be differentiated by the ratio of osmotic permeability (Pf )/diffusive permeability (Pd ):
- porous membranes: Pf /Pd >1
- lipid membranes with no pores, but allowing water diffusion by a dissolution-diffusion mechanism:
Pf /Pd = 1
- membranes containing narrow pores: Pf /Pd <1
Physiological importance of filtration and osmosis: water exchange in cells and tissues.
Dialysis: Dialysis is the artificial process of eliminating waste (diffusion) and unwanted water (ultrafiltration) from the blood. Our kidneys do this naturally. Some people, however, may have failed or damaged kidneys which cannot carry out the function properly - they may need dialysis.
In other words, dialysis is the artificial replacement for lost kidney function (renal replacement therapy). Dialysis may be used for patients who have become ill and have acute kidney failure (temporary loss of kidney function), or for fairly stable patients who have permanently lost kidney function.