General and Systematic Veterinary Pharmacology

FILTRATION

• Filtration is the passage of drugs through aqueous pores in the membrane or through paracellular spaces. This can be accelerated if hydrodynamic flow of the solvent is occurring under the hydrostatic or osmotic pressure gradient.
• Lipid insoluble drugs cross biological membranes by filtration if their molecular size is smaller than the diameter of the pores. Majority of the cells have very small pores and drugs with higher molecular weight will not be able to penetrate. However capillaries (except those in brains) have large pores and most drugs can filter through these pores.
• Passage of drugs across capillaries is dependent on the rate of blood flow through them rather than on lipid solubility of the drug or pH of the medium.
• Filtration seems to play almost a minor role in drug transfer within the body except for glomerular filtration, removal of drugs from CSF and passage of drugs across hepatic sinusoidal membrane.