Facilitated diffusion The passage of molecules and ions across a cell membrane with the aid of a specific carrier protein.  It is dependent on concentration gradient. Fetus The unborn offspring in the postembryonic period, after major structures have begun to form.  In humans this occurs from 8-9 weeks after conception until birth. Filtrate A substance that has passed through a filter.  As used in toxicokinetics, it usually pertains to the material that has passed through the glomerulus into the renal tubule. Filtration The passage of a solvent and dissolved substance through a membrane or filter.  In excretion, a portion of the plasma and dissolved materials undergo filtration through the glomerular filter (capillary bed). First-pass Effect The biotransformation of a substance in the liver after absorption from the intestine and before it reaches the systemic circulation.