Elimination The toxicokinetic process responsible for the removal or expulsion of a substance from the body. Endocytosis The process whereby a substance is engulfed and taken into a cell by an inward folding of the cell membrane, which detaches and moves into the cytoplasm. Enterohepatic circulation Also known as enterohepatic recirculation.  The cycling of a substance from the blood into the liver, then into the bile and gastrointestinal tract.  This is followed by re-uptake into the blood stream from the gastrointestinal tract, possibly after chemical or enzymatic breakdown. Enzyme A protein formed in living cells that acts as a catalyst for chemical reactions in cells. Enzyme Activation The increase in levels of an enzyme as the result of stimulation by another chemical substance.  Same as enzyme induction. Enzyme Inhibitor A substance which causes a decrease in levels of an enzyme. Epidermis The outer layer of the skin.  Also known as epithelium. Equilibrium A state of balance.  Opposing forces exactly counteract each other. Excretion A process whereby substances (or metabolites) are eliminated from the body. Exposure Contact with a foreign substance, usually by inhalation, ingestion, or skin contact. Exposure Dose The amount of a substance to which a person is subjected.