Capillaries The very small blood vessels that take blood from small arteries to small veins. Carcinogen A compound which is capable of causing cancer. Carcinogenic The ability of a substance to cause cancer. Carcinogenicity The complex process whereby normal body cells are transformed to cancer cells. Catalyst A substance that accelerates a reaction. Cell membrane The membrane composed of phospholipids, proteins, and cholesterol that form the outer boundary of a cell and regulates the movement of substances into and out of the cell. Cholestasis A liver condition in which excretion of bile salts via the bile duct is inhibited resulting in bile salts backing up into liver cells. Chronic Effect An effect that either shows up a long time after an exposure (the latency period) or an effect that results from a long-term (chronic) exposure. Compartment As used in toxicokinetics, compartment is a hypothetical volume of a body system wherein a chemical acts homogeneously in transport and transformation.  The body is composed of organs, tissues, cells, cell organelles, and fluids, any one or several of which may be referred to as a compartment. Concentration Gradient The relative amounts of a substance on either side of a membrane.  Diffusion occurs from a region of high concentration to a region of low concentration. Conjugate A metabolite that results form the joining of a Phase II molecule with a xenobiotic.  It is generally more water soluble that the original substance. Conjugation A metabolic process in which chemical groups are attached to foreign substances in the body, usually making the conjugated chemical more water soluble and easier to eliminate from the body. Covalent Bond The joining together of atoms that results from sharing electrons. Cytochrome P -450 An iron-protein complex with a maximum absorbance of visible light at 450 nm that functions as a nonspecific enzyme system during Phase I biotransformation reactions. Cytoplasm The fluid matrix of a cell exclusive of the nucleus.  Cytoplasm consists of a continuous aqueous solution (cytosol) and the organelles and inclusions suspended in it.  This is the site of most chemical activities within the cell. Cytosol The liquid medium of the cytoplasm, that is, cytoplasm without the organelles and nonmembraneous insoluble components.