Drugs must pass across cell membranes to reach their site of action. For example, drugs given by mouth must pass across the gut membrane to enter the bloodstream and then pass through the endothelium (lining cells) of vessel walls to reach the site of action in the tissues. This process is called absorption and may depend on lipid diffusion, aqueous diffusion, active transport, or pinocytosis – a process in which a cell takes in small droplets of fluid by cytoplasmic engulfment.