All living things are made of cells—small individually functional units which, in higher organisms, are organised into collections called tissues. A typical cell consists largely of cytoplasm, an aqueous liquid in which a wide range of biochemical processes occur. The cytoplasm is held as an intact unit by a cell membrane, which surrounds it and prevents it from mixing with its surroundings. Depending on the cell type and function, a number of other structures may be present, particularly a nucleus, in which the cell genetic information is stored in the form of DNA. Provision must be made for the supply and retention of substrates from extracellular sources and for the secretion of waste products that would otherwise accumulate in toxic amounts. The outer membrane of the cell must therefore allow penetration of some substances and not others, i.e. it must be selectively permeable. This is one of the most important features of the cell membrane.

Organs and tissues are collections of cells surrounded by specialised cell structures called epithelia, which can be thought of as the organ's 'outer membrane' in an analogous fashion to the membrane that surrounds the individual cell. Like cell membranes, they not only bound the organ, but also are the site for a wide range of transport, barrier and secretory processes which vary widely with the particular organ. Many epithelia protect organs from hostile environments (for example the skin or the contents of the stomach) and such cells generally have a rapid turnover and numerous barrier features.

In order for a drug to reach a site of action it must pass from an 'external' site (for example the surface of the skin or the small intestine) to an 'internal' site (the bloodstream or the cytoplasm of a particular cell group). In doing so it will have to pass through a number of tissues and epithelia, either by going through the cells themselves (and thus penetrating their plasma membranes) or by finding pathways between the cells. Overcoming these barriers to absorption is one of the most important considerations in the drug delivery process, and requires a detailed knowledge of the structure and behaviour of the cell membranes and epithelial tissues.

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