Cells are the smallest units that perform all vital functions in a living organism. Each cell maintains its own homeostasis. Homeostasis at the tissue, organ and organism level is a function of the combined activities of many different cells. The mammalian body contains two kinds of cells - somatic cells and germ cells. Germ cells are reproductive cells, that is, eggs or sperm.
Somatic cells make up the rest of the body.
Each cell is surrounded by watery fluid called extracellular or interstitial fluid. This is separated from the cell contents by a cell membrane. Within the membrane is cytoplasm, containing a nucleus, cytosol and cell organelles. The cell membrane separates the cell contents from the interstitial fluid. It regulates the exchange of substances with the extracellular environment.