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A great deal has been learnt about the way in which the various organs and tissues of animals perform their functions by removing them from the whole animal and examining their behaviour in isolation. Many physiological experiments involve the study of tissues dissected from animals and kept alive in special fluid media. By using such isolated tissues the effects of uncontrolled variables (e.g. changes in blood supply or changes in activity of other organs) can be prevented. One of the problems of this experimental approach is how to maintain tissues in a healthy and functional state once they have been removed from the body. This can be achieved by bathing or perfusing tissues with an appropriate physiological solution. Historically the optimal composition of such media were discovered largely by trial and error. The substances which must be present in such solutions, the ionic properties of the medium (including pH and osmotic pressure) were adjusted so that the cells of the tissue were kept healthy and remained functional. Perhaps not surprisingly, it is found that the best solutions have an ionic composition closely resembling that of mammalian serum.