Physiology is the branch of biology relating to how living organisms work. While often concentrating on the organ-level ("how does the heart work?"), it covers life from the whole organism down to the single cell and intracellular organelle. It overlaps with biochemistry and other life sciences, and is usually taken to include membrane biophysics. It can include every aspect of the function of an organism, from molecular mechanisms to the control of behaviour. In its applied aspects, physiology deals with the function and malfunction of parts of the human body with reference to health and disease (areas relating to medicine), how to improve crop yield (areas relating to plant sciences) as well as the practical problems of animal, plant and microbial performance and their responses to challenging conditions (areas relating to ecology).
Many areas of science have a hierarchy of levels of approach, and currently there is a tendency to look downwards rather than upwards, at mechanism in preference to the often less tractable problems posed by systems as a whole. Physiology is no exception, and the temptations to concentrate on molecular aspects of the subject - areas where new information is easier to come by, and where conceptual problems are less obvious - have never been stronger. But, as Research Councils are beginning to emphasize, the largest gaps in our knowledge are often how these mechanisms relate to function - and malfunction - in the organism as a whole. These questions are difficult to answer and sometimes, as in the case of the brain, difficult to formulate as well! Part of the training of a physiologist is to learn to think, argue and to see problems on a wider scale, without losing sight of the whole organism.