Physiology studies the mechanisms by which life carries out its essential functions. We can divide these into six categories, which make up the subject matter of physiology:
- Order: living organisms are highly complex and require a constant supply of nutrients to maintain their structure
- Metabolism: living organisms break down nutrients, using them for energy and to build new molecules
- Homeostasis: living organisms regulate their internal environment to maintain relatively constant conditions
- Responsiveness: living organisms detect and respond to changes in their internal or external environment
- Growth and Development: living organisms develop and increase in body size as they progress toward maturity
- Reproduction: living organisms produce like kinds from generation to generation
In a biology course, we would add evolution, the adaptation of a population of organisms to changing conditions. This is how we now understand the tree of life. Traditional physiology is concerned with individual organisms and not populations. For this reason, evolution is not a subject of physiology.
British schools teach a somewhat different list from the one I give above. An organism is alive only if it carries out all seven of the following: movement, reproduction, sensitivity, growth, nutrition, excretion, and respiration. The mnemonic for these is Mrs Gren.
Making a Difference in the World 