The breathtakingly simple facts of life.

How much oxygen do you consume in a day? If you do the arithmetic (proportion of air extracted as oxygen, times tidal volume, times respiratory rate, times minutes per day) it comes to around 500 litres per day. We are largely unaware that we consume so much gas, probably because we cannot see or feel it, just as fish presumably cannot see or feel water. We are of course far more aware of how much food we consume, because we can detect it with every one of our senses. It totals around 600 grams per day for the average healthy person. These gaseous and solid fuels enter our bodies by different and parallel tubes, but they end up interacting with each other in the same way as in any other internal combustion engine, with the release of energy and production of waste products along the way. We may use the citric acid cycle in our cells rather than cylinders, but the principle is the same. Essentially, the purpose of all this energy is self-replication. Living forms first evolved from inanimate objects – probably through chemical reactions in rocks – in order to manufacture replicas or near-replicas of themselves. Everything that they (and we) have done ever since has been directed towards the same result. Genes use energy to reproduce genes, cells use it to manufacture other cells, organisms use it to perpetuate their own kind, and families or groups use it to look after each other. We are in effect internal combustion engines dedicated to the production of identical or similar ones. This is self-evident, but also somewhat alien to our everyday thinking, even as doctors. Most of us never feel subjectively that our own lives are driven by such very basic principles. An orthopaedic surgeon who is replacing someone’s hip on the operating table, for example, is unlikely to be thinking about the oxidation of carbohydrates, proteins and fats or the preservation of kinship groups, let alone how the two processes are linked. If we do try to think in this way, we may suffer the kind of existential discomfort that leads some religious believers to dismiss these ideas as the work of the devil. Even without this denial, we may still feel that these facts, and the connection between them, are of no practical use in the day-to-day pursuit of medical work.