Why do living things die?

A living thing lasts for as long as it can pay for itself. A body holds its temperature, water, salt, sugar and oxygen steady while its neighbors keep writing on its boundary, and it has to answer every update, at a rate set by its own clock. A standing cost is a rate, a rate is energy, and the budget that pays it is finite. Walter Cannon, who named this holding homeostasis in 1926, had spent the First World War with wounded men dying of shock, and his list of the quantities a body holds steady was a list of the readings he had watched go. When the readings on a thing's boundary stop fitting any consistent world, the surrounding repair redistributes until the record closes again, with the boundary of whatever was doing the spending drawn somewhere else.

Also in Chapter 39: Why Is a Bacterium Freer Than an Atom?.