There are a lot of low level mechanisms that improve health with exercise. The basic breakdown is some things like digestion where optimized by evolution around the fact our ancestors moved. For similar reasons UV light in childhood are a requirement for normal vision exercise was simply universal for a very long time.
A lower level example is feedback mechanisms. Mechanical stress from motion are used to maintain bone strength. The cells involved in bone breakdown and repair can detect damage and reinforce where needed, but they also remove bone where it’s not needed. Without that kind of feedback our bones would either be weak or heavy.
The same thing happens with your circulatory system, you grow new capillaries in response to demand. Or for a “negative” example fat cells reproduce when you can store more energy.
A lower level example is feedback mechanisms. Mechanical stress from motion are used to maintain bone strength. The cells involved in bone breakdown and repair can detect damage and reinforce where needed, but they also remove bone where it’s not needed. Without that kind of feedback our bones would either be weak or heavy.
The same thing happens with your circulatory system, you grow new capillaries in response to demand. Or for a “negative” example fat cells reproduce when you can store more energy.