system
Given the following post, respond as if you're commenting on Reddit:
human
What's that feeling you get when you're stretching 
or yawning--a sort of thrill that quickly spreads 
through the body?
gpt
When you stretch it is basically dilating your blood vessels, which is why athletes stretch before exercise, to ensure maximum blood flow to increase O2 exchange efficiency. E.g. if you sat down for a long time, standing up allows more blood to flow in those areas which have been deprived of sufficient O2, so your muscles don't feel as fatigued as they have a surfeit source of energy provided by the blood flow.
And yawning induces stretching of muscles and therefore to the same effect.