system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I sometimes run out of breath and get tired while walking slowly up stairs, but I feel fine after running at a good pace for 10km?

I don't smoke nor do I have any health issues.
gpt
walking up stairs your body is quickly thrown out of homeostasis (simply put, a steady state where it is supplying organs and muscles with the blood it needs). Because you're only thrown out of homeostasis for a few seconds, the body starts to adapt, raising blood pressure and opening more blood vessels - which the action of your moving legs helps drive the blood around the body. Your body is still attempting to adapt and then you stop, but your blood vessels are still wide open and are now struggling to push blood back to the brain and lungs.

In a 10k run your body reaches a homeostatic state while running, so you aren't as tired after your run. But after a run you keep moving...If you were to stop dead after a hard 10k though, you would likely faint for the exact same reason.