Answer:
There are two answers that explain this question: Pressure and oxygen
Explanation:
When someone lives in a place, it normally produces red blood cells onto a certain quantity, and this quantity is enough for the body at that place. But when Jon goes hiking up in a hill, there's less oxygen for him to breath, so he starts having problems to breath, and he feels it's heavier than before. The pressure too changes, not that much, but it does, and both of these combined, red blood cells are not produced as they should be, the body needs to adapt to a new environment, and that why that happens.
