system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens to your body if you don't exercise?

Like your organs, tissues etc. if you don't workout for long periods of time at all.
gpt
Your body wants to be as energy efficient as possible.

If you don't exercise, your body has no reason to expend effort, resources and energy on maintaining bigger muscles and a highly capable cardiovascular system - so it doesn't.

Your muscles will slowly atrophy down to the minimum needed to your activity level. Your cardiovascular system will lose peak performance capacity until it hits the level you require. Your daily calorific requirements will drop, meaning you're likely to put on weight without an associated diet change. This fat can put stress on organs depending on how much and where.

Combining the muscle loss with degradation in ligaments and tendons - and whether any weight is gained - your body can begin to struggle to support you even doing almost nothing. You'll start getting neck, back, hip and knee pain and disorders.