system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: What does improve in ones body when training cardio ?
gpt
Cardiovascular exercise improves the heart's ability to pump blood efficiently. The body wants to expend the least amount of energy to meet its own metabolic needs, and the heart takes a lot of energy to pump rapidly. 

Repeated cardiovascular exercise has the effect of stretching out and strengthening the heart tissue, enabling it to pump more blood in less time with less beats while still meeting the body's metabolic demands.