system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 What is exactly stamina? What happens when it gets trained? Do your lungs get better and more efficient at pumping air?
gpt
There is some truth to what the other comments claim. However, the most important limiting factor for your body's ability to use oxygen (VO2 max) is your heart and its ability to pump blood. This is true for everyone except those with lung conditions and elite athletes. On average, we only use 60% of our lungs' maximum capacity when our heart is maxed out. So, to increase our stamina, we exercise our heart.

When we exercise our heart, it can pump more blood per contraction. Your heart can pump more blood > more oxygenated blood is pumped into the body > your muscles have access to more oxygen > your muscles can maintain a force over a longer time period before producing lactic acid.

It is true that the muscles also get better at using the oxygen (more blood vessels are created, more mitochondria, et cetera), but this is not as significant.

This is a heavily simplified explanation and also has to do with lactic acid and proton clearance by the liver, to name an example.