Why does the body need to be trained for cardio? What does your body do when at first u can’t run 1 mile but after a while u can run 10?

Question

Why does the body need to be trained for cardio? What does your body do when at first u can’t run 1 mile but after a while u can run 10?

In:

Answer 1

Your heart is used to a non effort life so it supplies a small ammount of oxygen to your muscles, blood and eventually brain. When you start exercising, the heart starts pumping more but it’s not used to it yet and can only do it for a small ammount of time. As the effort builds up, your brain gets to “breathe” less and that’s when you start crouching, breathing through your mouth for the necessary oxygen.

(Advanced info: if you keep going tired as you are, you will start losing your vision because of the lack of oxygen)