system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is 'fitness'? As in, why is working out easier the more you work out?

Let's assume you have two twins. One gets regular exercise and hits the gym every day. The other is a couch potato. Let's also assume that the fitter twin eats more calories and does only cardio, so they more or less have similar weight and muscle mass. Still, the one who gets regular exercise is able to run for longer, will break less of a sweat and have a lower heart rate.

What are the physiological differences when someone is 'fit' versus 'out of shape'?
gpt
When we do cardio, blood is more directed towards the muscles you are working out (and away from places that aren’t doing a whole lot at that time).

The blood flow is increased. The volume of blood is also increased when it is returning to the heart.

Over time, the left ventricle enlarges to adapt to the larger blood flow. Now this ventricle can hold more blood and eject more blood per beat (even when resting). This decreases the heart rate over time.

There is also evidence that exercise stimulates the production of new blood vessels. This creates more efficient circulation.

Also, there is evidence that at the same time that cardio increases the number of blood vessels, resistance training increases the size of those blood vessels. This allows for fewer blockages and more efficient blood flow.

The new blood flow (with fresh oxygenated blood) helps keep your other body parts healthy (like your organs).