system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is physically happening within our bodies that makes performing cardiovascular exercise easier and easier every time we do it consistently?
gpt
A number of things.

One, as you perform cardiovascular endurance exercise, your heart starts pumping more blood per beat. This is called "increased stroke volume" and it's what eventually leads to a lowered blood pressure and lower heart rate whole resting. More blood = more oxygen per stroke.

Two, your muscles get better at utilizing the oxygen  they're given. They can either become more permeable to oxygen (taking more up per second) or you can actually grow more mitochondria (meaning more O2 used per given unit of time) or both.

Three, cardiovascular endurance exercise is inherently fatiguing. As you perform more and more of it over a longer period of time, your body gets better at flushing out the metabolic waste materials that cause acute fatigue.

There are a few more reasons, but those are the basics.

Real ELI5: as you train, your heart pumps more oxygen, your muscles ~~use~~ need less oxygen, and you don't get as tired as easily.

**Edit:** Wording change to be more clear.