For example you could be super in shape from a muscular perspective but have really bad cardio and vice versa.
How is cardio different from physical shape and what exactly happens in your body that makes you have more cardio?
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I used to be able to answer this really well, but been a few years
Cardio will make the heart bigger by about 50%. Think about swapping a little 4cyclder engine for a big v8. The volume inside the heart has gotten bigger, meaning it can pump more blood. More blood means more energy. Meaning go faster for longer.
Lifting weights makes the heart bigger too, but it increases the heart muscles, not the volume. It does this to deal with the pressure increases that come with lifting weight. Think of this as the engine mount. If you chuck that v8 into your little car it will shake the car apart. So in this case, the heart walls get bigger, thus increasing pressure, to push the blood to where it needs.
Other things come into play, for instance, hermatcrit (red blood cells) increase, by up to about 3% of total blood volume (doesn’t sound like a lot, but it works out to about an increase of 10% extra oxygen). RBC’s transport oxygen and energy to the muscles, so using our car analogy, we’re now running on premium fuel.
Cardio is also region/activity specific. If you cycle lots, you’re body adjusts by creating new blood vessels in your legs, to allow more blood. This helps a little if you start running, but not so much if you start swimming. Because the body ‘learns’ to use the muscles and blood in a certain fashion.
Finally there is waste removal. This gets a bit more complex, but this is things like lactate buffering, and expelling co2, and honestly I can’t remember it well enough to ELI5, but it boils down to body gets good at removing waste product through breathing.
Now for the more complex: the big thing is heart. The average body needs about 5-6L of blood per minute, with a normal heart volume of about 80ml per pump, and a heart rate of 75beats is 80ml x 75bpm = 6L/min. During exercise, the heart increases by 50%, so is now pumping out 120ml, and let’s say it gets to 200bpm= 120ml x 200= 24L/min. But let’s say you’re fit. Real fit, and you’re heart at rest is 120ml. You still only need 6L at rest, so you’re resting heart rate is about 50bpm. But you start exercising, and your heart increases by 50%, you’re now pumping 180ml per beat, and you still hit the 200bpm, it’s now 180 x 200= 36L /min which is a shit load more work you can do.
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