I once saw a Nova documentary -- Marathon Challenge -- in which they trained a group of "average people" to run the Boston marathon, in 40 weeks.
One of the characteristics which they measured was "VO2max" i.e. the rate at which people consume oxygen (measured by wearing a respirator/mask on a treadmill and measuring the difference between oxygen in and oxygen out).
They said that was a good overall/single measure because it measures efficiency of:
- Lungs transpiring oxygen, and
- Heart pumping blood, and
- Muscle tissue using oxygen
They said that, with exercise/training, tissues becomes more heavily "vascularised": i.e. capillaries become larger or denser (or something like that), so blood is more easily delivered to the tissues.
Quoting from the transcript:
When we measure somebody's VO2 max, it's a very interesting number, because it is really complicated and there are a lot of different factors.
So it's how well the heart is beating; it's how well the vessels are expanding, how elastic they are; how many capillaries there are to bring the oxygenated blood to the muscles. So it's one number that shows us an overall good health of the entire cardiovascular system.
So in nine short weeks, what's happened? What's changed?
The runners' hearts are more efficient, filling up faster between beats and pumping more blood with less energy. They might even be slightly bigger.
Certainly the heart is working a bit better. But by far, the majority of the changes are happening with the vessels, the plumbing of the body.
Arteries and veins have become more elastic, easing blood flow. And down at the level of the muscle cell itself, there are more tiny capillaries, meaning faster delivery of oxygen.
Even inside the cell, energy production has been ramped up by mitochondria, the structures that transform fat, carbohydrate and oxygen into energy.
As you become more and more trained, the muscle actually starts making more mitochondria and also making them larger so that they can actually process and break down more fuels for energy.
So in nine weeks, from their hearts to the tiniest enzymes in their cells, these bodies were transformed.
The human body is an amazing organism. And what we see is that when you don't use things, you lose that body tissue.
I think it's not only about the delivery of oxygen (and other nutrients) to the tissues, but also the removal of the spent by-products.
There may be some difference in gross musculature too -- people say that cyclists look as if they have strong legs -- but legs are already the biggest muscles in the body.
Another consideration is that if or when circulation fails (when you're unwell) it tends to fail in the legs first -- causing "edema" or "peripheral neuropathy" -- I think the body privileges e.g. the circulation to the brain, and the legs get the left-overs (also the feet are more distant, and circulation has to work against gravity).
In case it isn't clear, the body adapts to training: e.g. if you move weight[s] the muscles become stronger, and you train aerobically then your aerobic capacity (that "VO2max" again) increases.
I think it's worth reading the whole transcript, by the way.