All right then. How many of the 60,000 miles of blood vessels have a smooth muscle layer, or the ability to expand and contract? I guess all but the capillaries, which means not all 60,000.
My argument still applies if those vessels have a smooth muscle layer which is actually doing the work of expanding and contracting, or for any other structure which performs that function.
It is *those* cells that are doing the bulk of the work of being alive, and damage to *those* cells can come about through the means that I discussed. The endothelium experiences the problem first, if it lives between the muscle and the source of the sugar and oxygen, the blood, and the problem happens to be that the blood doesn't *have* enough sugar or oxygen in it. If *anything* interferes with this muscle's ability to get sustenance (even a plugged up endothelium, if you can have such a thing), the problem will happen.
OK? Does that make more sense? The question of how the body burns up all that energy, and why do we feel like we have no energy, should be clearer when you look at it that way. It's expanding and contracting all the blood vessels that takes all the work.
For capillaries flow is controlled in a different way. The point to remember, is that the flow control itself requires energy
, uses oxygen and sugar, and is vulnerable when the supply is low. If the flow gets stuck in the off position, damage.