Yes, “all of the stuff” is usually in any given area that they’d sample from.
Think about it this way: your blood circulates through your whole body. You don’t have a separate circulatory system with its own pump and veins going to each different body region, right? All of your blood is pumped by one pump and it loops around and around through your whole body. Because of that, all the blood everywhere is constantly intermixing. There’s blood coming back from your arms and legs and stomach and brain, all arriving into the same tube going into the heart, dumping any waste into the lungs, then being sent back out again. A certain blood cell might go out to the brain one time, then back to the heart, then happen to go down a different branch and end up in your foot on the next loop.
Basically I’m trying to say the blood you’d get from a major artery/vein is roughly homogenous, and does indeed have cholesterol, triglycerides, glucose etc. all dissolved and carried in it. Blood carries all the stuff that’s coming from and going to anywhere, and it’s all intermixed because of how the plumbing system works.
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