How are doctors able to determine the levels of say your cholesterol, triglycerides, glucose etc. through a few blood samples from one site? I can’t imagine all of the stuff would be in the one area you’d take a blood sample from, right?
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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.
The blood circulation system is sort of like a carburetor in that it’s designed to maximize diffusion thought the change of blood pressure and speed. If your body wasn’t excellent at this process, oxygen wouldn’t be carried to the cells and random bits of your body would die.
>I can’t imagine all of the stuff would be in the one area you’d take a blood sample from, right?
But it is.
Things that end up in your blood are never dumped in one go but released over time, and constantly getting mixed on top of that since everything passes through the heart again eventually. And triglycerides/glucose are especially prevalent –> easy to measure.
For perspective: the research lab I work in can do measurements of specific antibodies off of just a few drops of capillary blood from fingersticks. It performs exactly as when you get the sample from a 10 mL venous draw instead — we tested that extensively — and from that whole blood sample, we dilute >1000x further for actual measurement, too. That’s how homogenously distributed things are, and you can really go very far before you run into the sort of sampling error you’re thinking of (for my lab’s assays, the sensitivity of the test becomes a problem before sampling error does).
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