Deleted my other question, realized it was too vague and not relevant to my actual question.
In: Biology
The blood from a cut comes out when you cut many small blood vessels, letting the blood in them escape.
This blood is no different from the blood in a vein, but you damage lots of tissue to get it. Also the small damaged vessels tend to clot quickly, so large volumes are hard to get.
Nurses find a vein because they don’t want to damage large areas of tissue to get blood, and don’t want the bleed to clot before they get the blood they need, so they draw from a larger blood vessel (a vein).
The blood you see when you cut your arm is from the tiny blood vessels called capillaries that are in your skin. They’re so tiny there’s not a way to efficiently collect the quantity of blood needed. Hitting a vein is the best way to do so.
The thing is, you absolutely *can* (in theory) inject things directly into something like muscle tissue. Most small-needle injections like flu shots operate this way. And it will diffuse into your bloodstream just fine. But it takes a lot of time. Your body isn’t just a big liquid blob that can stir itself around, what actually happens to that injection is it creates a cavity inside your body that is now full of fluid and under pressure. And it stays there, pressing against things, until it slowly flows away by means of nearby capillaries and the lymphatic system. This is why some shots hurt long after it’s happened. It’s quite literally like having a big, painful pimple or cyst deep under your skin for several hours. Finding a vein to kick it in directly allows one to skip that step entirely.
And now that I’ve typed that all out, I realize you’re not asking about getting things *in*, but getting blood *out*. That’s more of a method for getting the volume they need in a reasonable amount of time, while doing the least amount of damage to your body. You *can* match the draw rate of a blood transfusion needle with equally severe paper cuts, but that’s gonna be a *lot* of cuts. Really deep cuts. You have to expose enough capillaries to match the flow rate of an entire artery. And then you have to find a way to collect all of that into one syringe safely and cleanly. And then you have to plug it up and allow it to heal. Siphoning directly from a vein is simply more efficient in almost every way.
Also, this is just my speculation (disclaimer: NOT A PHLEBOTOMIST or anything remotely resembling one) but another advantage of drawing blood straight out of the vein is that you can get access to deoxygenated blood. I have no idea if this matters in any kind of blood drawing, but if you simply sucked blood out from a paper cut that had been exposed to air, the red blood cells in it would become oxygen-enriched. Oh, and contaminated with whatever else is in the air, too. That probably does actually matter. With a needle probing deep, you deny almost any chance of external contamination, assuming your needle and puncture site are thoroughly cleaned.
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