eli5. How come you don’t continue to bleed from the hole in your vein after you get your blood drawn?
In: Biology
Have you ever seen video or pictures of a flash flood? There’s lots of water, but also lots of trees and sticks and other stuff caught up in the water. When we think of a flood, we think of water, but there’s lots of other stuff in there. Big things, like sticks and trees, and little things, like tiny pieces of dirt suspended in the water, which is why the water is brown and not clear. You can see a video of a flood like that here: [https://www.youtube.com/watch?v=ORJtxkuD62E](https://www.youtube.com/watch?v=ORJtxkuD62E)
Blood is kind of like flood water — it has all sorts of extra stuff mixed in, just like flood water has tree branches and tree trunks. Of course these things are very very tiny, so we can’t see them without a microscope, but they are there. And so when a small hole gets poked in a vein, the “tree branches and tree trunks” inside your blood end up shoved up against that hole and they get all tangled up and stuck to each other until they have formed a “plug” that plugs up the hole. (And it will stay there while our body repairs the hole and while other stuff in the blood comes along and eventually breaks down the plug.) Our blood is meant to have all of these extra little things floating it in — things to clump together and things to break down the clumps. Your body actually follows a very carefully balanced recipe for how many extra things to keep in the blood. The main blood components responsible for starting the plugging up of small holes are called “platelets.” Some peoples’ bodies don’t make enough of some of these different “extra things” [like platelets] that are supposed to be in the blood (or the things they make don’t stick together very well), and that can lead to problems with bleeding too long from just a small cut, or getting bruising when they just barely bump into something. Or bleeding when they just brush their teeth. That last example really helps to show how important platelets are; without working platelets we’d all be bleeding when we brush our teeth. Other peoples’ bodies might actually be too prone to forming those platelet plugs and so their doctor might put them on a medicine that makes the platelets not work so well. This can help improve their health by preventing bad clots inside the body that these people are prone to, but then if they get a cut they can bleed a lot more than other people because the medicine works so well.
The protein in the blood that makes platelets stick together is called von willebrand factor (“VWF”). You may have heard that we have “blood types.” There are four types in the “ABO” blood group: A, B, AB, and O. And scientists have shown that around [30% of the variation](https://pubmed.ncbi.nlm.nih.gov/3919575/) in VWF levels is due to blood type — people with type O blood have the least sticky platelets! (Which is just a fun fact I think is kinda cool.)
SOURCE: AM DOCTOR, HAVE KIDDO
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