Eli5: Why do medical people get the blood through the vein in the arm?

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Why can’t they just stuck the syringe anywhere to get blood? Is there something different getting the blood directly from the vein?

In: Biology

Because there not blood everywhere in a significant volume and the vein is the return flow to the heart so it’s not stealing blood full of oxygen and nutrients. It’s also a thin pipe with very low pressure compared to arteries and they heal very easily, while providing all the volume needed for that 5-10 mL pull in the shortest amount of time. All in all it’s the best place.

Blood is only in the vasculature (veins and arteries). We do sometimes take blood from arteries. Veins are good enough for most purposes though, and the pressure inside veins is lower wich means it will bleed less when the needle is taken out.

Source: I’m a paramedic.

You might be wondering that because blood comes out from everywhere if you get a cut or anything,even shallow ones. But thats not pure blood,so to speak. In veins and arteries is blood,which is used as the standard from our body to test. After this blood enters capillaries,it leaks into surrounding tissue,and hence gets its nutrients taken and some waste material added. This fluid then gets taken into our circulatory system later on via lymphatic system(not imp here). So if you cut anywhere,you get small amount of blood that has nutrients taken away and waste material added,plus blood from the multiple small capillaries and other arterioles(small arteries, don’t contribute much to shallow cuts’ bleeding) or venules(small veins). So this blood does not have the exact composition that we need to test,its contaminated. Also,this contaminated blood will not give you the correct cell count of anything at all,as all the cells are mostly still in veins and arteries(they cannot pass into tissues via capillaries easily).RBC and platelets are near absent,and WBC less than in our blood.

So if you prick anywhere to get blood,you will get the wrong amount of nutrients,waste material and cells to measure,which will fuck up your medical reports to the point they will think you are not human.

As to why they use the vein in the arm, that particular one runs close to the skin near the inside of the elbow, so it’s convenient.