Why is blood drawn from veins rather than arteries?

doctors typically draw blood from veins because veins are more superficial (ie close to the surface of the skin) and therefore more easily accessible. additionally, arteries are under high pressure and there is a risk of major bleeding. lastly, you have less collateral circulation with arteries than veins. what that means is this (let's use your arm as an example): there are many different veins in your forearm that lets blood get from your hand back to your heart, so damaging any one of these veins isn't a big deal. however, there are only two arteries in your forearm that supplies blood to your hand (the radial and ulnar arteries). Arterial blood is sampled from the radial artery because it's bigger and more easily accessible. However, damaging this artery can be catastrophic if the ulnar artery is not well developed, because then your hand is not getting enough blood, and it can become gangrenous and die (this is a vascular emergency). That is why doctors will do an Allen's test (look it up on google... you can even try it on yourself) before doing an arterial stick to make sure you have good blood flow through your ulnar artery that can sustain your hand in case the radial artery is destroyed with the arterial stick. so for these reasons, doctors don't do an arterial blood draw unless they really need one, and the one test that comes to mind where you absolutely need arterial blood is the arterial blood gas test. this test is used when doctors need to know the oxygen saturation and pH of the arterial blood, typically in patients with respiratory failure and sometimes in type 1 diabetics with diabetic ketoacidosis (DKA).