The reasons for drawing blood from a vein are that the vein is located on the surface of the body and can usually be seen with the naked eye, and the pressure in the vein is low, making it less labor intensive to draw blood. The walls of the vein are also thinner for puncture and the bleeding usually stops soon after the needle is withdrawn from the puncture. In contrast, arteries are deeper and not easy to find, and the internal pressure is higher, so they bleed easily after puncture, and insufficient pressure can lead to large hematomas and sometimes even pseudoaneurysms, so except for very special blood gas analysis, the vast majority of blood draws are venous blood.