There are three general reasons why blood vessels cannot be found, obesity, shock and the occurrence of thrombotic superficial phlebitis: In the first case, for obese people, because they rarely engage in physical work and physical exercise, it leads to the atrophy of the blood vessels themselves, and the accumulation of fatty tissue and thickening of the skin within the skin, which will cover the blood vessels and make them hard to find on the skin. In the second case, shock, when the blood pressure drops and the perfusion pressure decreases leading to a reduction in arterial blood supply and venous return, the blood vessels are deflated and cannot be found under the skin. In the third case, if thrombotic superficial phlebitis has occurred, then the thrombus in the superficial vein will cause the vessel to occlude or even disappear, and it will also be difficult to find the vessel.