Blood draws have no effect on the body. When the body is sick, that is, when it is not feeling well, it will be examined in the hospital, and the doctor will usually issue a laboratory test to find out if there is any disease in the body. Blood tests include routine blood tests, biochemical tests, coagulation tests, hepatitis B tests, and so on. The amount of blood drawn is usually very small, only 2ml or 5ml at most for each test tube. For the whole body, 30ml of blood is very little. Sometimes when you are well enough to donate blood, you can donate 200ml or even 400ml, so the blood draw will have no effect on your body.