I have very visable vains but when i go to have blood taken hardly any blood comes out can u tell me how i can produce more blood as im always cold

Stop cigarettes: exercise increases blood flow and results in growth of veins (over months). squeezing a ball many times, a few times a day is the usual recommended one, but any use of hands/arms (blogging?) will increase veins in the areas used. Fat on top of the veins makes it tricky to find. Cigarettes, female, younger, migrainers, have vessels that react (being calm and hot towel to area helps).