Taking blood. Nurse's hands using a tube to fill test tubes with blood samples. The clamp in her right hand will be used to stem blood flow through the tube while transferring the tube to the next test tube. The blood is being taken from a vein in the patient's arm. This is part of a therapeutic draining of blood session. A common reason for the removal of blood is to treat high amounts of iron in the body. The blood is extremely rich in iron.