Blood sampling. Nurse use a syringe with a tube and stop needle (butterfly needle, green, upper right) to extract a blood sample from the arm of a patient. The stop needle has a protruding collar to stop it when the needle has been pushed a prescribed distance into the vein. It also allows extra stability when taping the needle to the arm. Here, 50 millilitres have been drawn out of the patient's arm. Blood samples are subjected to many tests in modern hospitals, helping to confirm a medical diagnosis or identify a disorder.