Abstract: A blood syringe includes a hollow barrel an external thread formed on one end and a circular internal flange formed on the inside of the other end. A piston with a hollow stub extending from one end is slidably mounted in the barrel. The stub contains a female thread and includes a ratchet stop defined on the free end of the stub. A plunger includes a first end and a second end. A male thread is formed on one end of the plunger and is inserted into the barrel. A button is formed on the other end of the plunger outside the barrel. The male thread on the plunger screws into the female thread in said stub. One end of the male thread has a ratchet wing opposite to the ratchet stop. The ratchet wing and the ratchet stop are engaged with each other after the male thread is screwed into the stub. A connecting block includes an open end and a closed end. A female thread is defined in the open end to securely receive the external thread of the barrel.