The apparatus is provided for sealing a vascular puncture tract by forming the autologous plug within the puncture tract, and extruding that plug into the puncture tract. The apparatus of the present invention forms an autologous blood plug by drawing blood into the apparatus from a vessel, mixing a blood congealing agent with the drawn blood, and ejecting a plug formed from the clotted blood within the puncture tract. Also provided are various closure elements to isolate the drawn blood from the vessel during mixture with the blood congealing agent, and to facilitate placement of the apparatus relative to the vessel.