Abstract: A means to prevent backflow during vena-puncture by employing a one way valve in conjunction with an evacuated tubular element which has at its opened end a resilient stopper which is used to house or contain said valve and which acts as a sealant for the system. The stopper allows penetration by a special two-ended cannula which is threaded into a holder used to hold the tubular element during vena puncture. Blood flows from the vein, through the cannula, into the hollow portion of the stopper containing the valve and subsequently into the tubular element for storage. During blood collection, blood is prevented from backflowing by the closing action of the valve. In this manner contamination to the patient by particulate matter within the tubular element is prevented.