Novel devices which can be used to both collect blood samples from and administer medical fluids to a patient on a repeated and continual basis using one rather than multiple needle insertions. The device typically includes a main tubing segment confluently connected to a cannula for insertion in the patient's vein. A syringe port and a volumeter for collecting blood branch separately from the main tubing segment. The device is used to collect blood by attaching an empty blood collection syringe to the syringe port, inserting the cannula in the patient's vein, allowing passive flow of blood from the main tubing segment into the volumeter under intrinsic venous blood pressure and capillary action, and then facilitating active flow of blood from the volumeter into the blood collection syringe by extending the syringe plunger. The device may be used to administer medical fluids to the patient through the main tubing segment from a medical fluid syringe or catheter attached to the syringe port.