A small, portable, constant withdrawal device is connected to tubing, including a catheter, whose internal walls are coated with heparin. The catheter is inserted intravenously through a disposal needle into a subject such as a human being. The subject may then move about for a selected period when blood is being slowly withdrawn at a prescribed rate and collected in a container within a housing supporting the device. The collected blood may then be analyzed to permit the measurement of the integrated concentration of glucose, growth hormone or other material in blood. In addition, a portable microdiffusion chamber is incorporated between the indwelling catheter and the extra corporal tubing and is electrically connected through a sensor probe to an associated sensory responsive device. This permits continuous analysis of the withdrawn blood to determine the in vivo concentration of circulating concentrations of materials in the blood.