A medical device includes a vascular access device with an access port having a septum and a slit. The slit is formed on the inner surface of the body of the septum where the access port receives an access device that is separate from the vascular access device through the slit of the septum. A pivoting member in communication with the access port pivots when the port is accessed by an access device. The medical device may be used to control the volume displacement of a chamber within the medical device by decreasing the volume of the chamber by inserting a substance having a mass into the chamber, pivoting a structure associated with the chamber, and increasing the volume of the chamber simultaneously and commensurately with the mass of the substance inserted into the chamber.