Implantable access port including fluid handling features

An access port for subcutaneous implantation into a body of a patient is disclosed. The port is typically subcutaneously connected to a catheter, a distal portion of which is disposed within a vein or other vessel of the patient. The port is configured with enhanced fluid handling features to improve fluid flow therethrough while reducing the likelihood of clotting or occlusions in the attached catheter, thus improving system patency. In one embodiment, for instance, an implantable access port is disclosed and comprises a body defining a reservoir, a needle-penetrable septum covering an opening to the reservoir, a stem defining an outlet to the reservoir, and a deformable element included in the reservoir. The deformable element is operably connected to a main portion of the septum and deforms in response to displacement of the septum to counteract a change in volume within the reservoir and prevent blood ingress into the catheter.