Abstract: An implantable device for treating hypervolemia includes an expandable chamber, a rigid chamber coupled to the expandable chamber, a first valve in fluid communication with both the expandable chamber and the rigid chamber, a second valve in fluid communication with the rigid chamber and an exterior of the implantable device, and an osmotic fluid. The expandable chamber includes a first semipermeable membrane. The rigid chamber includes a piston. The first valve has an open position to permit fluid flow between the expandable chamber and the rigid chamber. The second valve has an open position to permit fluid flow from the rigid chamber to the exterior of the implantable device. The osmotic fluid has a higher osmotic concentration than bodily fluid. The osmotic fluid is designed to absorb water from the bodily fluid through the first semipermeable membrane.