Device and system for restricting fluid flow in physiological vessels

Provided is a device and system for restricting fluid flow in physiological vessels of humans or animals, the device being configurable in a first mode to be passively propellable by fluid flow within a physiological vessel and in a second mode to at least partially occlude a physiological vessel, the device having a first cross-sectional size in the first mode and a second cross-sectional size in the second mode, wherein the second cross-sectional size is greater than the first cross-sectional size. The system comprises: a plurality of the devices described above,a power source for powering the devices; and a controller comprising one or more processors for controlling the devices.