The use of reconfigurable hardware (HW) can improve the processing performance of many systems, including Wireless Sensor Networks (WSNs). Moreover, reconfigurable devices permit remote and runtime HW reconfiguration, which implies benefits in WSNs deployment and maintainability and, finally, cost reduction. In this paper, WSN node runtime reconfigurability is tackled from several aspects. First, the sensor node includes a commercial reconfigurable device, a Field Programmable Gate Array (FPGA), that permits to take advantage of the tools and support provided by the industry, while exploiting the inherent hardware parallelism. Second, two software (SW) and hardware reconfiguration scenarios are defined along with a support middleware. Third, in order to provide runtime reconfigurability to the WSN node, a complete runtime reconfigurable system has been defined and designed for the FPGA included in the node. Fourth, the HW reconfiguration cost has been evaluated, as well as the cost of transmitting new HW configurations and SW programs through the network, based on a set of defined parameters. Finally, the feasibility of the runtime reconfigurable system has been demonstrated with a use case.