Health Monitoring Systems (HMS) are among the most notable applications for Wireless Body Area Networks (WBANs). Online medical systems can use data collected in real-time by WBANs miniaturized sensors to early detect critical health events. In this sense, the alerts HMSs send to a central server must be prioritized by the network, ensuring lower delays and loss rates for medical alert packets. This paper proposes a platform based on Software Defined Wireless Networks (SDWN) for on-demand medium access prioritization of medical alert packets based on the IEEE 802.11 Medium Access Control programmatic management. The platform has been evaluated in a real wireless network environment, where multiple wireless stations contended for medium access. In a high dense scenario, we have observed a 34.65% reduction in the average delay of the prioritized medical alert packets compared to non-prioritized ones and a 58.62% decrease in the packet loss rate.