The IEEE 802.15.4 medium access control (MAC) is a widely used protocol for low rate wireless personal area network because of its vital features, such as low power consumption, low data rate, short-range and real-time communications. Its unique guaranteed time slot (GTS) mechanism makes it an appropriate communication protocol in healthcare monitoring and other critical applications. Nevertheless, this GTS mechanism has few a constraints. One of the key limitations is the scarcity of GTS slots and the static nature of the GTS allocation mechanism. In this paper, we propose a scheme called the Finest Superframe interval and GTS provision scheme (FSGPS) that aims to reduce the limitations of IEEE 802.15.4 MAC to make it more suitable for time-sensitive services. To address the impacts of the quasi-static nature of WBAN, we in this paper consider temporal path loss variation in our simulations that accounts for real-time variation associated with the nodes. Through extensive simulations, we show that our proposed FSGPS outperforms IEEE 802.15.4 MAC in respect of several key performance metrics.