Developing safe and reliable systems goes beyond simply specifying what they should do but also understand how they might fail. This paper proposes a framework for capturing both, functional requirements safety requirements of safety-critical systems. Our framework uses Extended Finite State Machines to formally model both types of requirements. We introduce the concept of a “failing state,” which is integrated into the normal behavior model, providing a comprehensive view of component operations. To demonstrate the practicality of our framework, we apply it to a safety critical healthcare device: the Continuous Glucose Monitoring System. This case study will show how our approach can enhance the design and analysis of safety-critical systems and ensuring their operation reliability.