In this paper we introduce a formalism to specify the behavior of biological systems. Our formalism copes with uncertainty, via fuzzy logic constraints, an important characteristic of these systems. We present the formal syntax and semantics of our variant of fuzzy automata. The bulk of the paper is devoted to present an application of our formalism: a formal specification of the heart that can help to detect abnormal patterns of behavior. Specifically, our model analyzes the heartbeats per minute and the longitude of the RR waves of a patient. The model takes into account the age and gender of the patient, where age is considered to be a emph{fuzzy} parameter. Finally, we use real data to analyze the reliability of the model concerning the diagnosis and prediction of potential illnesses.
