This study examines how lower secondary school students understand the circulatory system, using the structure-behavior-function (SBF) framework for conceptual representation. It evaluates the progress of students' understanding after interventions with two different teaching approaches, one using a biology textbook supported by augmented reality (AR) technology and the other using only a textbook as a main source of information. The data analysis is based on the assumption that systemic understanding demands the perception of three system dimensions: the components forming the system at all levels of organization (its structures), the interactions and mechanisms between them (its behavior), and the function as a whole outcome (its phenomena). The results indicate that both learning approaches contribute to a higher level of understanding the circulatory system. The group using AR-supported educational materials showed statistically significant better improvements in their knowledge of the circulatory system, including all three components of the SBF framework.