This paper presents a study to analyze the use of augmented reality (AR) for delivering multimedia content to support the teaching and learning process of the digestive and circulatory systems at the primary school level, and its impact on knowledge retention. Our AR application combines oral explanations and 3D models and animations of anatomical structures. A validation study was conducted with fourth grade students in order to evaluate the effect of our tool on knowledge retention. In this study, we attempt to verify whether students using the AR application retained more concepts that those learning the topic in a traditional setting. Results show an increased knowledge retention on students using AR multimedia contents as opposed to those following a traditional course, which validates AR technology as a promising tool to improve students' motivation and interest, and to support the learning and teaching process in educational contexts.