In this paper, we analyse the conceptual and procedural difficulties that university students of the Primary Education Degree have about respiratory physiology. To carry out this study a questionnaire was designed in order to assess the knowledge of these future teachers before and after studying the first subject of natural sciences of the degree. The results obtained revealed that the surveyed students have a low level of knowledge about the subject before their arrival at university, which became more evident when they were subjected to procedural and/or comprehension exercises. It has also been found that after completing the subject course, the level of knowledge increases significantly; however, there are still basic conceptual errors and a notable deficit in certain skills or competences on the part of these students that must be resolved.