This study investigates what knowledge pre-service student teachers bring into natural science courses about organ systems and what knowledge they believe is important when teaching in the future. The empirical data were collected from a group of pre-service student teachers specializing in grades 1–3 and grades 4–6. The data were collected from a drawing of a human figure. The students were instructed to draw the internal organs inside the figure and describe what happens in the body from the time a person drinks water until the person urinates. The survey also includes knowledge questions with multiple-choice and open-ended questions. The results indicate that the student teachers’ knowledge shows that it is challenging to connect more than two organ systems. One fifth of the students had the knowledge to compare three organ systems, but one-third of the students gave non-scientific explanations. But there was no clear link between knowledge of the pathway of water through the body, knowledge of the importance of drinking water for the body and knowledge of the kidney function. The pre-service teachers identified challenges in their future profession, such as limitations in their own knowledge. However, few considered students’ interest to be a challenge.