This study examined student language in learning the human circulatory system, to understand the challenges they face in representing the knowledge in this topic. Data for this study comprised students' written responses to a range of structured test items. Responses to 17 test items from 70 students across four classes were analysed using tools adapted from the Systemic Functional Linguistics framework using the seven categories of: "Process" error, "Participant" error, "Descriptor" error, "Circumstance" error, "Connective" error, Missing word and Irrelevant word. The analysis revealed six problems in language use that occurred in all four classes taught by three different teachers. Students in the study encountered most difficulties with the use of "Participants." Reasons for the prevalence of this error type are proposed. The problematic nature of student language use is discussed in relation to both the conceptual demands inherent in the topic and the scientific practice of language generation in science. Examining the student language illuminates the need for students to understand the rationale for the finely distinguished terminology found in this topic.