A case study about carbon monoxide poisoning was used help students gain a greater understanding of the physiology of oxygen transport by the blood. A review of student answers to the case questions showed that students can use the oxygen-hemoglobin dissociation curve to make meaningful determinations of oxygen uptake and delivery. However, the fact that many students had difficulty locating the effect of carbon monoxide poisoning in the process of external respiration suggests that these students have not built a robust model of how oxygen distributes itself between the plasma and hemoglobin. This suggests that more determined emphasis on how oxygen enters the blood and how it is partitioned between hemoglobin and the plasma would help students develop more accurate mental models of how oxygen moves from the lungs to the tissues.