In respiratory physiology, students must have a good idea about Dalton's Law to understand the effect of water vapor pressure on the partial pressure of the respiratory gases. This illumination has two aims: the first is to introduce a student's misconception about the application of Dalton's Law in respiratory physiology, and the second is to explore the potential effect of previous incomplete knowledge on the formation of this misconception. A survey about student feedback on these topics showed that this misconception is very common, hidden, and needs to be corrected to help students learning.