During interviews preservice nurses were asked to manipulate a range of such nursing devices as syringes, drain bottles, intravenous giving sets and aspirators. They were questioned about their knowledge of aspects of fluid dynamics. They were then asked to measure the interviewer’s blood pressure and asked about the pressures they observed and about their knowledge of blood flow and the function of the sphygmomanometer. They were also questioned about respiration and the function of the respirator. It was found that those who had alternative conceptions of atmospheric pressure, fluid pressure, fluid flow and equilibrium states were essentially unable to provide explanations for the function of simple nursing equipment or such physiological phenomena as blood flow, blood pressure and breathing.