Students have little opportunity to observe or experiment with blood filtration in the nephron. Thus, we have developed a modeling activity on the blood filtration in the nephron for middle school students. The students present their mental models of the principles of blood filtration in small groups. They then participate in a hands-on activity to conduct the blood filtration process using a syringe filter, then revising their initial models based on the activity and data analysis. Through this modeling activity, the students can build their knowledge about the excretion principle.