<![CDATA[The problem-based learning model is based on a constructivist paradigm, which is very student-centred and focuses on the student learning process (student centered learning). It is expected that student learning outcomes can be improved by using the PBL (Problem Based Learning) model. This research is a study of classroom behaviour. This research was conducted in the 2022/2023 academic year in class XI of SMA Swasta MIPA Imelda Medan. The research subjects were 38 students of class XI MIPA, 29 females and 9 males. Based on the results of two cycles of classroom behaviour research, the quality of the learning process and student learning outcomes can be improved by using the PBL model on respiratory system material. This model can also be trialled on other materials to see its significance. Key words: PBL Model, Learning Outcomes, and Respiratory System. ABSTRAK Model pembelajaran berbasis masalah berdasarkan paradigma konstruktivis, yang sangat berpusat pada siswa dan menitikberatkan pada proses belajar siswa (student centered learning). Diharapkan hasil belajar siswa dapat ditingkatkan dengan menggunakan model PBL (Problem Based Learning). Penelitian ini merupakan studi tentang perilaku kelas. Penelitian ini dilaksanakan pada tahun ajaran 2022/2023 di kelas XI SMA Swasta MIPA Imelda Medan. Subjek penelitian adalah 38 siswa kelas XI MIPA, 29 wanita dan 9 pria. Berdasarkan hasil penelitian perilaku kelas dua siklus, kualitas proses belajar dan hasil belajar siswa dapat ditingkatkan dengan menggunakan model PBL pada materi sistem pernapasan. Model ini dapat pula diujicobakan pada materi lain untuk melihat signifikansinya. Kata kunci: Model PBL, Hasil belajar, dan Sistem Pernapasan]]>
