Complex sentence: Pulmonary Fibrosis is when abnormal scar tissue forms on the epithium in lungs causing them to thicken causing a greater diffusion pathway for the oxygen (restrictive lung disease).
Split into simpler sentences: Pulmonary Fibrosis is when abnormal scar tissue forms on the epithium in lungs causing them to thicken. This creates a greater diffusion pathway for oxygen (restrictive lung disease).