Respiratory System Posted on 18th December 20159th April 2022 by ThePD (The Project Definition) Respiratory System is a breathing system including the lungs and air passages that is a biological system used for gas exchange in animals and plants. Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window)Click to share on LinkedIn (Opens in new window) Related Posted in HSE and tagged health. ThePD (The Project Definition) ThePD has been developing the Preferred Project Definitions based on the actual project execution and operation experiences and knowledge with the Project Language, and sharing with you daily basis.