OpenStax
Open Stax: Anatomy & Physiology: Organs and Structures of the Respiratory System
Students learn and understand the structures that make up the respiratory system, and then apply that knowledge to understand the process of oxygen and carbon dioxide exchange.
Biology Pages
Kimball's Biology Pages: Vertebrate Lungs
This personal page defines and compares the gas exchanges by the lungs of amphibians, reptiles, birds, and mammals. Offers detailed images.