What is the structure of the human gas exchange system- how do we breathe in and out?

Named structures : trachea, bronchi, bronchioles, alveolinote: surface area increases as you go down levels, highest SA at alveoli
Breathing in : diaphragm contracts and external intercostal muscles contractWhy? volume increase and pressure decrease in thoracic cavity (relative to atmosphere) resulting in air moving in
Breathing out - Diaphragm relaxes and internal intercostal muscles contractWhy? volume decrease and pressure increase in thoracic cavity (relative to atmosphere air = air moves out