Describe the flow of air through the respiratory system.

Diaphragm contracts, which increases volume of thorax as ribs/ lungs move up and out.
Pressure decrease draws air into the lungs (through mouth, trachea, bronchi, bronchioles into alveoli.)
Useful gases (Oxygen) are absorbed into the blood through diffusion in alveoli.
Harmful gases in blood (CO2) diffuse across alveoli to be breathed out.
Diaphragm relaxes which causes volume of thorax to decrease, which pushed air out through the mouth.

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