Oxygen goes in through mouth or nose and into larynx, and then into the trachea.
Trachea goes into two bronchi, one in each lung.
The bronchus split into narrow tubes called bronchioles
Alveioli are at the end of bronchioles, and the alveioli are covered in capillaries.
In capillaries, oxygen diffuses in and carbon dioxide diffuses out of the lungs and is removed from the body during exhalation.
Lungs during breathing
The diaphragm is the muscle underneath the lungs
During exhalation, diaphragm relaxed
During inhalation, diaphragm contracts and flattens out allowing lungs to open up
The larger the volume, the lower the pressure
Pressure gradient goes from high to low and oxygen flows down the pressure gradient
When pressure in lungs is lower than the pressure outisde the lungs, air flows in
When pressure in lungs is higher than the pressure outside the lungs, deoxygenated air flows out