Alveolar sacs are the part of the respiratory system where the exchange of air happens.
Inhalation is the process where oxygen is taking in to the respiratory system thru the nasal passageways and to the tiny alveolar sacs of the lungs. This process also expands the lungs and the diaphragm heightens or elevates upward.
The counterpart of inhalation is exhalation, where the carbon dioxide is taken out of the lungs in opposite of the oxygen passageways.
This two exchange is called the gas exchange or respiration which happens in the respiratory system responsible for enabling and providing oxygen throughout the body.