Question

What is the breath rate in human beings?

Answer 1

It is necessary that $O_2$ has to be continuously provided to the cells and $CO_2$ produced by the cells have to be released . This process of exchange of atmosphere $O_2$ with $CO_2$ produced by the cells is called breathing commonly known as respiration .

Complete answer: Breathing involves two stages : inspiration during which atmosphere air is drawn in and expiration by which the alveolar air is released out .
Inspiration is initiated by the contraction of diaphragm which increases the volume of the thoracic chamber in the anterior posterior axis , the contraction of the external intercostal muscles lift the ribs and sternum causing an increase in the volume of thoracic chamber in the dorsal ventral axis the overall increase in the thoracic volume causes a similar increase in pulmonary volume and increase in pulmonary volume decreases the entire pulmonary pressure to less than atmospheric pressure which forces the air from outside to move into the lungs that means inspiration .
Expiration is initiated by the relaxation of the diaphragm and the intercostal muscles return the diaphragm and sternum to their normal position reduces the thoracic volume and thereby the pulmonary volume this leads to an increase in intrapulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs that means expiration .
The breath rate of human beings is 16 breath per minute .

Additional Information: The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere .
There is negative pressure gradient

Note: Oxygen is utilised by the organism to indirectly break down simple molecules like glucose and amino acids , fatty acid to drive energy to perform various activities . Carbon dioxide which is harmful is also released during the above catabolic reaction.