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Theory:

During the inhalation (breathing in) of air, a chemical reaction occurs between the inhaled oxygen and digested food within the body to produce carbon-di-oxide gas, water vapour and energy.
Every organism on earth needs energy to carry out various processes in the body like movement, growth and repair.
Respiration is a process by which oxygen in the air reacts with digested food to form carbon-di-oxide, water vapour and energy. This can be represented in the form of an equation as,
 
Food + OxygenCarbondioxide+Water+Energy
 
respire.jpg
Inhalation of oxygen
  
During respiration, carbon-di-oxide formed in the body gets dissolved in the blood and exhaled through the lungs.
 
Both inhalation and exhalation contains the same substance, air, in different proportions. But only nitrogen is present in the same amount. In this process, oxygen is more in the inhaled air, whereas, carbon-di-oxide is more in the exhaled air.
 
Composition of air during inhalation and exhalation:
 
Component
Inhaled air
Exhaled air
Nitrogen
\(78\%\)
\(78\%\)
Oxygen
\(21\%\)
\(21\%\)
Carbon-di-oxide
\(0.03\%\)
\(4\%\)
Water vapour
Variable amount
Amount increases in
exhaled air
Noble gases
\(0.95\%\)
\(0.95\%\)
Dust
Variable amount
None
Temperature
Room temperature
Body temperature