Theory:

In the presence of your teacher or your parent try closing your nostrils and mouth tightly and look at a watch. Find out for how long could you hold your breath by closing them.
By this small activity we can know that we cannot survive for long without breathing.
What is breathing?
The process of taking in oxygen-rich air present in the surroundings and giving out the air that lacks oxygen into the surroundings with the help of respiratory organs is known as breathing.
It is a process that occurs continuously in the living organisms as one cannot survive without breathing.
The process of breathing can be broken down into two steps inhalation and exhalation.
  
Inhalation:
The process of taking in the air, which is rich in oxygen is known as inhalation.
 
Exhalation:
The process of giving out the air that is rich in carbon dioxide is known as exhalation.
 
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This process will be explained in detail in the upcoming theories.
Important!
As we know we breathe in oxygen and we breathe out oxygen. When we hold our breath for sometime and then release it we start breathing heavily as the amount of oxygen required to produce energy in the body decreases.
Breathing rate
It is the number of times a person breathes per minute.
During the process of breathing both inhalation and exhalation takes place alternately.
An inhalation that is followed by an exhalation is known as breath.
In human beings the rate of breathing or breathing rate is not always constant. This rate increases when there is high demand of energy by the body.
During these type of conditions when the humans breathe faster more oxygen is supplied to the cells so that the process of cellular respiration speeds up and more energy is released.
In such cases a person feels drowsy indicating the lack of oxygen supply in the body. This the reason why a person yawns.
An adult human being breathes 15 to 18 times in a minute on an average. When a person performs heavy exercises the breathing rate increases up to 25 times per minute so that more oxygen is inhaled.