•    Exchange of gases or taking in O2 and giving out CO2 with the help of respiratory organs is called breathing. 
  •    The taking in of air rich in oxygen is called inhalation and giving out CO2 rich air is called exhalation. 
  •    It is a continuous process which goes on all the time and throughout the life of an organism.
  •    The number of times a person breathes in a minute is called the breathing rate. 
  •    Normal breathing of adult human beings at rest is about 15-18 per min. But it can change according to requirement of O2 by the body. 
  •    Thus, whenever a person needs extra energy breathing rate increases. 
  •    So, during heavy exercise, the breathing rate can increase upto 25 times per minute and also we take deep breaths to inhale more oxygen. 
  •    As a result more oxygen is supplied to our cells. 
  •    So speeds up the breakdown of food and more energy is released. 
  •    That is the cause we feel hungry after a physical activity.


Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen.The breathing of all vertebrates with lungs consists of repetitive cycles of inhalation and exhalation through a highly branched system of tubes or airways which lead from the nose to the alveoli.[3] The number of respiratory cycles per minute is the breathing or respiratory rate, and is one of the four primary vital signs of life. Under normal conditions the breathing depth and rate is automatically, and unconsciously, controlled by several homeostatic mechanisms which keep the partial pressures of carbon dioxide and oxygen in the arterial blood constant.