Respiration: 

• The body needs energy to perform various vital activities. 
• This energy is obtained mainly by oxidation of carbohydrates & fats. Oxidation of these substances consumes O₂ & release CO₂. The process of consumption of O₂ and the release of CO₂ is termed as respiratory exchange.
• According to Carter, respiration involves external respiration, internal respiration & cellular respiration. 
• Mammals exhibit aerial respiration which needs a respiratory system. 
• The respiratory system concerns with an intake of O₂ and giving out of CO₂.

Types of Respiration: 

The respiration is of two types: 

• External respiration: It takes place in the lung alveoli. taking in of O₂ and giving out of CO₂ is termed as external respiration or Breathing or Ventilation. It is a physical process. 
• Internal respiration: In taking of O₂ by the body cells and production of CO₂ and ATP is called internal or cellular respiration. 

The internal respiration is of two types: 

• Aerobic respiration: Cellular respiration in the presence of O₂ is called aerobic respiration. 
• Anaerobic respiration: The cellular respiration in lack of O₂ is called anaerobic respiration.