We need energy to live. Although food, water and air are the sources of our life energy, air is the most vital of them all because without it there is no survival, and no life. It is no wonder that breath (air) by itself is generally referred to as life energy.
Solid food that we eat, liquids that we drink and air that we breathe provide the raw materials for the production of energy which we use to live. Essence of air that we breathe is oxygen which must be absorbed by red blood cells to be circulated to the various parts of the body. Similarly the essence of solid and liquid food in the form of nutrients must also be conveyed to our blood stream to be circulated to various parts of the body.
Inhalation and exhalation of air through the nostrils is external breathing. Internal breathing happens in the alveolar parts of the lungs. In internal breathing, oxygen from the alveoli is absorbed by the arterial blood to be circulated to the various parts of the body and carbon dioxide from the returning venous blood diffuses into the alveloi.to be exhaled.
Food and drink are propelled from the mouth down the gastrointestinal tract where it is converted into a semi-fluid form from which nutrients are absorbed by the lymphatic system and the blood stream to be circulated to various parts of the body. The process of absorbing the nutrients from food and expelling the food waste through the lower end is called digestion.
Using the nutrients and oxygen absorbed and circulated by our blood stream to the various parts of our body, energy is produced by every one of our more or less 40 trillion cells using a process called metabolism.