Being the biological fire that governs metabolism, agni encompasses all the changes in the body and mind from the dense to the more subtle. Such changes include the digestion and absorption of food, cellular transformations, assimilation of sensory perceptions and mental and emotional experiences. Agni therefore covers whole sequences of chemical interactions and changes in the body and mind. Digestive abilities being related to the strength of agni.
Agni and pitta are closely connected. While both are hot and light, agni is subtle and dry. The heat energy to help digestion contained by pitta is agni. Pitta is therefore the container and agni the content. Agni is acidic in nature and stimulates digestion. It is subtly related to the movement of vata. In every tissue and cell agni is present and is necessary for maintaining the nutrition and auto-immune mechanism. By destroying micro-organisms, foreign bacteria and toxins in the stomach and the intestines.