By the coalition of Prana and other airs, a reaction ensues and the heat generated thereby is known as the internal heat of the human system which causes digestion of food.
प्राणानां संनिपातात् तु संनिपातः प्रजायते। ऊष्मा चाग्निरिति ज्ञेयो योऽन्नं पचति देहिनाम्॥