Heat lives within the head (brain) and protects the body. The vital air called Prana, living within the head and the heat that is there, create all sorts of exertion.
श्रितो मूर्धानमात्मा तु शरीरं परिपालयन्। प्राणो मूर्धनि चाग्नौ च वर्तमानो विचेष्टते॥