The vital air called Prana, carrying a current of heat, goes (from the head) downwards to the extremity of the anal canal and thence goes up once more. Returning to the head again, it once more sends down the heat it carries.
अग्निवेगवहः प्राणो गुदान्ते प्रतिहन्यते। स उर्ध्वमागम्य पुनः समुत्क्षिपति पावकम्॥