Srimad-Bhagavatam: Canto 7 - Chapter 1 - Verse 25
खे खानि वायौ निश्वासांस्तेज:सूष्माणमात्मवान् ।अप्स्वसृक्श्लेष्मपूयानि क्षितौ शेषं यथोद्भवम् ॥ २५ ॥
khe khāni vāyau niśvāsāṁstejaḥsūṣmāṇam ātmavānapsv asṛk-śleṣma-pūyānikṣitau śeṣaṁ yathodbhavam
A sober, self-realized person who has full knowledge should merge the various parts of the body in their original sources. The holes in the body are caused by the sky, the process of breathing is caused by the air, the heat of the body is caused by fire, and semen, blood and mucus are caused by water. The hard substances, like skin, muscle and bone, are caused by earth. In this way all the constituents of the body are caused by various elements, and they should be merged again into those elements.
To be self-realized, one must understand the original sources of the various elements of the body. The body is a combination of skin, bone, muscle, blood, semen, urine, stool, heat, breath and so on, which all come from earth, water, fire, air and sky. One must be well conversant with the sources of all the bodily constituents. Then one becomes a self-realized person, or ātmavān, one who knows the self.