The tale of the Ganga in the Puranas is not the tale of an external geographic entity but, rather, it is the tale of a (special) river-like microcosmic entity that has importance in the cardio-respiratory framework.
The Landscape of the Puranas.
Source of Image Used in Developing Graphic:
In the metaphorical framework of the Puranas, the term "river" seems to denote a flow (see illustration above) such as the flow in the blood vessels. Therefore, we may have such "topographical" entities as venous "rivers" and "rivers" formed out of "juice!" Therefore, the veins are the rivers in the Puranas.
The Ganga represents venous circulation. It washes away the "sins" of beings! The poet-anatomists of the Puranas view the Ganga as descending from Brahmaloka, the highest region of the (body) universe. This is the region of the brain. The internal jugular vein (Ganga) drains the brain and, exiting the skull, descends from the cervical (heavenly) region into the pulmonary zone—the "earth” of the Puranas.
...she is spoken of as threefold, three-pathed...is said to issue from the world of Brahman and to fall like milk from Mount Meru into the lake of the moon, which her own fall has created, after being upheld for one hundred thousand years on Siva's head.
In the passage above, the "world of Brahman" is the brain. The (venous flow of the) internal jugular vein (Ganga) issues from it and falls, as it were, from the trachea (mount Meru). Its "waters," then, are upheld on the brachiocephalic vein ("Siva's head?") before it falls into the right atrium of the heart ("the lake of the moon").
The Ganga thus descends from the sky and falls, as it were, into the earth. But the Ganga cannot directly do so. First, it has to fall into the "sphere of the moon" (the heart) and only then does it re-emerge from it to enter the "earth" (the respiratory zone). This is in the form of the pulmonary artery.
According to the Bhagavata Purana:
According to the Bhagavata Purana:
Later on, it (the stream) descends by the path of gods (i.e. sky), teeming with multitudes of thousands of crores of celestial cars, to the sphere of the Moon. After flooding the lunar sphere, the stream (of the Bhagavatpadi) flows down to the city of Brahma (on the summit of Mount Meru).
The heart is also known as Patala, the nether-world, in the Puranas. And, therefore, the Ganga is three-fold. It flows through all three regions of:
- the sky (cervical region)
- earth (the lungs) and
- the nether-world (heart).
She is three-fold as the river of sky, earth, and the lower regions, tripathaga, trilokaga, etc.
The passage above refers to the venous flow (Ganga) as the internal jugular vein in the cervical region (“sky”), as the (deoxygenated) blood in the heart ("lower regions") and as the pulmonary artery in the lungs ("earth").
The river Ganga is said to divide over mount Meru into four (principal?) branches, all flowing towards the "ocean:"
There, on mount Meru, it is divided into four branches under four names:--Sita, Alakananda, Caksu and Bhadra, and it (i.e, these branches) flows towards four quarters and enters into the ocean, the Lord of big and small rivers.
The venous flow of the heavens—the "river" Ganga—now emerges from the intra-cardial region as the pulmonary trunk. The Ganga (as the main pulmonary artery) divides above (anterior to) mount Meru (the trachea) into four branches—the four pulmonary arteries (see figure above), corresponding to the lobes of the lungs.
 Epic Mythology, Edward Washburn Hopkins, pp. 5-6.
 Bhagavata Purana, 5.17.4, Tagare (trans).
 Epic Mythology.
 Bhagavata Purana, 5.17.5, Tagare (trans).