On the Identity of the Sisumara, Dhruva and the Others

 The Sisumara seems to be the entire pulmonary arterial tree, starting with the pulmonary trunk; it is upheld by the heart (hrdi narayana). Dhruva is situated on the tail of the Sisumara i.e. the pulmonary trunk. It is easy to see what the revolution of Dhruva would refer to; it would mean following either the left pulmonary arterial course or the right pulmonary arterial one.  

"Sisumara:" The pulmonary arterial tree

The Vayu Purana (Ch. 52, v. 99, p. 362) leaves no room for any doubt that Dhruva is identified with the pulmonary artery. It says:

"Among Dhruva, Agni and Kasyapa Dhruva is the most excellent. It alone whirls round at the top of the mountain Meru."

This would mean that Dhruva is identified with the primary pulmonary artery ("most excellent" in the hierarchy) since it is this entity that "whirls" at the top of Meru (principal bronchus). 

The pulmonary arterial flow of deoxygenated blood is the Ganga; therefore Ganga which divides into four streams on the summit of Meru would have an association with Dhruva.  And so it has for it is written (in the chapter on descent of the Ganga) that in the region called Visnupadi, the Ganga's waters are held on the head by Dhruva.