The Three Divisions of the Ganga

The deoxygenated pulmonary flow, the Ganga, first travels through the "sky," the foremost part of the pulmonary arterial conduit network. Then it is borne by the "wind" in the secondary division of the pulmonary arterial tree (corresponding to the bronchioles?). And, finally, this flow, assuming the name of "Sita," enters the "fire" at the level of the acinus.

She is three-fold as the river of sky, earth, and the lower regions, tripathaga, trilokaga, etc.

Sita's Entering Fire in Lanka 

The deoxygenated flow, enters the "fire" of the pulmonary arterial conduit in the "lower" regions –the acinus –and becomes exceedingly pure.