The Sun Sucks the Water of the Entire World

 According to the Brahmanda Purana (chapter 22, 12), "Presided over by Dhruva, the sun takes up water and showers it."  

 The lungs suck, as it were, the "water" of deoxygenated blood coming in to the heart from the systemic circulation. This oxygen-deprived flow enters the lungs by means of the pulmonary trunk ("presided over by Dhruva"). The deoxygenated blood of the body enters into pulmonary circulation. The sun (lungs), in this manner, sucks the water of the entire world:

With the mass of his rays [bronchial conduits] and accompanied by the wind [pulmonary artery] all round, the sun takes away the water of the entire world.

This "water" of deoxygenated blood is reoxygenated in the lungs – in the acinar region – and the "water" now returns to the heart for re-entering into systemic circulation as a rich and vivifying liquid. It is the pulmonary vein – the moon of the Puranas – that does this work of completing the pulmonary circulatory loop. 

The moon [pulmonary veins] transmits the entire water [flow of blood] drunk by the sun [lungs].

Therefore the function of the lung is not to change the volume of the "water" coming into it but to change the character of the liquid flowing through it: 

There is no total destruction of water. The same water gets transformed.

The terms "usna" (hot) and "sita" (cold) undoubtedly refer to deoxygenated and oxygenated blood respectively. The deoxygenated flow is always imagined as "deadly," as "fierce," "fiery" and "hot" – the pulmonary arterial conduit, as noted elsewhere, is given the code of "fire" – while the polar opposites of these concepts are associated with the oxygenated flow. 

The Usna (hot virility) [deoxygenated blood] oozes out of the sun [bronchial tree], and the Sita (chill virility) [oxygenated blood] functions from the moon [pulmonary veins].

The next few lines declare the supreme fact that this entire apparatus of the lungs is a peerless – almost magical – creation of the Lord:

 It is for the sake of sustenance of all the worlds [bodies / regions of the body] that this cosmic Maya has been created (by the lord).

Cloud Formation 

But the "sun" does not merely take up "water" and shower it (v. 12 above). Before such showering can take place, the "clouds" need to be formed. There is a process of combining of entities. The bronchial conduit and the pulmonary arterial conduit taken together, are referred to as "cloud" and it is the showering of these two entities in unison that gives rise to "rain" (oxygenated blood). 

The concept of "cloud," properly speaking, pertains to the respiratory zone (the acinus) and not the conducting zone. However, reading the extracts from the Puranas, one gets the impression that even the conduits in the conducting zone are referred to as "clouds." 

For clouds to be formed, both the bronchial conduits and the pulmonary arterial flows would need to be "vaporized." This means that the bronchial tubes have now, at the acinus, become so thin and wispy that they have become as subtle as vapor. Correspondingly the pulmonary flows  too have become extremely "emaciated."

The pulmonary arterial conduit is seen as a great cosmic heater (in contrast to the pulmonary veins which are imagined as a cooler) which is transforming the liquid poured into it – the deoxygenated blood – into smoke. 

At the same time, by virtue of its close proximity to the bronchial tree, it is also seen as incinerating the entire creation. We will see later how this image forms the basis of the practice of the yajna. 

The bronchial tree comprises both weighty conduits like the bronchi which are relatively unmoving. These make up the "immobiles" of the creation (such as the mountains and the giant trees). On the other end, we have the smaller bronchial conduits divested of cartilage – the bronchioles and the respiratory bronchioles – which are like birds. They are light and relatively "moving" – they dilate and constrict much. These are the "mobiles" of the creation. 

22. (The Ganga), the great rivers [pulmonary arterial flows] beginning with Bhadrasoma and the waters [ deoxygenated blood] (humids) that flow within the bodies [bronchial conduits] of all living beings mobile and immobile, become smoke (i.e. vapour).

23. When the mobiles and immobiles [bronchi and the bronchioles] get heated (and burnt) they spread everywhere as vapour.

As a result of this "vaporization" of conduits within the lungs, the clouds are formed in the respiratory zone: 

24. Clouds are formed thereby. It is remembered that the abode (waters) is full of clouds (or, is in the form of clouds).

It may be because the respiratory zone of the lung is full of "clouds" that it is referred to as the "waters" in the Puranic texts.This line seems to give the definition of the "waters."