Cows, Clouds and Respiratory Bronchioles

 The respiratory bronchioles are the cows; (the alveoli are their udder) ; their milk, the flow of oxygen, is collected in the capillaries (vessels).

The respiratory bronchioles are also clouds; as the clouds give rain (flow of oxygen), so the cows give milk (oxygenated blood).

The cow and the clouds in the Puranas both refer to the respiratory bronchiole in the acinar region of the lungs. 

The term "Gokula" or "Vraja" refers to a station of cows and herdsmen. Therefore it is the respiratory bronchiolar region that is given the name of "Gokula" (or "Vraja"). This region is situated at the distal (remote, peripheral) end of the bronchial tree--in the countryside, as it were,--away from the all the larger conduits, the cities and the palaces, where the fine constructions have given way to mere hutments.

On the Rein-Cords Fixed to the Chariot of the Sun

 The lungs is, essentially, the hierarchy of bronchial conduits. Starting from the level of the primary bronchus down to the level of the acinus, there are, broadly speaking, three classes of conduits – the bronchi, the bronchioles and the respiratory bronchioles. Now, the "sun" of the Puranas, strictly speaking, is the neural entity innervating this network of conduits. The nerves trunks emanating from this chief innervating entity coursing along the  bronchi are imagined by the ancient anatomists, those keen watchers of the landscape of the microcosm, as the "sun" himself traveling in his resplendent (bronchial) chariot.

Now, as previously noted, a chariot has a carriage which is connected to components that enable it to move. Thus we have a wheel and the horses which do the job of pulling (extending) the carriage and moving it forward. In the context of the bronchial chariot, these would be the lower order bronchi or could even be the bronchioles. The idea sought to be conveyed, through a very sophisticated metaphor, is that the lungs is essentially a set of interconnected entities; the lower order bronchi and the bronchioles extend the bronchial conduit and carry the innervating entity to the furthest and deepest (distalmost)  regions of the lung. Of course, the wheel and the horses do not pull and move the carriage directly. They are fitted to two other components of the chariot, the axle and the yoke. The former connects the wheel while to the latter the horses are fitted. 

In the following lines taken from the Brahmanda Purana, this bronchial chariot is said to have two "rein-cords" fastened to it, at the extremities of the axle and the yoke. These are the pulmonary arteries. They are said, further, to "revolve" in Dhruva which we have identified previously as the pulmonary trunk, but which may, in a more strict sense, stand for a particular neural entity. 

"Revolving" is used by the Puranic authors in the sense of "enclosing." An entity (such as a conduit) is said to "revolve" around another entity if it "encloses" it  or travels along all its "sides" or "directions." The lower order bronchial conduits, in this manner, "revolve" around the higher order bronchi, accompanied by their respective pulmonary arterial conduits which remain fixed, ultimately, in Dhruva, the pulmonary trunk. 

76. There are two rein-cords [pulmonary arterial conduits] fixed to the extremities of the yoke and the axis [bronchial conduits] of the chariot. Those two rein-cords revolve in Dhruva [pulmonary trunk] (and perform the function) of a pair of wheels. 

79. The extremities of the yoke and the axis [bronchial conduits] of the chariot revolve all round along with the rein-cords [pulmonary arterial conduits] like the rope fixed to a peg.

It must be noted that the pulmonary arterial conduits are not literally fixed to the bronchial conduits. It is only at the level of the alveoli that there is some sort of a direct connection. What the author seeks to indicate here, perhaps, is the physiological interdependence of these two entities. Anatomically, this "fixing" can only be in the sense of "as it were."