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.

The Animals in the Puranas

 It is the different components and sub-structures of the bronchial tree that masquerade as the different animals like the cow, the bullock, the tortoise, the horse, etc.

In the Mahabharata (Book 6: Bhishma Parva: Jamvu-khanda Nirmana Parva: Section IV), we have Sanjaya narrating to Dhrtarastra the fourteen species of animals:

These fourteen, O king, complete the tale of domestic and wild animals, mentioned, O lord of earth, in the Vedas, and on which the sacrifices rest.

The mobile and the immobile creatures (7 wild animals + 7 non-wild ones + 5 kinds of vegetation = 19) taken together with the five great elements make up the group of twenty four, collectively referred to as the “gayatri:”

Twenty-four in all, these are described as Gayatri (Brahma) as is well-known to all. He who knows these truly to be the sacred Gayatri possessed of every virtue, is not liable, O best of the Bharatas, to destruction in this world.