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.