The Concept of "Islands" (dvipa)

The dichotomous branching model of the bronchial tree is utilised by the ancient anatomists to construct a mandala representation in which the primary bronchus (Mt Meru) is placed at the centre. 

The concept of "islands" (dvipa) in the Puranas relates to the anatomical arrangement of the lung. Such an "island" is a lung in miniature--a segment bounded by the “ocean” of the pulmonary vein. These “islands” become more numerous as the airways branch dichotomously.

Bounded by the pulmonary veins on all sides (represented by the bronchi), each segment of the lung is like an island.

 "Jambudvipa" is a region of the lung. "Jambu" is the bronchial "tree" that grows, as it were, on the top of the lobar bronchus.

The bronchi represent the different sides or the directions of and within a particular continent. Each continent is like an island because in whichever "direction" one chooses to travel, one finds oneself surrounded by the "ocean" of the pulmonary vein. All "sides" are bounded or enclosed by the pulmonary vein. It is this fact that is represented in abstracted form in a mandala diagram. Each ocean in reality is a collection of oceans bounding a collection of islands. 

There appears to be a lack of consensus among anatomicians and researchers as to whether the branching pattern of the human bronchial tree is dichotomous or monopodial. The anatomists of the Puranas seem to have accommodated diverse viewpoints. While they seem to recognise that the branching within the bronchial region is quite irregular and varied with a number of peculiar configurations – some of which give rise to interesting metaphors, when it comes to the less discernible bronchiolar region, they have unhesitatingly opted for a regular dichotomous model. And the latter has found expression in the well-known concentric circular diagram of the seven island-continents.