On the Various Meanings of "Earth"

 The learned describe the very same lotus as the terra firma, the Maharsis call it the lotus born of Narayana. Rasā, also known as Padmā Devi, is the earth. The weighty portions of the lotus are the mountains.

It is the bronchial artery, emanating from the Aorta, and supplying oxygenated blood to the bronchial tree (see image below), that is referred to as the "earth" or Padma Devi or "bhu devi" in the Puranas. 

The bronchial arteries supply oxygenated blood and nourish the bronchial conduits and the neural entities that accompany them. They are represented in various (usually feminine) forms in the Puranas. By virtue of their nourishing and life-sustaining nature, they are the clay, as it were, out of which the world is built.

The bronchial tree is the lotus born of the navel of Narayana. There seems to be a number of meanings of the term "earth" in the Puranic literature, all referring generally to the lungs and the human bronchial apparatus, but differing slightly in respect of the precise entity referred to. Sometimes the entire bronchial lotus is meant when the term "earth" is used. Sometimes, when talking from the standpoint of the maha bhutas, "prthvi" would be the bronchial arteries containing the subtle element (tanmatra) of "smell" (gandha) represented by the oxygenated blood. In those contexts, when the triune division of the world is being considered, the realm of "bhu loka" would stand only for the acinus, the truly respiratory portion of the lung, over which "rain" (shower of oxygen molecules) occurs; and so on. In the quote above, taken from the Matsya Purana, it is the bronchial arteries that are being referred to as the "earth." Rasa / Padma Devi is the bronchial artery.