nadyah samudras ca tayos
tustih pustis tad-asraye
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When there was a desire to have food and drink, the abdomen and the intestines and also the arteries became manifested. The rivers and seas are the source of their sustenance and metabolism.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The controlling deities of the intestines are the rivers, and those of the arteries, the seas. Fulfillment of the belly with food and drink is the cause of sustenance, and the metabolism of the food and drink replaces the waste of the bodily energies. Therefore, the body’s health is dependent on healthy actions of the intestines and the arteries. The rivers and the seas, being the controlling deities of the two, keep the intestines and the arteries in healthy order.