Isosmotic - When two solutions with the same osmotic pressure are isosmotic. Remember - Lactose is the major osmole in milk. Synthesis of lactose in the Golgi results in an impermeant small molecule that is formed effectively at an extracytoplasmic site. This flow of water probably induces an electrical potential difference across the apical membrane when the secretory vesicles fuse with the apical membrane and release their contents. The apical membrane is impermeant to Ca, citrate and phosphate - these do not flow back into the cell.
Ducts are impermeable to the major constituents of milk.
There is no resorption of water in the ducts. Ducts act only as a conduit for moving milk from alveoli to the teat and the large ducts for passive storage of milk between milk removals. In Advanced Dairy Chemistry, Volume 1, Proteins. P. F. Fox, Ed. Elsevier Science Publishers Ltd, New York.