A two-part coupling structure includes a male coupling part and a female coupling part which are mutually engageable to open fluid communication between a fluid-storage vessel and a conduit. The coupling parts are disengageable from one another to discontinue fluid communication between the vessel and the conduit, and also to close communication between ambient and each of the vessel and conduit. The retention volume of fluid which is trapped between the connected male and female coupling parts and which is exposed to or lost to ambient when the coupling parts are disconnected is very small. Because of particular structural features, the coupling parts will function reliably even with fluid having chunks or particulates of non-fluid material therein.