atrial naturiuretic factor is encapsulated within hollow acidic proteinoid microspheres which are stable and protective in the acidic stomach of a mammal but which dissolve at the near neutral ph of the blood . microspheres having diameters of less than about 10 microns readily penetrate the gastrointestinal mucosa and release the anf in the bloodstream in physiologically active form . such gastric administration significantly extends the period of effectiveness of anf as a diuretic and blood pressure suppressant .