A pump for dispensing a liquid product, including a pump body and a dispensing head manually movable on the latter, the head including a cylinder forming a dosage chamber of the pump, slidingly mounted in the pump body, and a channel allowing the passage of the product or air from the dosage chamber towards an ejection orifice. The rod includes means for airtightly sealing the channel consisting of a peripheral shoulder capable of coming to a rest against a complementary sealing seat arranged in the cylinder, the opening of the channel being performed by separating the sealing seat from the cylinder opposite the shoulder of the rod after the rod reaching its stop early during the axial movement of the cylinder in the pump body.