Abstract: Process for introducing a fluid into a cavity or a blood vessel of a human or animal body, particularly for gas insufflation in preparation for or during a minimally invasive surgical procedure or investigation of the body cavity or blood vessel, the fluid being conveyed from a pressurised gas store (11,12) through a fluid supply line (4) and an introduction instrument (T) of a fluid introduction system (1) into the body cavity or blood vessel and the internal pressure produced in the body cavity or blood vessel being determined from the insufflation pressure measured at a measuring point in the fluid introduction system and a correction magnitude reflecting the drop in pressure between the measuring point and the body cavity or blood vessel, wherein the internal pressure is determined substantially without interrupting the introduction of fluid from substantially continuously detected measurements of the insufflation pressure and the fluid volume introduced per unit of time (fluid flow), the gas flow yieldin