A medicinal-technical device for introducing fluids into body cavities uder determination and control of the body's internal pressure with medicinal-technical methods, comprising: a controllable fluid pump, a supply line, a pressure sensor in the supply line, a medicinal instrument to be connected to the supply line, wherein the fluid can flow out of the body cavity through at least one second line, at least one microprocessor, at least one memory with ζ 2 values of different medicinal instruments, and at least one software, which is used for carrying-out the method for determining the drag coefficients ζ 1 and ζ 2, wherein the pressure measured by the pressure sensor is an input variable of a mathematical estimation system that mathematically describes a state space, which estimates the actual pressure in the body cavity and controls the power of the pump using this estimated value, wherein the drag coefficients ζ 1 and ζ 2 of the medicinal instrument needed for the estimation of the pressure are determined by that during the start-up of the pump, the pressure trend is evaluated for a certain time, wherein the drag resistance ζ 1 is determined as a function of the measurable speed of rotation, the measurable flow pressure p 1, the non-measurable static pressure p 2 in the body, and a pre-deter-' mined value for the drag resistance ζ 2 according to following equation: ς 1 = Δ p − ς 2 ⋅ n 1 n 1 2 = p 1 − p 2 − ς 2 ⋅ n 1 n 1 2 under consideration of the loss term p 1 = Δp + p 2 for n 1 > 0, characterized by that the drag resistance ζ 2 is considered constant within certain speed ranges and is selected from the memory by a short, constant speed delivery based on the pressure rise, therefrom a characteristic line is determined, and the characteristic line is stored in the memory unit of the pump. a controllable fluid pump, a supply line, a pressure sensor in the supply line, a medicinal instrument to be connected to the supply line, wherein the fluid can flow out of the body cavity through at least one second line, at least one microprocessor, at least one memory with ζ 2 values of different medicinal instruments, and at least one software, which is used for carrying-out the method for determining the drag coefficients ζ 1 and ζ 2, wherein the pressure measured by the pressure sensor is an input variable of a mathematical estimation system that mathematically describes a state space, which estimates the actual pressure in the body cavity and controls the power of the pump using this estimated value, wherein the drag coefficients ζ 1 and ζ 2 of the medicinal instrument needed for the estimation of the pressure are determined by that during the start-up of the pump, the pressure trend is evaluated for a certain time, wherein the drag resistance ζ 1 is determined as a function of the measurable speed of rotation, the measurable flow pressure p 1, the non-measurable static pressure p 2 in the body, and a pre-deter-' mined value for the drag resistance ζ 2 according to following equation: ς 1 = Δ p − ς 2 ⋅ n 1 n 1 2 = p 1 − p 2 − ς 2 ⋅ n 1 n 1 2 under consideration of the loss term p 1 = Δp + p 2 for n 1 > 0, characterized by that the drag resistance ζ 2 is considered constant within certain speed ranges and is selected from the memory by a short, constant speed delivery based on the pressure rise, therefrom a characteristic line is determined, and the characteristic line is stored in the memory unit of the pump.