Abstract: An apparatus for and a method of extracting physiological liquids and adding of a coagulation preventing element thereto such to control the added quantity of the coagulation preventing agent in accordance with the quantity of flow of the physiological liquid such that at all times a set ratio between the flow quantity of the physiological liquid and of the coagulation preventing agent is maintained. This allows an improvement of the yield and of the durability of the physiological liquid. The apparatus comprises an emitter element, a transducer including two oscillators which are to be applied onto a plastic hose flowed through by the physiological liquid, comprises further a demodulating circuit which generates a signal which is proportional to the velocity of speed within the plastic hose, comprises further a converter which generates a DC-signal which is proportional to the flow quantity. The measuring proceeds without contact and without any influence onto the medium being measured.