fig1 to 3 show the connection of the end portion of first duct means 1 which may also be referred to as either an infusion line or as infusion duct means for conveying a flow of a medical liquid to a tubular portion 2 of second duct means which may also be referred to as receiving duct means constituting a portion of a circuit for circulating blood outside the body the terminal portion of the first duct means 1 is constituted by a flexible tube in accordance with the invention a first type of medical liquid flow detector is constituted by a blood detector 3 disposed on the tube 1 in the vicinity of the tube 2 substantially to intercept the axis 4 of the duct 1 the distance between the blood detector 3 and the tube 2 may be adjusted as a function of the response time desired for the flow detector as explained in greater detail below when describing the operation of the flow detector if a minimum response time is desired then the blood detector 3 is placed as close as possible to the tube 2 given the physical size of the selected blood detector the blood detector 3 may be constituted by any conventional detector capable of detecting the presence of a liquid in a duct for example it may be an optical detector delivering colorimetry or turbidimetry data in which case the tube 1 should be made of transparent material or else it may be an inductive detector or a capacitive detector because optical detectors are reliable and inexpensive it is particularly advantageous to use an optical detector in the detector of the invention the operation of this detector is based on taking advantage of two characteristics of blood first its facility for.