An apparatus for monitoring whether or not a distal end of a tube is positioned inside a blood vessel includes a heating element configured to heat the tube distal end, and a sensor arrangement configured to generate a measurement signal indicative for heat transferred by exterior of the distal end. The apparatus further includes a comparator configured to compare the measurement signal with a reference level. The reference level equals a value attained by the measurement signal in response to a minimum flow velocity in the blood vessel. Further, a system is configured to exchange a liquid with a mammal via a blood vessel. The system includes the apparatus having the tube.
