The invention relates to a device and a method for measuring a blood constituent in blood for an extracorporeal blood treatment device, comprising a dialyzer or a filter, and a hose line system comprising hose lines that are pervious to electromagnetic radiation. The kinetics of a liquid flowing at a measuring site in one of the hose lines of the hose line system is varied by varying the flow properties of the liquid in the hose line at the measuring site, particularly by stopping a blood pump arranged in the hose line and/or by closing a shut-off member in the hose line. The invention analyzes the intensity of the light entering the hose line at the measuring site and the light exiting the hose line at the measuring site.