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 (1) which is separated into a first chamber (3) and a second chamber (4) by a semi-permeable membrane (2), or comprising a filter, and a hose line system (I, II) comprising hose lines (5, 7 10, 11) that are pervious to electromagnetic radiation. The invention is based on the fact that the kinetics of a liquid flowing at a measuring site in one of the hose lines (5) of the hose line system (I) is varied. This can be achieved by varying the flow properties of the liquid in the hose line at the measuring site, particularly by stopping a blood pump (6) arranged in the hose line (5) and/or by closing a shut-off member (22) in the hose line. The actual analysis of the measuring data obtained for determining the concentration of the blood constituent is then carried out according to the known methods that use a pressure cuff at the finger of the patient, wherein the invention however analyzes the intensity of the light entering the hose line (5) at the measuring site and the light exiting the hose line at the measuring site.
