A system for determining blood flow within a blood vessel, the system comprising: - a fluid infusion unit (4, 10, 11) for continuously infusing a fluid into the blood vessel with an infusion rate such that a mixture of the fluid and the blood is generated within the blood vessel, comprising a tube with a tube opening, wherein the tube is adapted to guide the fluid into the blood vessel such that the fluid leaves the tube through the tube opening, - a temperature values determining unit (14, 21) for simultaneously measuring several temperature values at several locations along the length of the blood vessel, which are distal from the tube opening location, such that a temperature profile along the length of the blood vessel is determined and for determining, based on these measured several temperature values, a first temperature value at a first location along the length of the blood vessel and a second temperature value at a second location along the length of the blood vessel such that the first temperature value is indicative of the temperature of the fluid and the second temperature value is indicative of the temperature of the mixture, - a blood flow determining unit for determining the blood flow based on the determined first and second temperature values and the infusion rate - an assessment unit for determining a quality value being indicative of a degree of mixing of the fluid and the blood based on the measured several temperature values, and adapted to determine a quality value indicating a sufficient degree of mixing, if a spatial variation of the several temperature values is smaller than a predefined spatial threshold. - a fluid infusion unit (4, 10, 11) for continuously infusing a fluid into the blood vessel with an infusion rate such that a mixture of the fluid and the blood is generated within the blood vessel, comprising a tube with a tube opening, wherein the tube is adapted to guide the fluid into the blood vessel such that the fluid leaves the tube through the tube opening, - a temperature values determining unit (14, 21) for simultaneously measuring several temperature values at several locations along the length of the blood vessel, which are distal from the tube opening location, such that a temperature profile along the length of the blood vessel is determined and for determining, based on these measured several temperature values, a first temperature value at a first location along the length of the blood vessel and a second temperature value at a second location along the length of the blood vessel such that the first temperature value is indicative of the temperature of the fluid and the second temperature value is indicative of the temperature of the mixture, - a blood flow determining unit for determining the blood flow based on the determined first and second temperature values and the infusion rate - an assessment unit for determining a quality value being indicative of a degree of mixing of the fluid and the blood based on the measured several temperature values, and adapted to determine a quality value indicating a sufficient degree of mixing, if a spatial variation of the several temperature values is smaller than a predefined spatial threshold.