A system for determining a cardiovascular parameter in a patient, wherein the system is adapted to work in conjunction with an extracorporeal blood treatment device (ECBTD, 10) connectable to a patient's vascular system via an afferent line and an efferent line, wherein the ECBTD comprises a first circuit with at least one pump arranged between the afferent and the efferent line for pumping the blood of the patient, the system comprising: a) a liquid-filled, second circuit thermally connected to the first circuit of the ECBTD via a heat exchanger, b) temperature changing means for generating a controlled temperature change in the second circuit, c) a temperature sensor TS2up arranged in the second circuit upstream of the heat exchanger, and a temperature sensor TS2down arranged in the second circuit downstream of the heat exchanger, d) a computer system connected to the temperature sensors TS2up and TS2down and to the temperature changing means, which is adapted to induce a temperature change within the first circuit of the ECBTD via the temperature changing means, to record each temperature as a function of time and to evaluate the respective temperature curves T2up(t) and T2down(t), wherein the computer system is further adapted to compute a relation of T2up(t) and T2down(t)1 in order to determine and control the characteristics of the time-dependent changes in the temperature generated by the temperature changing means, wherein T2down(t)1 is derived from temperature data measured by TS2down at a first time, and to determine the cardiovascular parameter of the patient from the relation of T2up(t) and T2down(t)2), wherein T2down(t)2 is derived from temperature data measured by TS2down at a second time. wherein the ECBTD comprises a first circuit with at least one pump arranged between the afferent and the efferent line for pumping the blood of the patient, the system comprising: a) a liquid-filled, second circuit thermally connected to the first circuit of the ECBTD via a heat exchanger, b) temperature changing means for generating a controlled temperature change in the second circuit, c) a temperature sensor TS2up arranged in the second circuit upstream of the heat exchanger, and a temperature sensor TS2down arranged in the second circuit downstream of the heat exchanger, d) a computer system connected to the temperature sensors TS2up and TS2down and to the temperature changing means, which is adapted to induce a temperature change within the first circuit of the ECBTD via the temperature changing means, to record each temperature as a function of time and to evaluate the respective temperature curves T2up(t) and T2down(t), wherein the computer system is further adapted to compute a relation of T2up(t) and T2down(t)1 in order to determine and control the characteristics of the time-dependent changes in the temperature generated by the temperature changing means, wherein T2down(t)1 is derived from temperature data measured by TS2down at a first time, and to determine the cardiovascular parameter of the patient from the relation of T2up(t) and T2down(t)2), wherein T2down(t)2 is derived from temperature data measured by TS2down at a second time. a) a liquid-filled, second circuit thermally connected to the first circuit of the ECBTD via a heat exchanger, b) temperature changing means for generating a controlled temperature change in the second circuit, c) a temperature sensor TS2up arranged in the second circuit upstream of the heat exchanger, and a temperature sensor TS2down arranged in the second circuit downstream of the heat exchanger, d) a computer system connected to the temperature sensors TS2up and TS2down and to the temperature changing means, which is adapted to induce a temperature change within the first circuit of the ECBTD via the temperature changing means, to record each temperature as a function of time and to evaluate the respective temperature curves T2up(t) and T2down(t), wherein the computer system is further adapted to compute a relation of T2up(t) and T2down(t)1 in order to determine and control the characteristics of the time-dependent changes in the temperature generated by the temperature changing means, wherein T2down(t)1 is derived from temperature data measured by TS2down at a first time, and to determine the cardiovascular parameter of the patient from the relation of T2up(t) and T2down(t)2), wherein T2down(t)2 is derived from temperature data measured by TS2down at a second time.