A device for blood treatment by means of extracorporeal circulation comprising: - a filter having a primary compartment and a secondary compartment separated by a semipermeable membrane, - a blood circuit comprising an arterial line designed to withdraw from the patient, the primary compartment of the filter and a venous line designed to return to the patient, - a dialysate circuit comprising the secondary compartment of the filter and a draining line configured to let the whole of the used liquid getting out of the filter to go towards a drain, - a first bag in fluid communication with the draining line, - a second bag in fluid communication with the draining line, - at least a first gravimetric weighing means associated with the first bag, - fluid flow rate adjusting means (31, 32, 33, 34) acting upon the draining line, wherein the adjusting means comprise a first adjusting member acting upstream from the two bags (11, 12), a second adjusting member acting between the two bags (11, 12), and a third adjusting member acting downstream from the two bags (11, 12), - a control unit connected to the first gravimetric weighing means and to the fluid flow rate adjustment means (31, 32, 33, 34), - wherein the control unit is capable to: i. receive the weight signals from the first gravimetric weighing means, and ii. piloting the fluid flow rate adjustment means (31, 32, 33, 34) so as to fill one of the bags (11, 12) with liquid while the other bag (12, 11) is emptied of the liquid, and vice versa, and iii. piloting the first adjusting member so as to ensure the presence of a substantially continuous flow rate during treatment. - a filter having a primary compartment and a secondary compartment separated by a semipermeable membrane, - a blood circuit comprising an arterial line designed to withdraw from the patient, the primary compartment of the filter and a venous line designed to return to the patient, - a dialysate circuit comprising the secondary compartment of the filter and a draining line configured to let the whole of the used liquid getting out of the filter to go towards a drain, - a first bag in fluid communication with the draining line, - a second bag in fluid communication with the draining line, - at least a first gravimetric weighing means associated with the first bag, - fluid flow rate adjusting means (31, 32, 33, 34) acting upon the draining line, wherein the adjusting means comprise a first adjusting member acting upstream from the two bags (11, 12), a second adjusting member acting between the two bags (11, 12), and a third adjusting member acting downstream from the two bags (11, 12), - a control unit connected to the first gravimetric weighing means and to the fluid flow rate adjustment means (31, 32, 33, 34), - wherein the control unit is capable to: i. receive the weight signals from the first gravimetric weighing means, and ii. piloting the fluid flow rate adjustment means (31, 32, 33, 34) so as to fill one of the bags (11, 12) with liquid while the other bag (12, 11) is emptied of the liquid, and vice versa, and iii. piloting the first adjusting member so as to ensure the presence of a substantially continuous flow rate during treatment. i. receive the weight signals from the first gravimetric weighing means, and ii. piloting the fluid flow rate adjustment means (31, 32, 33, 34) so as to fill one of the bags (11, 12) with liquid while the other bag (12, 11) is emptied of the liquid, and vice versa, and iii. piloting the first adjusting member so as to ensure the presence of a substantially continuous flow rate during treatment.