A countercurrent haemodialyser has a membrane area of 1.2 m, and the overall mass transfer coefficient is estimated to be KL = 1.05 x 10-6 m/s. (a) Calculate the extraction ratio if the blood flow rate is 300 ml/min, and the dialysate flow rate is ten times as high. (b) Calculate the time of reduction of blood urea from 220 mg to 20 mg if the blood volume in the patient's body is 5.5 litre.