Biomedical Engineering: Bridging Medicine and Technology 1st Edition by Veronique Tran,Mark Saltzman

Exercise 6

The following setup is a simplified version of hemodialysis, where Q = Flow rate of fluids (mL/min); B = Concentration of urea in blood; D = Concentration of urea in dialysate; and the subscripts i = in and o = out.
a. Identify the system of the dialyzer and draw boundaries around it.
b. Write a mass balance equation for urea in this system.
c. Knowing that this is an actual system with a patient connected to the dialysis machine, which two variables would you have control over?
d. Because urea is a waste product toxic to the body if accumulated, what would you set Di, the concentration of urea in the dialysate, to be?
e. At a particular moment, suppose the patient's blood flow rate out to the
dialysis machine is 300 mL/min, with a urea concentration of 300 mg/L.
You set the dialysate flow rate to be 1400 mL/min, and you measured
the urea concentration of the exiting dialysate to be 54 mg/L. What
concentration of urea does the blood going back to the patient contain?
f. We have analyzed the system of the dialyzer, but we are more interested
in knowing the concentration of urea within the patient over time. Set up
a mass balance equation for the system of the patient