Chapter 4, Problem 4.33P

An artificial kidney is a device that removes water and waste metabolites from blood. In one such device, the hollow fiber hemodialyzer, blood flow's from an artery through the insides of a bundle

of hollow' cellulose acetate fibers, and dialyzing fluid, which consists of water and various dissolved salts, flows on the outside of the fibers. Water and waste metabolites—principally urea, creatinine, uric acid, and phosphate ions—pass through the fiber walls into the dialyzing fluid, and the purified blood is returned to a vein.

Dialysate

At some time during a dialysis the arterial and venous blood conditions are as follows:

Arterial (entering) Blood Venous (exiting) Blood

Flow Rate	200.0 mL/min	195.0 mL/min
Urea (H2NCONH2) Concentration	1.90 mg/mL	1.75 mg/mL

1. Calculate the rates at w hich urea and water are being removed from the blood.

If the dialyzing fluid enters at a rate of 1500 mL/min and the exiting solution (dialysate) leaves at approximately the same rate, calculate the concentration of urea in the dialysate.

Suppose w^re want to reduce the patient's urea level from an initial value of 2.7 mg/mL to a final value of 1.1 mg/mL. If the total blood volume is 5.0 liters and the average rate of urea removal is that calculated in Part (a), how long must the patient be dialyzed? (Neglect the loss in total blood volume due to the removal of water in the dialyzer.)

Exploratory Exercises—Research and Discover

The device described in this problem is external to the body and requires that the patient travel to a dialysis center several times a week. What types of challenges would be encountered if an artificial kidney were implanted in the patient’s body?

Briefly describe current state-of-the-art designs for artificial kidneys.