Question

The patient was under acute renal failure for which the patient underwent renal dialysis to externally filter the blood via osmosis. Dialysis involves the passage of arterial blood through a tube with a semi-permeable outer membrane. This membrane only allows ions/metabolites/electrolytes and water to pass through. Surrounding the tube is another larger tubing containing dialysate (assume pure water). Answer the following questions regarding osmosis and the function that dialysis serves in this case.

a.In which direction is the driving force for osmosis during dialysis and why?

b.How would the initial driving force for osmosis change (increase/decrease) if dialysis was performed less frequently? Why?

c.In this patient, blood urea nitrogen (BUN) was measured at 23 mg/dl upon admission. Calculate the diffusion rate (J) of BUN across the dialysis membrane, assuming the following:

Purea(membrane) = 1.5 x 10-6 cm/sec

[BUN]dialysate= 0 at t=0

Membrane thickness = 40 μm

Amembrane = 1.5 m2

d.Describe how BUN will change in the blood following dialysis. Justify your answer.

Answer 1

A)The driving force for the osmosis during dialysis is from the tube with semi-permeable membrane(containing patient's blood) to the larger tube containing pure water because the concentration of BUN;electrolytes etc is higher in blood than water.So osmosis will occure from the area of higher concentration to the area of lower concentration.

B)If dialysis was performed less frequently; the concentration of BUN in blood will rise to higher levels.So the initial driving force for osmosis will increase.

C)Rate of diffusion = k* A* (C1-C2) / t

k is the diffusion coefficient ( 1.5*10^-6 cm /sec)=1.5 *10^-8 m/sec

A= surface area of the membrane= 1.5 m^2

C1-C2= 23-0 =23 mg/dl

t=thickness of the membrane= 40 micrometer =40* 10^-6

so rate of diffusion= 1.5*10^-8*1.5*23 /40*10^-6

=1.29 *10^-2

D)During dialysis ; urea will move frok blood to the larger tube.So BUN will decrease in the blood during dialysis.