Substance Blood entering / mg per 100 cm3 Dialysis fluid / mg per 100 cm3 Blood leaving / mg per 100 cm3 urea 180 0 30 glucose 90 90 90 salts 360 350 350 A ...
Question 1 Report
| Substance | Blood entering / mg per 100 cm3 | Dialysis fluid / mg per 100 cm3 | Blood leaving / mg per 100 cm3 |
|---|---|---|---|
| urea | 180 | 0 | 30 |
| glucose | 90 | 90 | 90 |
| salts | 360 | 350 | 350 |
A person whose kidneys have failed can be kept alive using a dialysis machine, which removes waste substances from the blood. During dialysis the patient's blood flows on one side of a partially permeable membrane while a special dialysis fluid flows on the other side. Table 1.1 compares the concentrations of some substances in the blood entering the machine, in the dialysis fluid, and in the blood leaving the machine.
(a) Name the main waste substance that must be removed from the blood during dialysis. [1](b) State the name of the type of membrane used in the dialysis machine. [1]
(c) Using Table 1.1, explain how urea moves from the blood into the dialysis fluid. [2]
(d) Explain why the dialysis fluid contains glucose at the same concentration as normal blood. [2]
(e) Describe how the concentration of the blood leaving the machine differs from the blood entering it. [3]
(f) Suggest why the dialysis fluid is replaced continuously with fresh fluid during treatment. [2]
(g) State two ways in which a kidney transplant is an advantage over regular dialysis for a patient. [2]
Answer Details
This question tests how a dialysis machine removes waste by diffusion across a partially permeable membrane, using the concentration data in Table 1.1.
(a) The main waste substance removed during dialysis is urea. [1]
(b) The membrane used is a partially permeable membrane. [1]
(c) How urea moves into the dialysis fluid [2]: urea is at a higher concentration in the blood (180 mg per 100 cm\(^3\)) than in the dialysis fluid (0 mg per 100 cm\(^3\)) [1], so it diffuses from the blood into the fluid down the concentration gradient. [1]
(d) Why the fluid contains glucose at the same concentration as blood [2]: this means there is no concentration gradient for glucose between the blood and the fluid [1], so the useful glucose is not lost from the blood by diffusion into the fluid. [1]
(e) How the blood leaving differs from the blood entering [3]: the blood leaving contains much less urea (30 compared with 180 mg per 100 cm\(^3\)) [1]; its salt concentration has been corrected towards normal (from 360 to 350 mg per 100 cm\(^3\)) [1]; and its glucose concentration stays the same at 90 mg per 100 cm\(^3\), so no glucose is lost. [1]
(f) Why the fluid is continuously replaced [2]: fresh fluid keeps the urea concentration in the fluid low [1], so a steep concentration gradient is maintained and urea keeps diffusing out of the blood. [1]
(g) Two advantages of a transplant over regular dialysis (1 mark each) [2]: there is no need for frequent, time-consuming hospital dialysis sessions; and the person can live a more normal life because the transplanted kidney works continuously.
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