Fig. 1 shows a simplified dialysis machine used by a patient with kidney failure. Table 1 gives solute concentrations before the blood enters and after it l...
Question 1 Report
Fig. 1 shows a simplified dialysis machine used by a patient with kidney failure. Table 1 gives solute concentrations before the blood enters and after it leaves the dialyser. The dialysis solution is continuously replaced.
| substance | blood entering / mmol dm-3 | blood leaving / mmol dm-3 | dialysis solution / mmol dm-3 |
|---|---|---|---|
| urea | 18.0 | 7.0 | 0.0 |
| glucose | 5.0 | 5.0 | 5.0 |
| sodium ions | 145 | 140 | 140 |
(a) Name the process by which urea moves from the blood into the dialysis solution. [1]
(b) Calculate the decrease in urea concentration in the blood. Show your working. [2]
(c) Explain why glucose is included in the dialysis solution at 5.0 mmol dm-3. [2]
(d) Describe why fresh dialysis solution must continually enter the machine. [2]
(e) State one component of blood that cannot pass through the membrane. [1]
(f) Explain why dialysis does not fully replace all functions of a kidney. [2]
Answer Details
(a) Urea moves into dialysis solution by diffusion. [1] It moves from a higher concentration in blood to a lower concentration in the solution.
(b)
\[18.0-7.0=11.0\text{ mmol dm}^{-3}\]
The decrease is 11.0 mmol dm-3. [2]
(c) The dialysis solution contains glucose at 5.0 mmol dm-3, the same concentration as normal blood glucose. Therefore there is no net diffusion of glucose out of the blood, so useful glucose is not lost. [2]
(d) Fresh dialysis solution keeps the urea concentration outside the membrane low. This maintains a steep concentration gradient, allowing urea to continue diffusing out of the blood. [2]
(e) A red blood cell cannot pass through the membrane. White blood cells, platelets, and plasma proteins are also acceptable. [1]
(f) Dialysis cannot selectively reabsorb and regulate substances as precisely as a kidney. It also does not make hormones or carry out all kidney functions continuously. [2]
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