5.16 How the kidneys make urine
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Which tube carries urine from a kidney to the bladder?
Choose one answer.
Correct Not quite: the answer is Ureter
Mark scheme
B (ureter)
Model answer
B: the ureter.
Describe how the kidneys produce urine.
Mark scheme
- (blood is) filtered (1)
- small molecules / water, glucose, ions and urea pass into the tubules (1)
- selective reabsorption (1)
- of (all) glucose, some ions and (some) water (1)
- urea / excess water / excess ions left form urine (1)
Max 4
Model answer
The kidneys filter the blood: small molecules such as water, glucose, ions and urea are forced out of the blood into the kidney tubules. Then useful substances are selectively reabsorbed back into the blood: all the glucose, some ions and as much water as the body needs. The urea, excess ions and excess water left behind form urine.
The table shows the concentration of some substances in the blood plasma, the filtrate and the urine of a healthy person (g per dm³).
| Substance | Plasma | Filtrate | Urine |
|---|---|---|---|
| Protein | 80 | 0 | 0 |
| Glucose | 1.0 | 1.0 | 0 |
| Urea | 0.3 | 0.3 | 20.0 |
(a) Explain why there is no protein in the filtrate. (b) Explain why there is glucose in the filtrate but not in the urine. (c) Calculate how many times more concentrated urea is in the urine than in the filtrate.
Mark scheme
(a) protein molecules are too large / big to be filtered (1) (b) - glucose is small enough to be filtered (1)
- all the glucose is (selectively) reabsorbed (into the blood) (1)
(c) - 20.0 ÷ 0.3 (1)
- = 67 (times) (allow 66.7) (1)
Model answer
(a) Protein molecules are too large to pass out of the blood during filtration. (b) Glucose molecules are small, so they are filtered, but all of the glucose is then reabsorbed into the blood, so none is left in the urine. (c) 20.0 ÷ 0.3 = 66.7, so about 67 times more concentrated.
Urea is much more concentrated in the urine than in the filtrate.
Explain why.
Mark scheme
- (most of the) water is reabsorbed (1)
- urea is not (re)absorbed / stays in the tubule (1)
Model answer
Most of the water in the filtrate is reabsorbed into the blood, but the urea is not, so the urea becomes much more concentrated.
A urine test shows glucose in a person's urine.
Suggest what this might show, and explain why glucose is present.
Mark scheme
- (the person may have) diabetes (1)
- blood glucose is too high so not all the glucose can be reabsorbed (1)
Model answer
The person may have diabetes. Their blood glucose is so high that the kidneys cannot reabsorb all the glucose that is filtered, so some is left in the urine.
Name the blood vessel that brings blood to the kidney and the one that carries blood away from it.
Mark scheme
- renal artery (brings blood) (1)
- renal vein (takes blood away) (1)
Model answer
The renal artery brings blood to the kidney and the renal vein carries it away.
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