Answers · Excretion and the kidneys

Excretion and the kidneys — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. UK1 · Final check

Model answer. A. The only tube that reaches the outside is W, the urethra, so the nurse's tube passes along W. Urine is stored in V, the bladder. A person has two ureters, one from each kidney; N is one of them. M, a kidney, makes urine but does not store it.

A full answer includes

I chose row A.
I identified W as the tube the nurse's tube passes along: the urethra.
I identified V as the organ that stores urine: the bladder.
I identified N as the tube a person has two of: a ureter.
I did not choose row B: N runs from a kidney to the bladder and does not reach the outside.

Also accept

Watch for

“Row B: N; V; W.” — N runs from a kidney down to the bladder and does not reach the outside: it is a ureter. W, the urethra, leads out, and there is only one urethra.
“Row C: W; M; N.” — M is a kidney. It makes urine but does not store it: the urine flows down N into V, the bladder.
“Row D: N; M; W.” — N does not reach the outside, M does not store urine, and a person has only one tube like W.

2. UK2 · Final check

Model answer. A. The extra salt gives the body more ions than it needs, so they are in excess, and the kidneys excrete them: the urine carries more ions. Cycling makes the muscles respire faster, so they make more carbon dioxide, which the lungs breathe out.

A full answer includes

I chose row A.
I did not choose row C: the extra salt puts ions in excess, and the kidneys excrete them.

Also accept

Watch for

“Row B: more ions; the same carbon dioxide; kidneys.” — Cycling makes the muscles respire faster, so they make more carbon dioxide, which the lungs excrete.
“Row C: the same ions; more carbon dioxide; kidneys.” — The extra salt puts ions in excess. The kidneys excrete excess ions, so the urine carries more.
“Row D: more ions; more carbon dioxide; lungs.” — Ions are excreted in urine by the kidneys. The lungs excrete carbon dioxide.

3. UK3 · Final check

Model answer. A. In the picture the row of dots lies in the outer layer, near the curved edge, so the dots are in the cortex. The fan shapes lie further in, between the dots and the cream space in the middle, so they are the medulla. The cream tube leaving the bottom of the kidney is the ureter, which would carry urine to the bladder.

A full answer includes

I chose row A.
I identified the region with the dark dots as the cortex.
I identified the region with the fan shapes as the medulla.
I identified the cream tube at the bottom as the ureter.
I did not choose row B: in the picture the dots lie in the outer layer, outside the fans.

Also accept

Watch for

“Row B: medulla; cortex; ureter.” — Look at the picture again: the dots are in the outer layer, near the curved edge, so they are in the cortex. The fans are further in: the medulla.
“Row C: cortex; medulla; urethra.” — The tube that leaves a kidney is a ureter. The urethra leaves the bladder.
“Row D: medulla; space where urine collects; ureter.” — The fans are solid kidney tissue, not the open cream space in the middle, and the dots are in the outer layer: the cortex.

4. NK1 · Final check

Model answer. A. Water and small molecules such as glucose, urea and ions pass through the filter into the capsule, so less plasma and less glucose flow out of the glomerulus than flow in. Protein molecules are too large to pass the filter, so all the protein that flows in also flows out.

A full answer includes

I chose row A.
I worked out that less plasma flows out, because water passes into the capsule.
I worked out that the mass of protein is the same, because protein is too large to pass the filter.
I worked out that less glucose flows out, because glucose passes into the capsule.
I did not choose row D: glucose is filtered even though the body needs it.

Also accept

Watch for

“Row B: less plasma; less protein; less glucose.” — Protein molecules are too large to pass the filter, so all the protein that flows in also flows out.
“Row C: the same plasma; the same protein; less glucose.” — Water passes the filter too, and most of the filtrate is water, so less plasma flows out than flows in.
“Row D: less plasma; the same protein; the same glucose.” — The filter lets every small molecule through, useful or not. Some glucose passes into the capsule, so less flows out in the blood.

5. NK2 · Final check

Model answer. A. No glucose is left, so all of it was reabsorbed. Only 3 of the 300 water particles are left, so most of the water (99%) was reabsorbed. 30 of the 60 ion particles are left, so some of the ions were reabsorbed and the rest leave in the urine.

A full answer includes

I chose row A.
I checked that no glucose is left in A's urine: all of it was reabsorbed.
I checked that only 3 of the 300 water particles are left: most of the water was reabsorbed.
I checked that 30 of the 60 ion particles are left: some of the ions were reabsorbed.
I did not choose row B: in B every ion was taken back, not some.

Also accept

Watch for

“Row B: 3 water; 0 glucose; 0 ions.” — No ions are left, so every ion was taken back. A healthy nephron reabsorbs only some of the ions; the excess ones leave in the urine.
“Row C: 3 water; 12 glucose; 30 ions.” — 12 glucose particles are left in the urine. A healthy nephron reabsorbs all of the glucose.
“Row D: 150 water; 0 glucose; 30 ions.” — Half of the water is left. A healthy nephron reabsorbs most of the water, so very little is left.

6. NK3 · Final check

Model answer. Most of the water is reabsorbed along the tubule, so anything that stays behind becomes more concentrated. Some of the ions are reabsorbed too, as many as the body needs, so their mass in the urine is much less than was filtered and their concentration rises only a little: 13 ÷ 7.2 ≈ 1.8 times. Far less of the urea is taken back, so most of it ends up in the much smaller volume of water: 17 ÷ 0.28 ≈ 61 times. The kidney does not add urea; taking water away concentrates it.

A full answer includes

I explained that most of the water is reabsorbed, which makes everything left more concentrated.
I explained that some of the ions are reabsorbed with the water.
I explained that far less of the urea is reabsorbed: much of it stays in the tubule.
I explained that the urea therefore ends up in much less water, but the ions do not.
I did not say that the kidney adds urea to the tubule.

Also accept

most water reabsorbed; some ions reabsorbed too; much of the urea stays; so urea is concentrated far more

Watch for

“The kidney adds urea to the tubule.” — All the urea in the urine was filtered in the glomerulus. Taking the water away concentrates it.
“The ions are not filtered as easily as urea.” — Ions and urea pass the filter equally: they have the same concentration in the filtrate as in the plasma. The difference comes later, along the tubule.

7. LK1 · Final check

Model answer. A growing child assimilates more of the amino acids: they are converted to new proteins, which build new cells and tissues. So fewer amino acids are in excess. Only excess amino acids are deaminated in the liver, so fewer are deaminated and less urea is made and excreted.

A full answer includes

I explained that the child assimilates more amino acids: they are built into new proteins.
I explained that the child uses these proteins to grow.
I explained that fewer of the child's amino acids are in excess.
I explained that fewer amino acids are deaminated.
I explained that less urea is made.
I did not say that the child's kidneys excrete less of the urea that is made.

Also accept

more amino acids assimilated for growth; fewer in excess; fewer deaminated; less urea

Watch for

“The child's kidneys remove less urea, so it stays in the blood.” — Urea is not kept in the blood. Less is excreted because less is made.
“The child digests less of the protein.” — Both digest the protein to amino acids. The difference is what happens to the amino acids.

8. LK2 · Final check

Model answer. A. The excess amino acids can no longer be deaminated, and the body cannot store them, so they build up in his blood. Urea is made in the liver only by deamination, so much less urea is made: there is less in his blood, and his healthy kidneys excrete less of it in his urine.

A full answer includes

I chose row A.
I worked out that amino acids build up in his blood, because the excess ones are not deaminated.
I worked out that less urea is in his blood and urine, because urea is made only by deamination.
I did not choose row C: the kidneys remove urea, they do not make it.

Also accept

Watch for

“Row B: amino acids higher; urea in the blood higher; urea in the urine less.” — The liver makes urea; it does not remove it from the blood. With less urea made, less is in his blood.
“Row C: amino acids higher; urea in the blood the same; urea in the urine the same.” — The kidneys only remove urea; they do not make it. It is made in the liver, so the damage means less urea in his blood and urine.
“Row D: amino acids the same; urea in the blood lower; urea in the urine less.” — The body cannot store excess amino acids. If they are not deaminated, they stay in the blood, so their concentration rises.

9. LK3 · Final check

Model answer. (a) The concentration rises steadily, by about 0.22 g per dm³ each day, from 0.26 to 1.13 g per dm³: more than four times as high by day 5. (b) His liver keeps deaminating excess amino acids and making urea each day. With both ureters blocked, urine cannot drain out of his kidneys, so the urea is no longer excreted and builds up in his blood. (c) Urea is toxic, so as its concentration rises it harms his cells; without treatment he becomes very ill.

A full answer includes

(a) I described a steady rise.
(a) I used the data: about 0.22 g per dm³ more each day.
(b) I explained that his liver keeps making urea from excess amino acids.
(b) I explained that urine cannot leave his kidneys down the blocked ureters.
(b) I explained that the urea is no longer excreted, so it builds up in the blood.
(c) I explained that urea is toxic.
(c) I explained that the rising urea harms his cells.
I did not say that the urea rises because his liver makes more of it.

Also accept

(a) steady rise, about 0.22 g per dm³ a day; (b) still made, but not excreted because urine cannot leave the kidneys; (c) toxic, harms cells

Watch for

“(b) His liver makes more urea when the ureters are blocked.” — The liver makes urea as before. It builds up because it is no longer excreted.
“(b) The urea stays in the bladder.” — The stones are in the ureters, above the bladder, so urine cannot even reach the bladder. The urea builds up in the blood.
“(c) It is dangerous because no urine is passed.” — The danger is the toxic urea building up in the blood.