Answers · Excretion and the kidneys

Excretion and the kidneys — return questions key

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

1. UR1 · Return

Options as numbered on the pupil sheet: 1 — excreted by the kidneys in urine; 2 — excreted through the lungs; 3 — egested in faeces, not excreted; 4 — kept by the body: none in urine.

Model answer. Carbon dioxide, a waste product of respiration, is excreted through the lungs. Urea, a waste product of metabolism, is excreted by the kidneys in urine. Glucose is needed by the cells, so the body keeps it: normal urine has none. Undigested fibre never left the gut and never entered the cells, so it is egested in faeces: that is not excretion.

A full answer includes

I matched carbon dioxide to the lungs.
I matched urea to the kidneys.
I matched glucose to kept by the body.
I matched undigested fibre to egested in faeces.
I did not match undigested fibre to excretion: it never entered the body's cells.

Also accept

carbon dioxide – lungs; urea – kidneys in urine; glucose – kept; undigested fibre – egested in faeces

Watch for

“Undigested fibre is excreted.” — Fibre passes along the gut without entering the cells, so getting rid of it is egestion.
“Glucose is excreted in urine.” — The body keeps glucose: normal urine has none.
“Urea is breathed out.” — Urea is dissolved in the blood and removed by the kidneys. The lungs excrete carbon dioxide.

2. UR2 · Return

Model answer. A. K leads out of the bladder (L) to the outside: it is the urethra. Urine from the kidney F still flows down H, a ureter, into L, above the stone. It cannot get past the stone, so it collects in L and cannot leave the body.

A full answer includes

I chose row A.
I worked out that the stone is below L, so urine from F still reaches L.
I worked out that urine collects in L because it cannot get past the stone.
I did not choose row B: H is not blocked, so urine does not build up in H.

Also accept

Watch for

“Row B: urine cannot reach L; it builds up in H; it cannot leave the body.” — K is below the bladder, not between F and L. H is clear, so urine still flows down it into L.
“Row C: urine reaches L; it builds up in L; it can still leave the body.” — The stone blocks K, the only tube out of L, so urine cannot leave the body.
“Row D: urine cannot reach L; it builds up in F; it cannot leave the body.” — The stone is below L, so the kidney's urine still flows down H into L.

3. UR3 · Return

Model answer. It rises. Her cells keep respiring, so they keep making carbon dioxide, which passes into the blood. Carbon dioxide is excreted through the lungs in the air breathed out. She is not breathing out, so it builds up in her blood until she breathes again.

A full answer includes

I predicted that the concentration of carbon dioxide in her blood rises.
I explained that her cells keep making carbon dioxide in respiration.
I explained that it is not excreted while she does not breathe out.
I did not say that it falls because she is not breathing in.

Also accept

rises; cells still make carbon dioxide; it is not breathed out

Watch for

“It falls, because she is not breathing in any air.” — Breathing in brings oxygen, not carbon dioxide. Her cells still make carbon dioxide, and it is not breathed out.
“It stays the same, because she is not moving much.” — Her cells respire all the time, even at rest, so carbon dioxide is still made.

4. UR4 · Return

Model answer. The narrow end of every fan points to the cream, branching space in the middle, so the tubes carry urine inwards into that space. The fans are the medulla. The cream tube that leaves the space is the ureter, which takes the urine to the bladder.

A full answer includes

I chose: into the cream space in the middle, which leads into the ureter.
I did not choose the reddish-brown outer band: the fans point away from it.

Also accept

into the cream space in the middle, which leads into the ureter

Watch for

“Out into the reddish-brown band, which leads into the ureter.” — The fans point away from the outer band. That band is the cortex; the ureter starts at the cream space in the middle.
“Out to the curved outer surface, where the urine leaves.” — Urine does not leave through the surface of the kidney. It leaves only through the ureter, at the inner dent.
“Into the blood of the renal vein, which carries it away.” — Urine stays out of the blood once it is made. It drains into the space in the middle, then down the ureter.

5. NR1 · Return

Model answer. (a) The blood is filtered in the glomerulus, into the capsule. (b) Any two of water, glucose and ions: they are small enough to pass the filter, and the body needs them, so they are reabsorbed later. Urea also passes, but it is waste.

A full answer includes

(a) I stated the glomerulus.
(b) I stated one useful substance: water, glucose or ions.
(b) I stated a second, different one.
I did not give urea in (b): it is filtered, but it is waste.

Also accept

(a) glomerulus; (b) two of water, glucose, ions

Watch for

“(b) Protein.” — Protein is not filtered: its molecules are too large.
“(a) The tubule.” — The tubule reabsorbs substances. The filtering happens in the glomerulus.

6. NR2 · Return

Model answer. A. The drink gives her body more water than it needs: the water is in excess. Her kidneys filter about the same volume, but the tubules take back a smaller percentage of the water, so more is left to form urine and the volume of urine rises. The drink has no glucose, so the glucose filtered is as usual, and all of it is still reabsorbed.

A full answer includes

I chose row A.
I predicted that a smaller percentage of the filtered water is reabsorbed.
I predicted that all of the filtered glucose is still reabsorbed.
I predicted that the volume of urine rises.
I did not choose row B: taking back more of the water would keep the excess water in the body.

Also accept

Watch for

“Row B: a larger percentage of the water; all of the glucose; urine falls.” — Taking back more of the water would keep the excess water in her body. The kidneys excrete excess water, so a smaller percentage is taken back and the urine rises.
“Row C: a smaller percentage of the water; less of the glucose; urine rises.” — Water does not change the glucose filtered. The tubules still take back all of it, so there is still none in her urine.
“Row D: the same percentage of the water; all of the glucose; urine the same.” — Then the excess water would stay in her body. To excrete it, the tubules take back a smaller percentage, so more urine is made.

7. NR3 · Return

Model answer. The water is reabsorbed but the urea stays, so all the urea in 120 cm³ of filtrate ends up in 1.6 cm³ of urine. The volume is 120 ÷ 1.6 = 75 times smaller, so the urea is 75 times more concentrated: 0.24 × 75 = 18 g per dm³.

Checked automatically

Concentration of urea in the urine: 18 g per dm³ (within ±0.1 g per dm³)
- Common wrong answer 0.0032 g per dm³: That divides. The urea ends up in less water, so its concentration rises.
- Common wrong answer 0.24 g per dm³: That is the filtrate's concentration. It rises as the water is reabsorbed.

A full answer includes

Method: I found how many times smaller the volume becomes: 120 ÷ 1.6 = 75.
Method: I multiplied the filtrate concentration by this: 0.24 × 75.
Answer: I gave 18 g per dm³.
I did not divide 0.24 by 75.

Also accept

18 g per dm³

Watch for

“0.0032 g per dm³.” — That divides by 75. The urea ends up in less water, so it becomes more concentrated.
“0.24 g per dm³.” — That is the filtrate's concentration. Water is reabsorbed and the urea stays, so it becomes more concentrated.

8. LR1 · Return

Model answer. Less urea. Her food brings the usual amino acids to the liver. This week her liver assimilates more of them: it joins them together into new plasma proteins. So fewer amino acids are left in excess. Only excess amino acids are deaminated, so fewer are deaminated, less urea is made and less is excreted. (The change is small, but its direction is clear.)

A full answer includes

I predicted that she excretes less urea.
I explained that more amino acids are assimilated: the liver joins them into plasma proteins.
I explained that fewer amino acids are left in excess.
I explained that fewer amino acids are deaminated, so less urea is made.
I did not predict more urea: joining amino acids into proteins does not make urea.

Also accept

less urea; more amino acids assimilated into plasma proteins; fewer in excess and deaminated

Watch for

“More urea, because making plasma proteins makes urea.” — Joining amino acids into proteins makes no urea. Urea is made only when excess amino acids are deaminated, and fewer are in excess this week.
“The same urea, because she eats the same food.” — The same amino acids arrive, but more are built into plasma proteins, so fewer are left to be deaminated.

9. LR2 · Return

Options as numbered on the pupil sheet: 1 — It enters a kidney in the renal artery.; 2 — It leaves the heart in the aorta.; 3 — It leaves the liver in the hepatic vein.; 4 — It is made in a liver cell from an excess amino acid.; 5 — It passes through the heart and the lungs..

Model answer. Urea is made in a liver cell from excess amino acids. It passes into the blood, which leaves the liver in the hepatic vein. That blood flows through the heart and the lungs, and the heart pumps it out in the aorta, and the renal artery, a branch of the aorta, carries it into a kidney.

A full answer includes

I put the urea being made in a liver cell first.
I put the urea leaving the liver in the hepatic vein second.
I put the urea passing through the heart and the lungs third.
I put the urea leaving the heart in the aorta fourth.
I put the urea entering a kidney in the renal artery last.
I did not start in the kidney: the kidney removes urea, it does not make it.

Also accept

made in a liver cell; hepatic vein; heart and lungs; aorta; renal artery into a kidney

Watch for

“It starts in the kidney.” — The kidneys remove urea from the blood. It is made in the liver.
“The renal artery comes before the heart.” — The renal artery branches off the aorta, which leaves the heart, so the heart comes first.

10. LR3 · Return

Model answer. A. The baby's liver makes urea from excess amino acids. The urea passes across the placenta into the mother's blood, and her kidneys remove it and excrete it in her urine. It must be excreted because urea is toxic: if it built up it would harm the cells of the mother and the baby.

A full answer includes

I chose row A.
I identified the baby's liver as the organ that made the urea.
I identified the mother's kidneys as the organs that excrete it.
I gave the reason that urea is toxic.
I did not choose row D: urea is a waste that harms cells, not something cells need.

Also accept

Watch for

“Row B: the baby's kidneys; the mother's kidneys; it is toxic.” — Kidneys remove urea; they do not make it. The baby's liver makes it.
“Row C: the baby's liver; the mother's lungs; it is toxic.” — The lungs excrete carbon dioxide. Urea is removed from the blood by the kidneys.
“Row D: the baby's liver; the mother's kidneys; her cells need it.” — Cells do not use urea. It is excreted because it is toxic.