Answers · Excretion and the kidneys

Getting rid of waste — answer key

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

1. UQ1 · Practice

Options as numbered on the pupil sheet: 1 — bladder; 2 — urethra; 3 — kidney; 4 — renal artery; 5 — ureter; 6 — renal vein.

Model answer. P is a kidney. Q, from the aorta, is the renal artery, and R, to the vena cava, is the renal vein. S runs from the kidney to the bladder, so it is a ureter. T is the bladder, and U, from the bladder to the outside, is the urethra.

A full answer includes

I matched P to kidney.
I matched Q to renal artery.
I matched R to renal vein.
I matched S to ureter.
I matched T to bladder.
I matched U to urethra.
I did not match S to urethra: S runs from a kidney to the bladder.

Also accept

P kidney; Q renal artery; R renal vein; S ureter; T bladder; U urethra

Watch for

“S is the urethra.” — S runs from a kidney to the bladder: it is a ureter. The urethra (U) leads from the bladder to the outside.
“Q is the renal vein and R is the renal artery.” — The renal artery (Q) comes from the aorta and brings blood to the kidney. The renal vein (R) takes blood to the vena cava.
“U is a ureter.” — U leads out of the bladder to the outside, so it is the urethra.

2. UQ2 · Practice

Model answer. Carbon dioxide made in respiration is carried by the blood to the lungs, and leaves the body in the air breathed out. The kidneys excrete urea, excess water and excess ions.

A full answer includes

I chose the lungs.
I did not choose the kidneys: carbon dioxide is not excreted in urine.

Also accept

the lungs

Watch for

“The kidneys.” — The kidneys excrete urea, excess water and excess ions in urine. Carbon dioxide leaves through the lungs.
“The bladder.” — The bladder only stores urine until it is passed. Carbon dioxide leaves in the air breathed out.
“The liver.” — The liver does not remove carbon dioxide from the body. The blood carries it to the lungs.

3. UQ3 · Practice

Model answer. The kidneys excrete urea, excess water and excess ions in urine. Normal urine has no glucose and no protein. Carbon dioxide is excreted through the lungs.

A full answer includes

I chose urea.
I chose excess water.
I chose excess ions.
I did not choose glucose: normal urine has none.

Also accept

urea; excess water; excess ions

Watch for

“Glucose.” — Normal urine has no glucose: the body keeps it.
“Carbon dioxide.” — Carbon dioxide leaves through the lungs, not in urine.
“Protein.” — Protein stays in the blood: normal urine has none.

4. UQ4 · Practice

Model answer. Burping is not excretion. The swallowed air only went into the stomach and came back out: it was not made by metabolism, and it is not a substance in excess in the body. Carbon dioxide made by muscle cells is a waste product of respiration, urea is a waste product of metabolism, and water drunk beyond what the body needs is a substance in excess, so removing each of these is excretion.

A full answer includes

I chose burping out air that was swallowed with a meal.
I did not choose passing urine after a big drink: removing excess water is excretion.

Also accept

burping out air that was swallowed with a meal

Watch for

“Breathing out carbon dioxide made by muscle cells.” — Carbon dioxide is a waste product of respiration in the muscle cells, so breathing it out is excretion.
“Passing urine after drinking more water than needed.” — The extra water is a substance in excess, and the kidneys excrete it in urine: that is excretion.
“Passing urine that carries urea made by the body.” — Urea is a waste product of metabolism, so passing it in urine is excretion.

5. UQ5 · Practice

Options as numbered on the pupil sheet: 1 — It flows down a ureter.; 2 — It leaves the body through the urethra.; 3 — It is stored in the bladder.; 4 — It arrives in the renal artery.; 5 — It is removed into the urine in a kidney..

Model answer. The red colour reaches a kidney in the blood of the renal artery. In the kidney it is removed into the urine. The urine carries it down a ureter to the bladder, where it is stored, and it leaves the body through the urethra.

A full answer includes

I put the renal artery first.
I put the kidney second.
I put the ureter third.
I put the bladder fourth.
I put the urethra last.
I did not put the renal vein first: the renal vein takes blood away from the kidney.

Also accept

renal artery; kidney; ureter; bladder; urethra

Watch for

“The renal vein brings the colour to the kidney.” — The renal vein takes blood away from the kidney. Blood arrives in the renal artery.
“The urethra comes before the bladder.” — The urethra leads out of the bladder. Urine reaches the bladder down a ureter.

6. UQ6 · Practice

Options as numbered on the pupil sheet: 1 — medulla; 2 — ureter; 3 — cortex.

Model answer. X is the cortex, the outer layer just under the surface. Y is the medulla, the striped cone-shaped sections inside it. Z is the ureter, which carries urine away from the space in the middle.

A full answer includes

I matched X to cortex.
I matched Y to medulla.
I matched Z to ureter.
I did not match X to medulla: the outer layer is the cortex.

Also accept

X cortex; Y medulla; Z ureter

Watch for

“X is the medulla.” — The medulla is the inner, striped region (Y). The outer layer is the cortex.
“Z is the urethra.” — The tube that leaves a kidney is a ureter. The urethra leaves the bladder.

7. UQ7 · Practice

Model answer. Urine leaves a kidney through its ureter, and a ureter ends at the bladder, where urine is stored. So the surgeon joins the new kidney's ureter to the bladder. The urine then leaves the body through the patient's one urethra, as before.

A full answer includes

I chose: the new kidney's ureter is joined to the bladder.
I did not choose the urethra: it leads out of the bladder and does not come from a kidney.

Also accept

the new kidney's ureter is joined to the bladder

Watch for

“The new kidney's ureter is joined to the urethra.” — The urethra leads out of the bladder. Urine sent straight into it would miss the bladder, so it could not be stored.
“The new kidney's urethra is joined to the bladder.” — A kidney has no urethra: the tube that leaves a kidney is a ureter. The one urethra leaves the bladder.
“The new kidney's renal vein is joined to the bladder.” — The renal vein carries blood away from a kidney, not urine. It must be joined to a vein.

8. UQ8 · Practice

Model answer. Volume of carbon dioxide = 6.0 × 3.5 ÷ 100 = 0.21 dm³. In cm³: 0.21 × 1000 = 210 cm³ in one minute. This carbon dioxide was made by respiration in the body's cells, carried to the lungs in the blood, and excreted in the air breathed out.

Checked automatically

Volume of carbon dioxide: 210 cm³ (within ±1 cm³)
- Common wrong answer 0.21 cm³: That is the volume in dm³. Multiply by 1000 to give cm³.
- Common wrong answer 21000 cm³: Divide by 100 for the percentage: 3.5% of 6.0 dm³ is 0.21 dm³.
- Common wrong answer 21 cm³: That is 6.0 × 3.5. Divide by 100 for the percentage, then multiply by 1000 for cm³.

A full answer includes

Method: I found 3.5% of the air: 6.0 × 3.5 ÷ 100 = 0.21 dm³.
Method: I changed dm³ to cm³: 0.21 × 1000.
Answer: I gave 210 cm³.
I did not leave the answer in dm³ (0.21).

Also accept

210 cm³

Watch for

“0.21 cm³” — That is the volume in dm³. Multiply by 1000 to give cm³: 210 cm³.
“21 000 cm³” — The percentage was not divided by 100. 3.5% of 6.0 dm³ is 0.21 dm³, which is 210 cm³.

9. UQ9 · Practice

Options as numbered on the pupil sheet: 1 — The space where urine collects.; 2 — The ureter.; 3 — The cortex.; 4 — The medulla..

Model answer. The needle first passes through the cortex, the outer layer just under the surface. Next it passes through the medulla, the striped cone-shaped sections inside the cortex. Then it reaches the pale space in the middle where urine collects, and from there it enters the ureter.

A full answer includes

I put the cortex first.
I put the medulla second.
I put the space where urine collects third.
I put the ureter last.
I did not put the medulla first: the outer layer is the cortex.

Also accept

cortex; medulla; space where urine collects; ureter

Watch for

“The medulla comes first.” — The medulla is the inner region. The needle meets the cortex, the outer layer, first.
“The ureter comes before the space where urine collects.” — Urine collects in the middle space first, then drains out into the ureter.

10. UQ10 · Practice

Model answer. Excretion is the removal of the waste products of metabolism and of substances in excess. Water drunk beyond what the body needs is a substance in excess, so the kidneys excrete it in urine, even though the cells did not make it.

A full answer includes

I stated that excretion also removes substances in excess.
I stated that water drunk beyond what the body needs is in excess.
I stated that the kidneys excrete this excess water in urine.
I did not agree that only substances made by cells are excreted.

Also accept

excretion also removes substances in excess; extra water is in excess; the kidneys excrete it in urine

Watch for

“Extra water only leaves in sweat, not in urine.” — Some water leaves in sweat, but the kidneys excrete excess water in urine.
“The pupil is right: only waste from cells is excreted.” — Excretion also removes substances in excess, such as extra water and extra ions.

11. UT1 · Apply

Model answer. The mass of ions in the urine decreases. With little salt in the food, the body takes in fewer ions than usual, so fewer of them are in excess. The kidneys excrete only the excess ions, so less leaves in the urine.

A full answer includes

I predicted that the mass of ions in the urine decreases.
I explained that fewer ions are in excess.
I explained that the kidneys excrete only excess ions.
I did not predict that the mass of ions stays the same.

Also accept

decreases; fewer ions in excess; kidneys excrete excess ions

Watch for

“The mass stays the same, because the kidneys always excrete the same amount.” — The kidneys excrete ions that are in excess. With little salt, fewer are in excess, so fewer leave.
“The mass rises, because the body is short of ions.” — A body short of ions keeps them. It does not excrete more.

12. UT2 · Apply

Model answer. The blood flowing into the lungs has more carbon dioxide. It has come from the body's cells, which made carbon dioxide in respiration. In the lungs, carbon dioxide diffuses out of the blood into the air in the alveoli and is excreted in the air breathed out, so the blood leaving the lungs has less.

A full answer includes

I predicted that the blood flowing into the lungs has more carbon dioxide.
I explained that carbon dioxide passes from the blood into the air in the alveoli.
I explained that the carbon dioxide is then breathed out.
I did not say that the blood leaving the lungs has more.

Also accept

blood going in; carbon dioxide leaves into the air and is breathed out

Watch for

“The blood flowing out has more.” — The lungs remove carbon dioxide from the blood, so the blood leaving them has less.

13. UT3 · Apply

Model answer. The left ureter is the only way out of the left kidney, so urine from the left kidney cannot reach the bladder and builds up above the blockage. The right kidney has its own ureter, so its urine still reaches the bladder and leaves through the urethra: the person can still pass urine.

A full answer includes

I explained that urine from the left kidney cannot reach the bladder.
I explained that it builds up above the blockage.
I explained that the right kidney still sends urine down the right ureter to the bladder.
I stated that the person can still pass urine through the urethra.
I did not say that no urine can reach the bladder.

Also accept

left kidney's urine cannot reach the bladder; right kidney and ureter still work, so urine still passes

Watch for

“No urine can reach the bladder.” — The right ureter is not blocked, so urine from the right kidney still reaches the bladder.

14. UT4 · Apply

Model answer. A. The cortex is a layer under the whole surface of the kidney, with the medulla inside it, so any cut through the kidney shows the cortex at the edge and the medulla further in. The ureter leaves only at the inner dent, so only pupil 1, whose cut passes through the dent, sees it.

A full answer includes

I chose row A.
I predicted that both pupils see the cortex and the medulla.
I predicted that only pupil 1 sees the ureter.
I did not choose row D: the ureter leaves only at the inner dent.

Also accept

Watch for

“Row B: cortex seen by pupil 1 only; medulla seen by both; ureter seen by pupil 1 only.” — The cortex lies under the whole surface, so pupil 2's cut also has cortex all round its edge.
“Row C: cortex seen by both; medulla seen by pupil 1 only; ureter seen by both.” — The medulla lies inside the cortex in the top part of the kidney too, and the ureter leaves only at the inner dent.
“Row D: cortex seen by both; medulla seen by both; ureter seen by both.” — Pupil 2's cut is above the inner dent, and the ureter leaves only there, so pupil 2 does not see it.