Year 8 Science · Unit 1 · Lesson 12
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Learning Goals
Match each organ to its job
Write the matching letter in the middle column. Each letter is used once.
| Organ | Letter | Job |
|---|---|---|
| Kidneys | A. Remove carbon dioxide from the blood when you breathe out. | |
| Ureters | B. Carries urine out of the body. | |
| Bladder | C. Filter the blood and make urine. | |
| Urethra | D. Releases sweat, which carries water, salt and a little urea. | |
| Lungs | E. Stores urine until it is released. | |
| Skin | F. Carry urine from the kidneys to the bladder. |
1. State what “balance” means for a living system, and give one condition inside the body that must be kept in balance.
Is it excretion?
Excretion means removing wastes that were made by the living thing’s own cells. Write Yes or No for each one, and give a reason.
| Substance leaving a living thing | Excretion? (Yes or No) | Reason |
|---|---|---|
| Faeces | ||
| Urea in urine | ||
| Carbon dioxide given off by a leaf at night | ||
| Fibre from a salad that passes out unchanged |
Cause and effect
Complete the chain for a person whose kidneys fail. Each box follows from the box before it.
Scenario: a kidney clinic
A clinic tests four patients. It measures the urea in a blood sample (in millimoles per litre, mmol/L) and how much urine each person makes in a day. For this test, the clinic uses a healthy range of 3 to 8 mmol/L.
| Patient | Blood urea (mmol/L) | Urine made per day (L) | Inside the healthy range? (Yes or No) |
|---|---|---|---|
| Ava | 5.0 | 1.6 | |
| Ben | 31 | 0.3 | |
| Chloe | 6.5 | 1.4 | |
| Dev | 17 | 0.7 |
Source: sample clinic results. Real healthy ranges differ slightly between laboratories.
2. Complete the last column of Table 1. Then calculate how many times higher Ben’s blood urea is than the top of the healthy range. Give your answer to one decimal place.
Answer times
3. Compare Ben with Ava. What pattern do you see across all four patients? Suggest a reason for the pattern.
4. Compare the digestive system with the excretory system. For each, say whether it is mainly an input or an output system, what it moves, and what would happen if it stopped working. Explain why faeces are not counted as excretion, and why a living thing needs both systems.
Match each organ to its job
Kidneys → C; Ureters → F; Bladder → E; Urethra → B; Lungs → A; Skin → D.
1. Keeping conditions inside the body within useful limits; for example the amount of water, salt or waste (urea) in the blood. [2 marks]
Is it excretion?
Faeces: No, they are mostly undigested food that never entered the body’s cells. Urea in urine: Yes, a waste made in the body and removed by the kidneys. Carbon dioxide from a leaf at night: Yes, a waste of respiration in the plant’s cells, released through the stomata. Fibre from a salad: No, it was never absorbed or made by cells.
Cause and effect
Box 2: urea, extra water and salt are no longer removed from the blood as urine. Box 3: wastes build up in the blood and the body’s conditions go out of balance.
Go Deeper
2. Last column: Ava Yes; Ben No; Chloe Yes; Dev No. 31 ÷ 8 = 3.875, so about 3.9 times. [3 marks: 1 column; 1 working; 1 answer to one decimal place]
3. Patients who make less urine have higher blood urea (Ben 0.3 L and 31 mmol/L; Ava 1.6 L and 5.0 mmol/L). Healthy kidneys filter urea out of the blood into urine. When the kidneys are not working well, less urine is made and less urea is removed, so it builds up in the blood. [3 marks: 1 link with data; 1 kidneys remove urea in urine; 1 poor filtering, urea builds up]
Show What You Know
4. Marking guide (points expected): 1 digestive system mainly input: breaks food down and absorbs nutrients into the blood; 1 excretory system mainly output: the kidneys filter the blood and remove urea, extra water and salt in urine; 1 if digestion stopped, cells would not get nutrients; 1 if excretion stopped, wastes would build up and cells would stop working properly; 1 faeces are mostly undigested food that was never inside cells, while excretion removes wastes made by cells; 1 both are needed so inputs and outputs stay in balance. [6 marks]
Common error to watch for: in Q3, saying the kidneys “make” urea. They remove urea that is already in the blood.