Year 8 Science · Unit 1 · Lesson 3
Challenge Worksheet
Learning Goals
Rate three answers
Three students answered the question “What does the circulatory system do?” Read all three.
Three student answers
Answer P: “It’s basically the heart. The heart moves blood, so that’s it.”
Answer Q: “Vessels carry the blood that the heart pumps, and the blood delivers oxygen and food to cells, so together they move substances round the body.”
Answer R: “The circulatory system is really important for keeping you alive.”
1. Write the letters from the weakest answer to the strongest. For the weakest answer, name one thing it leaves out.
2. Answer P contains a scientific error. State the error, and name the two components that P leaves out, with the role of each.
| Measurement | Typical value |
|---|---|
| Blood pushed out of the heart (to the body) in one beat | 70 mL |
| Heart rate at rest | 70 beats per minute |
| Total blood in the body | about 5 L (5000 mL) |
Source: common figures for an adult sitting still. Heart rate and blood volume differ between people.
3. Calculate how much blood the heart pumps to the body in one minute. Give your answer in mL and in L.
Answer mL, and L
4. Use your answer to Q3 to estimate how long it takes for an amount of blood equal to all the blood in the body to be pumped around once. Then estimate how many times this happens in one hour.
Answer minutes; times per hour
5. An adult has only about 5 L of blood, yet cells everywhere need oxygen every second. Using your Q4 answer, explain why the blood must keep coming back through the lungs, and what this means for how the two systems work.
Evaluate the claim
Someone claims
“Each organ system is really just its biggest organ. The lungs are the respiratory system and the stomach is the digestive system. The other parts are only extras.”
Check the claim against three parts. Fill in each part’s system and job, and what would fail if it were missing.
| Component | System | Its role | Without it, the system… |
|---|---|---|---|
| Trachea (windpipe) | |||
| Small intestine | |||
| Blood vessels |
6. Use your table to rewrite the claim so that it is scientifically correct.
7. Evaluate this statement: “The circulatory system is the most important organ system, because it keeps every other system alive.” Use the circulatory, respiratory and digestive systems in your answer, and finish with a clear judgement.
Rate three answers
1. Weakest to strongest: R, P, Q. R names no components, roles or function. [2 marks]
2. Error: the heart is only one component of the circulatory system, not all of it. Missing: blood vessels (carry blood through the body) and blood (transports substances such as oxygen and food). [3 marks]
Your Turn
3. 70 mL × 70 = 4900 mL per minute = 4.9 L per minute. [2 marks: 1 mL, 1 L]
4. 5 ÷ 4.9 ≈ 1.02, so about 1 minute. In one hour: 60 ÷ 1.02 ≈ 59, so about 60 times (accept 58 to 60). [2 marks: 1 each]
5. The same 5 L of blood goes round about once a minute. The cells take oxygen out each time, so the blood must pass through the lungs again to pick up more. The respiratory system must keep supplying oxygen and the circulatory system must keep carrying it, every minute, or the cells run out. [3 marks: 1 blood reused about once a minute, 1 role of each system, 1 so they must work continuously]
Evaluate the claim
Trachea: respiratory; carries air to and from the lungs; without it no air reaches the lungs, so no gas exchange. Small intestine: digestive; absorbs useful substances into the blood; without it food is broken down but not absorbed. Blood vessels: circulatory; carry blood through the body; without them pumped blood cannot reach the cells.
6. Model: “An organ system is a group of organs with different roles that work together on one major function. The lungs, trachea and other air passages together make the respiratory system, and the stomach and intestines together make the digestive system.” [2 marks: 1 several organs with different roles, 1 correct examples]
Show What You Know
7. Marking guide: 1 support (heart, vessels and blood supply every organ with oxygen and food and remove wastes); 1 what it needs from the respiratory system (the lungs put oxygen into the blood); 1 what it needs from the digestive system (the intestines absorb food into the blood, and heart muscle needs that food too); 1 component → role → function language with named organs; 2 justified judgement (only partly right: the systems are interdependent, so if one fails the others fail too; 1 if no reasoning). [6 marks]
Common error to watch for: in Q4, working out 4.9 ÷ 5 instead of 5 ÷ 4.9.