The Circulatory System: The Heart, Blood Vessels and Blood — Answers
Answer sheet with model answers and success criteria for the Year 9 the human body worksheet.
Success TutoringWorksheetsAnswer sheetYear 9 · Science · The Human BodyThe Circulatory System: The Heart, Blood Vessels and Blood — Answers
Part A: The heart
The heart is a muscular pump with four chambers. The upper chambers, the atria, receive blood. The lower chambers, the ventricles, pump blood out. Valves stop blood flowing backwards.
The right side receives deoxygenated blood from the body through the vena cava and pumps it to the lungs through the pulmonary artery. The left side receives oxygenated blood from the lungs through the pulmonary vein and pumps it to the body through the aorta.
Answer: Body → right atrium → right ventricle → lungs → left atrium → left ventricle → aorta (to the body)
- ARight atrium
- BLeft atrium
- CRight ventricle
- DLeft ventricle
Answer: Left ventricle
The left ventricle pumps blood all the way around the body, so it needs the strongest muscle.
- ATo make the blood red
- BTo stop blood flowing backwards
- CTo add oxygen to the blood
- DTo slow down the heart rate
Answer: To stop blood flowing backwards
Valves close behind the blood to keep it flowing in one direction.
Answer: True
It is unusual: most arteries carry oxygenated blood, but the pulmonary artery carries blood low in oxygen to the lungs to pick up oxygen.
Answer: Blood passes through the heart twice for each complete trip around the body: once in a circuit to the lungs (to pick up oxygen) and once in a circuit to the rest of the body (to deliver oxygen).
- AVena cava
- BPulmonary artery
- CPulmonary vein
- DAorta
Answer: Aorta
The aorta leaves the left ventricle and carries oxygenated blood to the body.
Answer: True
It carries blood that has just picked up oxygen in the lungs back to the left atrium.
Part B: Blood vessels and blood
- Artery
- Vein
- Capillary
- Red blood cell
- White blood cell
- Platelet
- Plasma
- Helps blood to clot at a wound
- Wall one cell thick, where substances are exchanged with cells
- Thinner walls and valves; carries blood back to the heart at low pressure
- Defends the body against pathogens
- Thick, elastic walls; carries blood away from the heart at high pressure
- Liquid that carries dissolved food, carbon dioxide and wastes
- Contains haemoglobin to carry oxygen
Answer: Artery → Thick, elastic walls; carries blood away from the heart at high pressure; Vein → Thinner walls and valves; carries blood back to the heart at low pressure; Capillary → Wall one cell thick, where substances are exchanged with cells; Red blood cell → Contains haemoglobin to carry oxygen; White blood cell → Defends the body against pathogens; Platelet → Helps blood to clot at a wound; Plasma → Liquid that carries dissolved food, carbon dioxide and wastes
Answer: So that the distance is very short, allowing oxygen and nutrients to diffuse quickly out of the blood into cells and carbon dioxide and wastes to diffuse in.
Answer: Blood in veins is at low pressure and may flow backwards (for example when returning upwards from the legs), so valves keep it moving towards the heart. Blood in arteries is pushed forwards at high pressure by the heart.
Part C: Heart rate
Each heartbeat sends a pulse of blood through the arteries, which you can feel at your wrist or neck. Heart rate is measured in beats per minute (bpm). To save time, count for 15 or 20 seconds and multiply up to one minute.
Answer: 72 beats per minute
- 60 ÷ 15 = 4
- 18 × 4 = 72 bpm
Answer: 75 beats per minute
- 60 ÷ 20 = 3
- 25 × 3 = 75 bpm
Answer: 4.9 L/min
- 70 × 70 = 4900 mL per minute
- 4900 ÷ 1000 = 4.9 L/min
Answer: 58 bpm
- 130 − 72 = 58 bpm
Part D: Explain
Model answer: During exercise, the muscle cells need more energy, which they release from glucose by cellular respiration: glucose + oxygen → carbon dioxide + water (+ energy). To do this faster, the muscles need more oxygen and glucose and must get rid of more carbon dioxide. The heart beats faster to pump blood to the muscles more quickly, and breathing becomes faster and deeper to take in more oxygen and breathe out carbon dioxide. After exercise, the heart rate and breathing rate stay high for a while because the body is still removing extra carbon dioxide and repaying the oxygen needed to break down waste products (such as lactic acid) built up during hard exercise.
Success criteria:
- Muscles need more energy from cellular respiration
- Respiration needs oxygen and glucose and produces carbon dioxide
- Faster heart rate delivers blood to muscles more quickly; faster breathing increases gas exchange
- Explains why rates stay high after exercise
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.