Teacher sheet · Ages 15–16 · about 45–50 minutes
Circulation, gas exchange and cardiac output
Lesson page: echoesof.science/school/heart-and-lungs/ages-15-16/ · Pupil worksheet: echoesof.science/school/heart-and-lungs/ages-15-16/print/
Learning aims
- Describe the double circulation and the cardiac cycle.
- Explain gas exchange at the alveoli and capillaries.
- Use CO = HR × SV.
- Plan and evaluate a safe exercise investigation.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: how much blood does your heart pump in one day? Estimate with 5 L/min. |
| 10 min | Read the text; route-of-blood diagram. |
| 20 min | Exercise and recovery practical. |
| 5 min | Calculations and evaluation. |
| 5 min | Quiz or calculator game. |
Materials for the experiment
- Stopwatch
- A step or low bench (about 20–30 cm high), or space for marching on the spot
- Results table
- Optional: a heart-rate monitor or fitness watch
Safety: Moderate exercise only. Pupils with a heart or breathing condition, an injury or who feel unwell do not exercise; they can time and record. Stop at once if dizzy, in pain or very short of breath. Do not press on the neck to find a pulse. Results are for learning, not health checks: no comparing or ranking pupils.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| All arteries carry oxygen-rich blood. | The pulmonary artery carries oxygen-poor blood to the lungs: arteries are defined by direction, away from the heart. |
| Only heart rate rises in exercise. | Stroke volume rises too; both increase cardiac output. |
| A lower or higher number than a classmate means a health problem. | Normal ranges are wide and vary between people; health questions go to a doctor. |
Worksheet answers
- Right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.
- Blood arriving is low in oxygen and high in CO₂, so steep concentration gradients drive diffusion across the thin walls of alveoli and capillaries; the large number of alveoli gives a large surface.
- Example: 72 × 70 = 5.0 L/min at rest, 120 × 70 = 8.4 L/min after; factor ≈ 1.7 (an underestimate, because SV also rises).
- Counting errors over 15 s (× 4 multiplies them); SV assumed constant; exercise intensity not controlled. Improvement: heart-rate monitor, fixed step rate with a metronome, repeat readings.
Quiz answers
- Which ventricle pumps blood into the systemic circuit? The left ventricle. The left side sends oxygen-rich blood to the body.[1]
- Systole is the phase when the heart… contracts and pushes blood out. Diastole is the relaxation and filling phase.[4]
- HR = 80 beats/min, SV = 70 mL. Cardiac output is… 5.6 L/min. 80 × 70 mL = 5600 mL/min = 5.6 L/min.[5]
- Why does oxygen move from the alveoli into the blood? Blood arriving at the lungs is low in oxygen. It diffuses down its concentration gradient.[2]
- Where does exchange with body cells take place? Capillaries. Capillaries are the sites of exchange.[4]
- Using 220 − age, the estimated maximum heart rate of a 16-year-old is… 204 beats/min. 220 − 16 = 204; it is only a rough estimate.[6]
Going further
Estimate how many litres of blood your heart pumps in a day at rest, and how many times it beats.
Sources used in this lesson
- [1] How the Heart Works: Anatomy (National Heart, Lung, and Blood Institute (NIH, USA)): reliability High
- [2] How the Lungs Work: The Respiratory System (National Heart, Lung, and Blood Institute (NIH, USA)): reliability High
- [3] How the Lungs Work: What Breathing Does for the Body (National Heart, Lung, and Blood Institute (NIH, USA)): reliability High
- [4] Biology, 40.3 Mammalian Heart and Blood Vessels (OpenStax, Rice University): reliability High
- [5] Anatomy and Physiology, 19.4 Cardiac Physiology (OpenStax, Rice University): reliability High
- [6] Watch your heart rate, but don't obsess about it (American Heart Association News): reliability Medium-high
Full list with links and a note on each source's reliability: echoesof.science/school/heart-and-lungs/sources/
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