Example 1: pulse from a count
A student counts 21 beats in 15 seconds. What is her pulse in beats per minute?
- There are four 15-second periods in a minute.
- 21 × 4 = 84 beats per minute.
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IB DemystifiedMYP Sciences
Circulatory system and blood
Your heart beats about 100 000 times a day, pushing blood through tens of thousands of kilometres of blood vessels. The circulatory system delivers oxygen and food to every cell, carries away waste, and helps defend the body, and keeping it healthy is one of the most important things you can do.
Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Blood vessels compared
- Circulation in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- describe the heart and the double circulation
- relate the structure of arteries, veins and capillaries to their functions
- describe the components of blood and their functions
- explain the effects of exercise and lifestyle on the heart
- analyse pulse data and design ethical investigations
- discuss blood donation and reducing heart disease
Before you start
You will use these skills. If any feel shaky, review them first.
- breathing and gas exchange (see Breathing and gas exchange)
- respiration (see Respiration)
- diffusion (see Diffusion, osmosis and active transport)
Key vocabulary
- Artery
- A thick-walled vessel carrying blood away from the heart at high pressure.
- Vein
- A thinner-walled vessel with valves, carrying blood back to the heart at low pressure.
- Capillary
- A tiny vessel with walls one cell thick, where substances are exchanged.
- Double circulation
- Blood passes through the heart twice per circuit: to the lungs and to the body.
- Haemoglobin
- The protein in red blood cells that carries oxygen.
- Coronary heart disease
- Narrowing of the arteries that supply the heart muscle.
Understanding the ideas
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The heart and double circulation
The heart is a muscular pump with two sides. The right side pumps deoxygenated blood to the lungs, where it picks up oxygen; the left side pumps oxygenated blood to the rest of the body. This double circulation keeps blood pressure high enough to deliver oxygen quickly.
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Blood vessels
Arteries carry blood away from the heart at high pressure and have thick, elastic walls. Veins return blood at low pressure and have valves to stop backflow. Capillaries have walls one cell thick, so oxygen, glucose and carbon dioxide can diffuse between the blood and the cells.
-
Blood
Blood contains red blood cells (carry oxygen using haemoglobin), white blood cells (defend against pathogens), platelets (help blood clot) and plasma (carries dissolved food, carbon dioxide, hormones and urea).
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Why does it matter?
During exercise, heart rate and breathing rate rise to supply muscles with more oxygen and glucose. Smoking, high blood pressure, obesity, diabetes and inactivity increase the risk of coronary heart disease, while regular exercise and a balanced diet reduce it.
-
What does it connect to?
This topic links breathing, respiration, digestion, immunity and health, and leads to more detailed study of the heart and disease.
Blood vessels compared
- Arteries: away from the heart; high pressure; thick muscular, elastic walls; narrow lumen.
- Veins: back to the heart; low pressure; thinner walls; wide lumen; valves.
- Capillaries: walls one cell thick; exchange of substances by diffusion.
- Left side of the heart has thicker muscle, because it pumps blood around the whole body.
Circulation in the real world
Blood donation centres and thalassaemia centres rely on regular donors. Doctors measure blood pressure and heart rate to check heart health. People living high in the northern mountains have extra red blood cells to cope with thin air.
Worked examples
Example 2: cardiac output
A heart pumps 80 cm³ per beat at 70 beats per minute. How much blood is pumped per minute?
- 80 × 70 = 5600 cm³.
- = 5.6 litres per minute.
Assessment tips
Questions on this topic often use diagrams of the circulation, vessel data and pulse measurements. Expect to:
- Describe the route of blood through the heart, lungs and body.
- Explain how vessels and blood cells are adapted to their functions.
- Analyse pulse and recovery data.
- Discuss health choices and blood donation.
Common mistakes: saying all arteries carry oxygenated blood (the artery to the lungs does not); saying veins carry only "blue" blood; confusing the roles of white blood cells and platelets; and forgetting that the right side of the heart appears on the left of a diagram.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: pulse and body position
Partially guided investigation · about 30 minutes · groups of three
- Research question
- Does body position (lying, sitting or standing) affect resting pulse rate?
- Scientific background
- When you stand, the heart must work harder to pump blood up to the brain against gravity, so pulse rate may change with body position.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Stopwatch, mat for lying down, chair; optionally a pulse monitor.
- Method
- Lie still for 3 minutes, then count your pulse at the wrist for 30 seconds and double it.
- Sit still for 3 minutes and measure again.
- Stand still for 3 minutes and measure again.
- Repeat the whole sequence twice and find the mean for each position; combine results from the class.
Safety. Stand up slowly to avoid dizziness. Anyone with a medical condition should not take part. Keep individual results anonymous.
Then evaluate: why did you wait 3 minutes in each position, and why is it useful to combine results from the whole class?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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