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B. Life Systems · Human Organ Systems

The Circulatory System: Heart, Blood and Blood Vessels

B2.1 B2.2 A1.2 A1.4About 45 minutes
Learning goals
  • I can describe the structure and job of the heart, blood vessels and blood.
  • I can trace the path of blood from the heart to the lungs and the body.
  • I can find and measure my pulse.
  • I can investigate how activity changes my heart rate.

Words to know

circulatory system
The heart, blood vessels and blood. It carries materials all around the body.
atrium
One of the two top chambers of the heart. Atria (more than one) receive blood.
ventricle
One of the two bottom chambers of the heart. Ventricles pump blood out.
artery
A blood vessel that carries blood away from the heart.
vein
A blood vessel that carries blood back to the heart.
capillary
A tiny blood vessel, thinner than a hair, where materials move in and out of the blood.
pulse
The beat you can feel in an artery each time your heart pumps.
heart rate
The number of times your heart beats in one minute.

Squeeze a tennis ball

If you have a tennis ball, squeeze it hard, then let go. Do it again and again. Your hand gets tired fast! Your heart squeezes like that about once every second, every minute of every day, your whole life, without getting tired.

Your heart is the pump of the circulatory system. Its job is to keep blood moving so that every cell gets what it needs.

The heart: a four-room pump

Your heart is a hollow muscle about the size of your fist. It sits in your chest between your lungs, tipped a little to the left.

Inside, it has four chambers (rooms):

  • two atria on top. They receive blood coming back to the heart.
  • two ventricles on the bottom. They pump blood out of the heart.

A wall down the middle keeps the right side and left side apart. Valves are like one-way doors that keep blood flowing in the right direction. The "lub-dub" sound of your heartbeat is the sound of the valves closing.

A stethoscope coiled on a wooden table next to a yellow tennis ball.
A stethoscope lets you hear the heart pumping. Squeezing a tennis ball shows how hard your heart works with every beat. AI-generated image
person's RIGHT sideperson's LEFT siderightatriumleftatriumrightventricleleftventriclevalvesfrom the bodyto the lungsfrom the lungsto the bodyblood low in oxygenoxygen-rich blood
The four chambers (simplified). The right side (on your left in a front view) gets blood from the body and sends it to the lungs. The left side gets oxygen-rich blood from the lungs and pumps it to the body. Valves keep blood moving one way.

Two trips for your blood

Your blood makes two loops:

  1. The right side of the heart pumps blood to the lungs. There the blood drops off carbon dioxide and picks up oxygen.
  2. The oxygen-rich blood comes back to the left side of the heart, which pumps it out to the rest of the body: brain, muscles, stomach, toes.
  3. The cells use up oxygen, and the blood comes back, low in oxygen, to the right side. Then it all starts again!
lungsright sideleft sideheartrest of the bodylow oxygenoxygen-richoxygen-richlow oxygen
The two loops of blood. The right side of the heart sends blood to the lungs; the left side sends oxygen-rich blood to the body. In a front-view diagram, the heart's right side is on your left.

Blood vessels: the body's roads

  • Arteries carry blood away from the heart. They have thick, stretchy walls because the blood in them is pushed hard.
  • Veins carry blood back to the heart. Many veins have little valves that stop blood from flowing backward.
  • Capillaries are tiny vessels that connect arteries to veins. Their walls are so thin that oxygen and nutrients pass out to the cells and wastes pass in to the blood.

Hint: Arteries go Away.

Part of bloodJob
plasmathe yellowish liquid part; carries nutrients, wastes and blood cells
red blood cellscarry oxygen
white blood cellsfight germs
plateletshelp blood clot to stop bleeding when you get a cut
What blood is made of. An adult has about 5 litres of blood.
Worked example

Let's measure a pulse

How can Jonah find his heart rate?

  1. Turn one hand palm up. Put the first two fingers of your other hand on the inside of your wrist, below the thumb. (Don't use your thumb: it has a pulse of its own that can confuse you.)
  2. Press gently until you feel a beat. That beat is your pulse. Each beat is one heartbeat pushing blood through an artery.
  3. A grown-up times 15 seconds while Jonah counts the beats: he counts 21.
  4. Multiply by 4, because there are four 15-second parts in one minute: 21 × 4 = 84.

Jonah's resting heart rate is 84 beats per minute. (For many 10-year-olds at rest, it is somewhere around 70 to 110.)

Safety first

When you exercise in an investigation, stop if you feel dizzy, sick or out of breath. If you have asthma or a heart condition, check with your grown-up first. Scientists always follow safety rules.

Just for fun

Why did the red blood cell pack a map?

It had a long trip! An adult's blood vessels, laid end to end, would stretch roughly 100,000 km, enough to wrap around Earth more than twice.

Hands-on activity

Heart rate investigation

You need: a timer, paper, a pencil, a grown-up. Optional: an empty paper towel tube.

  1. Question: How does activity change my heart rate?
  2. Predict: write what you think will happen and why.
  3. Find your resting pulse after sitting for 2 minutes. Count for 15 seconds and multiply by 4.
  4. Walk on the spot for 1 minute. Measure again right away.
  5. Do jumping jacks for 1 minute. Measure again right away.
  6. Sit and rest for 2 minutes. Measure again.
  7. Record your data in a table and draw a bar graph. Compare it to your prediction.
  8. Bonus: put one end of the paper towel tube gently against a grown-up's chest (on the left side, over their shirt) and your ear on the other end. Can you hear "lub-dub"? You made a simple stethoscope!

Grown-up note: stop if your child feels dizzy or unwell, and choose gentle activities if your child has a health condition. Never put anything inside the ear.

Your turn

1

What are the three main parts of the circulatory system?

2

Which chambers of the heart pump blood out?

3

Which side of the heart pumps blood to the lungs?

4

Which blood vessels carry blood away from the heart?

5

What do red blood cells carry?

6

Priya counts 23 beats in 15 seconds. What is her heart rate per minute?

7

What do valves in the heart do?

More practice: warm-up

1

What organ pumps blood?

2

How many chambers does the heart have?

3

What do white blood cells do?

4

Which blood vessels carry blood back to the heart?

5

What do you feel at your wrist when you take your pulse?

More practice: on your own

1

Trace one complete trip of blood starting in the right side of the heart.

2

Why do arteries have thicker walls than veins?

3

Why do capillaries have very thin walls?

4

Your heart rate is 76 at rest and 136 after skipping. By how much did it go up? Why?

5

You cut your finger. Which part of the blood helps stop the bleeding?

Challenge

1

Why does the blood need to go to the lungs before going to the rest of the body?

2

Ben measures his pulse for only 5 seconds and multiplies by 12. Ella counts for 30 seconds and multiplies by 2. Whose result is probably more accurate? Explain.

3

The left ventricle has a thicker wall than the right ventricle. Use what you learned to explain why.

Check your understanding

Quiz

1.

The bottom chambers of the heart, which pump blood out, are the...

2.

Blood picks up oxygen in the...

3.

Which blood vessels carry blood away from the heart?

4.

Which part of blood fights germs?

5.

Kofi counts 20 beats in 15 seconds. His heart rate is...

6.

Which side of the heart pumps blood to the body?

For parents and teachers

Big idea: the heart is a four-chambered muscle that pumps blood in two loops: right side to the lungs, left side to the body. Arteries carry blood away, veins bring it back, capillaries exchange materials.

How to help at home:

  • Help your child find a pulse at the wrist with two fingers, and time 15 seconds.
  • Make the bar graph together and talk about the pattern (up with exercise, down with rest).

Common difficulties:

  • Mixing up arteries and veins ("Arteries go Away").
  • Mixing up the heart's right and left on diagrams: in a front view, the heart's right side is on the viewer's left.
  • Believing veins carry "blue blood." Blood is always red; veins can look bluish through the skin.

Safety: Stop activity if your child feels dizzy or unwell.