What Is Blood? The Four Parts That Keep You Alive (Circulatory System for Kids)
⭐ Free stories for ages 5-10. Read in any order!
What Is Blood? The Four Parts That Keep You Alive (Circulatory System for Kids)
Keywords: what is blood made of for kids, red blood cells explained, white blood cells for kids, platelets for kids, plasma explained, circulatory system for kids ages 5–10
Before the story — for parents and teachers:
What is blood actually made of — and why is it red? This free science story explores the four components of blood through Uncle Daoud, who arrives with a single glass of water, some food colouring, and four small objects that change the way three children see the red liquid in their own veins. Part 33 of the Science Storyland series, continuing directly from Blog 32.
👉 Start from the very beginning — Part 1: The Mystery of the Watermelon Seed
The Story Begins
Uncle Daoud arrived the next evening with a small bag.
Not his usual coat-and-nothing. An actual bag — small, dark, zip closure.
Hamza had been watching from the window and was at the front door before the bell finished ringing.
"What's in the bag?"
"Good evening, Hamza."
"Good evening what's in the bag—"
"Shoes off first."
Hamza kicked his shoes sideways without looking at them. "What's in the bag?"
Uncle Daoud stepped inside, looked at the shoes scattered across the hallway, said nothing, and walked toward the kitchen.
Hamza followed him like a shadow.
At the kitchen table, Uncle Daoud opened the bag slowly — deliberately slowly, Hamza felt — and set out four objects in a line.
A small round button, slightly concave, reddish-brown.
A larger white bead.
A flat, irregular piece of clear plastic, crumpled slightly.
A small coin.
Then a clear glass.
Then a small bottle of red food colouring.
Hamza stared at them.
"...Is this a trick?"
"Go get Ali and Zara."
Zara came downstairs with her notebook already open, which Ali felt was slightly excessive. He came in, looked at the table, looked at Uncle Daoud, and sat down without saying anything.
He was doing the thing where he went quiet because he was already thinking.
Zara sat beside him. Hamza remained standing — he always stood at the beginning, before the conversation pulled him into a chair.
Uncle Daoud picked up the clear glass and placed it in the centre of the table.
"What colour is blood?" he said.
"Red," Hamza said immediately.
"Is it?"
Hamza opened his mouth. Closed it. "...Yes?"
"All of it?"
Hamza looked at Zara. Zara looked at the glass.
"I mean — when I've seen it, it's red," she said slowly. "But I've only ever seen it from a cut, so—" she paused. "Is it not all red?"
Uncle Daoud didn't answer. He picked up the red food colouring and held it above the glass.
"What you've seen from a cut," he said, "is the finished mixture." He let one drop fall into the water. It bloomed outward, red tendrils spreading through the clear liquid. "But blood is not one red thing. It is four things — and most people go their entire lives without knowing what those four things actually are."
He set the food colouring down.
Pointed at the button.
"Start here."
❤️ The Button
Hamza grabbed it first.
Turned it over. Felt the slight dip in the middle — concave on both sides, like someone had pressed their thumbs into both faces.
"Why's it shaped like that?" he said. "Like a squished ball?"
"What does it need to do?" Uncle Daoud said.
"Carry—" Hamza stopped. "Carry oxygen. You said that last time. Red blood cells carry oxygen."
"So if it's carrying oxygen — and it needs to squeeze through capillaries thinner than a human hair — what does the shape do for it?"
Hamza turned the button over again, frowning. Then — "Oh. It can bend. It can fold through narrow bits."
"And surface area," Ali said quietly, from across the table. He was still looking at the button in Hamza's hand. "More surface means more oxygen can attach."
Hamza held the button up. "Both things at once?"
"The shape solves both problems at once," Uncle Daoud said.
Zara had been writing. She looked up. "What makes it red though? The actual red colour — is that the oxygen?"
"Not quite." Uncle Daoud leaned forward slightly. "Inside these cells — there's a protein called haemoglobin. It contains iron. When iron binds to oxygen—"
"It goes red?" Hamza said.
"Bright red. Oxygenated blood — full of oxygen — is the bright red you see if you cut an artery. Blood that's given up its oxygen turns darker. Bluish-red."
Zara's pencil stopped. "Wait. So veins aren't blue?"
"Your skin makes them look blue from the outside. The blood inside is still red — just darker."
"I always thought—" Zara shook her head. "Okay. That's — I was wrong about that."
Hamza was still holding the button, staring at it.
"How much iron?" he said. "Like, how much iron is actually in me?"
"Enough to make a small nail," Uncle Daoud said. "If you extracted it."
Hamza went absolutely still.
"I have a nail in me."
"Distributed through approximately twenty-five trillion red blood cells, yes."
Hamza set the button down very carefully.
"That's going in the notebook," he said. He looked at Zara. "Write that down."
"I'm already writing it."
🤍 The White Bead
Uncle Daoud slid it toward Zara this time.
She picked it up. Larger than the button, smooth, white.
"White blood cells," she said, with the confidence of someone who has looked this up before. "They fight bacteria and—"
"What else?" Uncle Daoud said.
Zara paused. "...Viruses?"
"What else?"
She turned the bead over. "That's — that's all I know, actually."
Hamza leaned in. "There's more types?"
"Several," Uncle Daoud said. "Some patrol — circling through the blood looking for anything that doesn't belong. Some when they find a bacterium—"
Hamza interrupted. "What do they do when they find it?"
"Some of them engulf it."
Hamza stared. "They eat it?"
"They surround it completely and absorb it."
"They EAT bacteria." Hamza looked at Ali. "There are cells inside me that eat things."
Ali was leaning forward now, elbows on the table. "And the memory ones," he said. "You mentioned memory cells last time — the ones that remember past infections."
"Yes. Certain white cells keep a record of every pathogen the body has successfully fought. When the same one appears again, the response is much faster — sometimes fast enough that you feel nothing."
Zara had been writing, but she stopped and looked up slowly. "Is that — is that what vaccines do? Give the memory cells something to remember without actually making you sick?"
"Exactly that."
Ali pointed at Zara. "I was going to say that."
"You were thinking too slowly," she said.
"I was building up to it—"
"There's only five hundred," Hamza said.
Both of them stopped.
"What?" Ali said.
"He said one white cell for every five hundred red ones." Hamza looked at Uncle Daoud. "Right? They're way outnumbered."
"Roughly five hundred to a thousand red cells for every white one, yes."
"And they fight everything off anyway." Hamza sat back. "With those odds."
Uncle Daoud picked up the white bead and placed it next to the button — small white bead beside many invisible red ones, in their heads.
"They don't need to be everywhere," he said. "They respond. They move toward trouble."
"Like — emergency response," Ali said.
"Like exactly that."
🩹 The Clear Plastic
Uncle Daoud pushed the flat crumpled piece toward Hamza.
Hamza picked it up, turned it over, made a face.
"This one's weird. It's not even a proper shape."
"That's accurate, actually," Uncle Daoud said. "Platelets aren't proper cells. They're fragments."
"Fragments of what?"
"Broken-off pieces of larger cells. No nucleus. Irregular shape. And—" Uncle Daoud paused. "What do you think they do? Given how they look."
Hamza held the crumpled piece. Zara looked at her knuckle — an old scrape, nearly faded.
"Blocking things?" Hamza guessed. "Like—filling gaps?"
"Very close. When a vessel is damaged — when you cut yourself — platelets rush to the site. They stick together, layer on layer, building a plug." Uncle Daoud watched Hamza's expression. "At the same time, they trigger a chemical reaction. That reaction produces a protein — fibrin — which forms a mesh of fibres across the wound."
"Like a net," Ali said.
"And the net traps more platelets, more red cells — building up into what you see on the surface."
Zara looked up slowly. "A scab is — that's platelets and this fibrin thing and red cells all stuck together?"
"Yes."
She looked at her knuckle again. Touched it lightly.
"I thought it was just dried blood," she said. "I didn't know it was — organised."
"The reddish-brown colour—"
"Iron," Ali said immediately. "Haemoglobin in the trapped red cells."
Uncle Daoud nodded.
Zara was still looking at her knuckle. Not writing. Just looking.
"They did that," she said quietly. "While I was annoyed about the scrape. They were already — fixing it."
Nobody said anything for a moment.
Hamza set the clear plastic piece down gently, like it had become more important than it looked.
💛 The Coin
Uncle Daoud placed the coin in the centre of the table.
The children looked at it. Then at each other.
"That doesn't look like a cell," Hamza said.
"It isn't." Uncle Daoud picked up the water glass — the red-tinged water — and held it up. "What colour is this?"
"Red. Sort of yellowish-red," Ali said.
"The yellow," Uncle Daoud said, "is plasma."
Hamza frowned. "Plasma's in there?"
"Plasma makes up fifty-five percent of your blood. It's a pale yellow liquid — if you separated blood by spinning it in a machine, the cells would sink and the plasma would float on top. Yellow and clear."
"So blood is more than half liquid," Zara said, sitting back slightly. "I — genuinely didn't know that."
"And that liquid carries everything the cells cannot carry themselves." Uncle Daoud set the glass down. "Nutrients from digested food—"
Ali sat forward suddenly. "The plasma — that's what carries the glucose and amino acids. From the small intestine. That's what Mummy Lisa was talking about — nutrients absorbed into the blood—"
"Into the plasma," Uncle Daoud said.
"They're in there right now." Ali looked at the glass. "From dinner."
"Dissolved in the plasma. Being carried to cells that need them." Uncle Daoud tapped the coin. "Plasma also carries hormones, proteins, heat from active muscles — it distributes warmth around the body. And waste — carbon dioxide, dissolved in the plasma, on its way back to the lungs."
Hamza had been very quiet.
He was looking at all four objects lined up on the table. Button. White bead. Clear plastic. Coin.
"So." He pointed at each one. "The red cells carry oxygen using iron. The white cells fight things and remember things. The platelets fix cuts and make scabs. And the plasma carries nutrients and waste and warmth and everything else." He paused. "And all four of those are in my blood right now."
"In every drop," Uncle Daoud said.
Hamza looked at his finger. He pressed his thumb against the tip until the skin went white, then released it. A flush of pink — blood returning through capillaries.
"Five million red cells," he said softly. "In one drop."
"About that."
"And white cells. And platelets."
"And plasma," Zara added. She had stopped writing. Her pencil was resting on the notebook, and she was just looking at the table.
"All of that," Hamza said. "In the drop that came out of my finger when I got a paper cut last year."
He looked up.
"I was annoyed about the paper cut."
🔗 Where Everything Meets
Ali had been quiet for a few minutes — the kind of quiet that meant something was assembling itself.
"The plasma carries nutrients," he said. "The red cells carry oxygen." He was looking at the table but not really seeing it. "Both of those are in the blood. And the blood goes through the capillaries — to every cell." He stopped. "So the oxygen from the lungs and the glucose from the digestive system — they arrive at the same cell. Together."
"Through the same blood," Uncle Daoud said.
"And the cell uses them — both of them — to make energy." Ali looked up. "That's what a cell does with them? Makes energy?"
"That process," Uncle Daoud said carefully, "has a name. And it connects back to something you learned a long time ago."
Zara's pencil came back up.
She was frowning, thinking hard. "Photosynthesis," she said slowly. "Plants take sunlight, water, carbon dioxide — and make glucose and oxygen." She paused. "And cells take glucose and oxygen — and make energy and release carbon dioxide and water."
She stopped.
Stared at what she'd just written.
"It's the reverse," she said. "It's literally the reverse of photosynthesis."
The kitchen went very still.
Uncle Daoud stood, picked up his coat from the back of the chair.
"From a watermelon seed," he said quietly, looking at the three of them.
He left the four objects on the table. Picked up his bag without them.
At the door he paused.
"You've connected breathing, digestion, and circulation — across thirty-three stories — to the same two molecules that have been cycling between plants and animals for billions of years." He buttoned his coat. "I'd say you've earned tomorrow's story."
"What's tomorrow?" Hamza called after him.
"Everything moving together," Uncle Daoud said, from the hallway. "All at once. Finally."
The front door closed.
Hamza looked at the four objects on the table.
Then at his hand.
Then at Zara.
"We started with a seed," he said.
"And now we're inside a blood cell," Zara said.
From upstairs, Ali's voice:
"We're not done yet."
🎯 Kids Activity: "Build Your Own Blood"
You need: A clear glass, water, red food colouring, small red buttons or beads (lots — red blood cells), a few white beads (white blood cells — just a few), small crumpled pieces of tape (platelets — irregular shaped).
- Fill the glass halfway with water — this is your plasma
- Add a few drops of red food colouring
- Drop in your red beads — generously, they should be everywhere
- Add just three or four white beads — notice how few there are
- Add a few crumpled tape pieces
Look at your glass.
That is blood.
Ask yourself:
- Which part is carrying oxygen right now?
- Which part would rush to a cut?
- Which part is carrying nutrients from your last meal?
- What gives it the red colour — and what metal is involved?
👩🏫 Parent / Teacher Tip
This post introduces the four components of blood — red blood cells (haemoglobin, iron, concave shape, oxygen transport), white blood cells (multiple types, immune response, memory cells, antibodies, vaccines), platelets (clotting, fibrin, scab formation), and plasma (liquid carrier, nutrients, hormones, heat, waste) — through object-based discovery rather than direct instruction.
Zara's moment of admitting she was wrong about vein colour is intentional — it models the important idea that correcting a misconception is a natural part of learning.
After reading, discuss:
- "Why do red blood cells have no nucleus?"
- "What would happen if you had no platelets?"
- "How does plasma connect the digestive system to the circulatory system?"
IB Connections: How Our Bodies Work (cellular function, systems integration), Who We Are (understanding the self at cellular level), Learner Profile — Knowledgeable, Thinker, Inquirer.
🔥 What Comes Next
Later, Hamza sat alone at the kitchen table.
The four objects still in a line. Button. White bead. Clear plastic. Coin.
He picked up the button — the red blood cell — and held it between his fingers the way Uncle Daoud had turned his palm upward two nights ago, remembering the weight of a beating heart.
Hamza wasn't thinking about hearts right now.
He was thinking about the cycle.
Plants make glucose and oxygen from sunlight and air.
Animals eat the plants — glucose enters the blood in the plasma. Oxygen enters the blood in the red cells. Both travel together through sixty thousand miles of vessels to every cell. The cell uses them together to make energy — and releases carbon dioxide and water.
Which goes back to the plants.
Which use it to make more glucose and oxygen.
Which goes back to the animals.
He set the button down.
"We breathe out what plants breathe in," he said quietly. "And plants breathe out what we breathe in."
He hadn't planned to say it out loud.
But it felt like the kind of thing that needed to be said out loud.
Just once.
"Blood is not just what you see on a cut finger. It is oxygen and nutrients and defenders and repairers — four systems inside one system, all moving, all working, all the time, without a single instruction from you."
📚 This Is Part 33 of the Science Storyland Series
Circulatory System Arc:
- ✅ Part 31: The Heart — The Pump That Never Stops
- ✅ Part 32: Blood Vessels — The 60,000-Mile Network
- ✅ Part 33: What Is Blood? The Four Parts ← You are here
- ➡️ Part 34: How Oxygen and Nutrients Travel Together — Coming next
- Part 35: The Heart in Action — Exercise, Health and the Full Circle
👉 Read Part 32: Blood Vessels — The 60,000-Mile Network
👉 Start from Part 1: The Mystery of the Watermelon Seed
Science Storyland publishes free science stories for children ages 5–10 every week. Written for curious kids, IB classrooms, and parents who love learning alongside their children.
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