# What is the human heart?

An episode of [Science for Kids / Science's Not Boring: The Kids Science Podcast](https://invisiblenetwork.ai/podcast/1896928296), 1 Jul 2026, 15 min.

Audio: https://www.buzzsprout.com/2624612/episodes/19406111-what-is-the-human-heart.mp3

Put your hand on your chest... thump, thump, thump! Did you know that steady drumbeat is actually the hardest-working muscle in your entire body? Join Mira and Finn on an epic adventure inside the human body to discover exactly how your heart works! You won't believe the amazing science hiding just beneath your skin. Did you know your heart is only about the size of your fist, but it squeezes 100,000 times a day without ever taking a nap? We're going to shrink down and follow a single drop of blood as it zooms out of the heart, picks up fresh oxygen in your lungs, and races all the way down to your toes like a high-speed roller coaster! You'll find out how your body holds 60,000 miles of blood vessels—enough tiny tubes to wrap around the entire Earth more than twice! Discover why microscopic tunnels called capillaries force your red blood cells to line up single-file, and find out how a brilliant doctor unlocked the secret of the circulatory loop. Get ready for a mind-blowing journey inside you!

## Transcript

[0:46] SPEAKER_00: Welcome to Science Is Not Boring by Kidoply.com.
[0:49] SPEAKER_00: I'm Mira!
[0:50] SPEAKER_02: And I'm Finn.
[0:52] SPEAKER_00: Hey Finn, have you ever looked closely at the back of your hand or your wrist?
[0:56] SPEAKER_02: Yeah, I can see some faint blue lines under my skin.
[1:00] SPEAKER_02: They look like tiny rivers.
[1:02] SPEAKER_00: Those rivers are exactly what we're talking about today.
[1:05] SPEAKER_00: But if you took all of those tubes that carry blood inside your body and laid them end to end, do you know how far they would reach?
[1:13] SPEAKER_02: Er, well, they're mostly inside me.
[1:16] SPEAKER_02: So maybe they would stretch across the room?
[1:19] SPEAKER_02: Or wait, maybe across a whole football field?
[1:22] SPEAKER_00: Not even close.
[1:24] SPEAKER_00: They would stretch for an unbelievable 60,000 miles.
[1:28] SPEAKER_00: 60,000 miles?
[1:31] SPEAKER_00: Wait, my brain can't even picture that.
[1:33] SPEAKER_00: How far is that in real life?
[1:35] SPEAKER_00: Well, if you walked all the way around the equator of the Earth, that's about 24,900 miles.
[1:43] SPEAKER_00: So your blood vessels are long enough to wrap around the entire Earth more than twice.
[1:48] SPEAKER_00: Whoa, inside me?
[1:51] SPEAKER_00: That doesn't make any sense.
[1:52] SPEAKER_00: How does all that fit inside a human body?
[1:55] SPEAKER_00: Because some of them are microscopic! Today we are exploring the ultimate engine inside you, your heart.
[2:02] SPEAKER_00: And we're going to follow a single drop of blood on its incredible journey.
[2:07] SPEAKER_00: Let's go! I want to know how I'm secretly carrying enough tubes to tie up the planet.
[2:12] SPEAKER_00: So, put your hand right on the middle of your chest, just a little to the left side.
[2:18] SPEAKER_01: Okay, I feel a thump.
[2:22] SPEAKER_01: Thump.
[2:23] SPEAKER_01: Thump.
[2:24] SPEAKER_01: It's like a tiny drum inside me.
[2:27] SPEAKER_00: That steady drumbeat is your heart at work.
[2:30] SPEAKER_00: It's essentially a muscle, and if you make a tight fist with your hand, that is almost exactly the size of it.
[2:37] SPEAKER_00: Wait, my heart is only as big as my fist.
[2:40] SPEAKER_00: But it's keeping my whole body alive.
[2:43] SPEAKER_00: Exactly.
[2:44] SPEAKER_00: It only weighs about 7 to 15 ounces.
[2:47] SPEAKER_00: That's roughly the weight of a can of soup.
[2:50] SPEAKER_00: But it is the hardest working muscle in your entire body.
[2:54] SPEAKER_00: How hard does it work?
[2:55] SPEAKER_01: Like, does it ever get to rest or take a nap while I sleep?
[3:00] SPEAKER_00: Nope.
[3:00] SPEAKER_00: While you are fast asleep, your heart is still working.
[3:04] SPEAKER_00: It squeezes or beats about 100,000 times a single day.
[3:10] SPEAKER_00: 100,000 times?
[3:12] SPEAKER_00: In just 24 hours?
[3:14] SPEAKER_00: If I squeezed my hand a hundred thousand times, it would fall off.
[3:19] SPEAKER_00: It's made of a special type of muscle called cardiac muscle that never tires out.
[3:24] SPEAKER_00: And every time it squeezes, it pumps blood.
[3:28] SPEAKER_00: In fact, it pumps about 2,000 gallons of blood every single day.
[3:33] SPEAKER_00: 2,000 gallons?
[3:35] SPEAKER_00: That's enough to fill a huge backyard swimming pool.
[3:38] SPEAKER_00: Or like thousands of milk jugs.
[3:42] SPEAKER_00: Exactly.
[3:43] SPEAKER_00: For centuries, human beings were completely confused by this.
[3:47] SPEAKER_00: Ancient scientists actually thought the liver created blood, and the body just drank it up and consumed it like a plant drinks water.
[3:56] SPEAKER_01: Well, if we pumped 2,000 gallons, where would it all come from?
[4:01] SPEAKER_01: We'd explode if we were just making that much blood from scratch.
[4:04] SPEAKER_00: That's the big secret.
[4:06] SPEAKER_00: In the year 1628, a brilliant English doctor named William Harvey discovered the truth.
[4:13] SPEAKER_00: What did William Harvey figure out?
[4:15] SPEAKER_00: Did he have a microscope?
[4:17] SPEAKER_00: He didn't even have a powerful microscope.
[4:20] SPEAKER_00: He carefully observed animals and measured the blood.
[4:23] SPEAKER_00: He did the math and realised the heart pumped way too much blood for the body to just absorb.
[4:29] SPEAKER_00: So he proved that the blood isn't being consumed.
[4:32] SPEAKER_00: It's moving in a giant continuous loop.
[4:36] SPEAKER_00: A loop?
[4:37] SPEAKER_01: So it's the exact same blood, just getting recycled and pumped around and around in a circle.
[4:42] SPEAKER_00: You got it! It's a closed system.
[4:45] SPEAKER_00: The same blood goes out, does its job, and comes right back.
[4:49] SPEAKER_00: That loop sounds like an epic road trip.
[4:52] SPEAKER_00: Let's follow a drop of blood.
[4:53] SPEAKER_00: Where does the journey start?
[4:55] SPEAKER_00: Great idea! Imagine a single drop of blood returning to the right side of your heart.
[5:01] SPEAKER_00: At this point, it's a dark purplish-red colour because it just travelled through your whole body and dropped off all its oxygen and nutrients to your muscles.
[5:11] SPEAKER_00: It sounds completely exhausted.
[5:14] SPEAKER_00: Does the heart give it more oxygen?
[5:16] SPEAKER_00: Actually, the heart's first job is to pump that tire drop straight into your lungs.
[5:22] SPEAKER_00: The right ventricle gives a strong squeeze and sends it to the lungs.
[5:27] SPEAKER_00: Oh, because we breathe air into our lungs.
[5:30] SPEAKER_00: That's where the oxygen is.
[5:32] SPEAKER_00: Exactly! Inside the lungs, there are millions of tiny air sacs.
[5:38] SPEAKER_00: Our drop of blood passes right next to them and picks up fresh oxygen from the air you just breathed in.
[5:44] SPEAKER_00: When it grabs that oxygen, a chemical reaction happens, and the blood instantly turns bright cherry red.
[5:52] SPEAKER_00: So now it's supercharged, bright red, and ready to go.
[5:56] SPEAKER_00: Where does it travel next?
[5:58] SPEAKER_00: The supercharged drop zips back out of the lungs and pours into the left side of your heart.
[6:04] SPEAKER_00: Specifically the left atrium, which is like a waiting room.
[6:08] SPEAKER_00: And then it drops down into the pumping part.
[6:10] SPEAKER_00: Yep, down into the left ventricle.
[6:13] SPEAKER_00: The left side of the heart is super thick and strong.
[6:17] SPEAKER_00: It gives a massive, powerful squeeze and blasts that bright red drop out into a giant tube called the aorta.
[6:25] SPEAKER_00: The aorta?
[6:26] SPEAKER_00: That sounds like a crazy water slide.
[6:29] SPEAKER_00: It acts like the biggest water slide in your body.
[6:32] SPEAKER_00: The aorta is about as thick as a garden hose.
[6:36] SPEAKER_00: From the aorta, the blood shoots out to the rest of your body at high speed.
[6:41] SPEAKER_00: It races up to your brain to help you think, down to your stomach to help digest food, and all the way to the tips of your fingers and toes.
[6:50] SPEAKER_00: Bringing fresh oxygen to every single cell so they can stay alive and do their jobs.
[6:56] SPEAKER_00: You got it! And once it drops off that oxygen, it turns dark red and tired again and rides the tubes back up to the right side of the heart to start all over.
[7:06] SPEAKER_00: But what exactly are these tubes made of?
[7:09] SPEAKER_00: And how in the world do we have 60,000 miles of them if the aorta is as thick as a garden hose?
[7:16] SPEAKER_00: That's where it gets amazing.
[7:18] SPEAKER_00: The tubes are called blood vessels, and they come in three main types that get smaller and smaller.
[7:25] SPEAKER_00: First, you have the arteries.
[7:27] SPEAKER_00: Do arteries take the blood away from the heart?
[7:30] SPEAKER_00: Yes.
[7:31] SPEAKER_00: Arteries carry blood away from the heart.
[7:34] SPEAKER_00: Most of them carry high-speed, oxygen-rich blood, so they have thick, stretchy walls made of muscle to handle the intense pressure of the blood rushing through.
[7:44] SPEAKER_00: Okay, so arteries take the blood out.
[7:47] SPEAKER_00: What brings the tired blood back?
[7:50] SPEAKER_00: Those are called veins.
[7:51] SPEAKER_00: Veins are thinner, and because the blood is moving slower on the way back, veins have a special trick.
[7:58] SPEAKER_00: They have little one-way doors inside them called valves.
[8:02] SPEAKER_00: Why do they need doors?
[8:04] SPEAKER_00: Gravity! Imagine the blood trying to travel all the way up from your foot back to your chest.
[8:10] SPEAKER_00: The valves snap shut so the blood doesn't accidentally flow backward down into your toes.
[8:16] SPEAKER_00: That's genius! But wait, how does the blood get from the arteries into the veins?
[8:22] SPEAKER_00: Through the third type of tube called capillaries.
[8:25] SPEAKER_00: These are the craziest part of the whole system, and they are the reason you have 60,000 miles of vessels.
[8:32] SPEAKER_00: Why?
[8:33] SPEAKER_00: What's so crazy about a capillary?
[8:35] SPEAKER_00: They are incredibly tiny.
[8:37] SPEAKER_00: They are microscopic.
[8:39] SPEAKER_00: They are so incredibly thin that your red blood cells actually have to line up and squeeze through them single file.
[8:47] SPEAKER_00: Single file?
[8:49] SPEAKER_00: Like kids lining up at the door for recess.
[8:52] SPEAKER_00: Exactly.
[8:53] SPEAKER_00: They are so tiny they can weave through every single piece of tissue in your body.
[8:59] SPEAKER_00: That's why if you get a tiny paper cut anywhere, even on the very tip of your pinky, you see a drop of blood.
[9:06] SPEAKER_00: The capillaries are everywhere.
[9:08] SPEAKER_01: So if you laid all those microscopic tubes out, that's the 60,000 miles that wraps around the Earth twice.
[9:15] SPEAKER_00: Yes, and even with all that distance, a single drop of blood can make the entire round trip journey, from the heart to your big toe and back again in less than 60 seconds?
[9:28] SPEAKER_00: Less than a minute?
[9:29] SPEAKER_00: That is the fastest roller coaster ride in the universe.
[9:33] SPEAKER_02: So if the blood travels that fast when I'm just sitting here talking to you, what happens when I'm outside running around?
[9:40] SPEAKER_00: When you run, your leg muscles are working much harder.
[9:44] SPEAKER_00: They burn through oxygen super fast, so they send a panic signal to your brain saying, we need more oxygen right now.
[9:53] SPEAKER_00: And the brain tells the heart to step on the gas.
[9:56] SPEAKER_00: Exactly.
[9:57] SPEAKER_00: Your heart starts beating much faster and harder.
[10:00] SPEAKER_00: Normally, an adult heart pumps about five litres of blood every minute.
[10:05] SPEAKER_00: But when you exercise really hard, it can pump three to four times that amount.
[10:11] SPEAKER_00: Whoa! It goes into overdrive! Does every animal's heart work at this exact same speed?
[10:17] SPEAKER_00: The system is similar, but the speeds are totally different depending on the size of the animal.
[10:23] SPEAKER_00: Usually, smaller creatures have faster heart rates.
[10:26] SPEAKER_00: For example, a tiny shrew has a heart that beats up to a thousand times a minute.
[10:32] SPEAKER_00: A thousand times a minute?
[10:34] SPEAKER_00: That sounds like a buzzing bee! Haha, it does.
[10:38] SPEAKER_00: And even humans change as they grow.
[10:41] SPEAKER_00: When you were a tiny baby, your heart beat much faster than it does now, up to 150 times a minute.
[10:49] SPEAKER_00: Babies are always in overdrive.
[10:51] SPEAKER_00: What about huge animals?
[10:53] SPEAKER_00: Let's think about the biggest animal on Earth, the blue whale.
[10:57] SPEAKER_00: A blue whale's heart is completely gigantic.
[11:00] SPEAKER_00: It's about the size of a golf cart and weighs around 400 pounds.
[11:05] SPEAKER_01: A 400-pound golf cart inside its chest?
[11:08] SPEAKER_00: Yes, and because it's so massive, it beats incredibly slowly.
[11:13] SPEAKER_00: When a blue whale dives deep down into the cold ocean, its heart might only beat two times in a whole minute.
[11:20] SPEAKER_00: Just two times?
[11:22] SPEAKER_00: That is so slow it's barely even moving.
[11:25] SPEAKER_00: But every single beat pushes dozens of gallons of blood through massive vessels.
[11:30] SPEAKER_00: In fact, a blue whale's aorta is roughly the width of a dinner plate.
[11:35] SPEAKER_00: Wow.
[11:36] SPEAKER_00: The human heart might be much smaller, but it's still pretty amazing.
[11:40] SPEAKER_00: It is.
[11:41] SPEAKER_00: Your heart will beat over two and a half billion times in your average lifetime.
[11:46] SPEAKER_00: It never takes a vacation.
[11:48] SPEAKER_00: Two and a half billion?
[11:50] SPEAKER_00: I'm going to make sure I run and play a lot to keep it strong, since it works so hard for me.
[11:54] SPEAKER_00: Alright, Finn, it's quiz time! Let's see how much you remember about the amazing human heart.
[12:01] SPEAKER_00: I've got my thinking cap on.
[12:02] SPEAKER_00: I'm totally ready.
[12:04] SPEAKER_00: Bring it on.
[12:05] SPEAKER_00: First question.
[12:07] SPEAKER_00: About how many times does the human heart beat every single day?
[12:12] SPEAKER_00: Next one.
[12:16] SPEAKER_00: What is the name of the English doctor who discovered in the 1600s that blood moves in a continuous loop?
[12:27] SPEAKER_00: Question three.
[12:29] SPEAKER_00: Which type of blood vessel carries blood away from the heart?
[12:35] SPEAKER_00: Here's number four.
[12:37] SPEAKER_00: How small are the tiny microscopic blood vessels called capillaries?
[12:45] SPEAKER_00: Last question.
[12:47] SPEAKER_00: If you lined up all the blood vessels in a human's body, how many miles would they stretch?
[12:56] SPEAKER_00: Great job! Let's go through the answers and see how you did.
[13:01] SPEAKER_00: The first answer is your heart beats about 100,000 times every single day.
[13:08] SPEAKER_00: For question two, the English doctor who figured out the loop was William Harvey.
[13:14] SPEAKER_00: Number three, the answer is arteries are the blood vessels that carry blood away from the heart.
[13:21] SPEAKER_00: Question four.
[13:22] SPEAKER_00: Capillaries are so incredibly tiny that red blood cells have to travel through them single file.
[13:29] SPEAKER_00: And the final answer is they would stretch for an amazing 60,000 miles.
[13:35] SPEAKER_00: Enough to wrap around the Earth twice.
[13:37] SPEAKER_00: That was an awesome episode.
[13:39] SPEAKER_02: I still can't get over the fact that there are 60,000 miles of tubes secretly hiding inside of me.
[13:46] SPEAKER_00: It really is mind-blowing.
[13:48] SPEAKER_00: Science is amazing because it makes you look at yourself and the world in a totally different way.
[13:53] SPEAKER_00: Everything is more complex than it seems.
[13:56] SPEAKER_01: Hey, if you're listening right now and you thought the single file blood cells were as cool as I did, will you leave us a five-star review?
[14:05] SPEAKER_00: Yes, please do.
[14:06] SPEAKER_00: Leaving a review really helps the show.
[14:09] SPEAKER_00: Just scroll down on whatever app you're using right now and tap those five stars.
[14:13] SPEAKER_01: And don't forget to head over to our site kidopoly.com.
[14:17] SPEAKER_00: We've got tons of fun learning games and cool activities there to keep the science fun going.
[14:23] SPEAKER_00: That's right, our site, kidopoly.com, is packed with awesome stuff.
[14:27] SPEAKER_00: And if you want your name shouted out on the show or just want to tell us your favourite weird fact, send us an email at hello at kidopoly.com.
[14:36] SPEAKER_00: Keep your heart healthy, keep exploring, and keep asking big questions.
[14:42] SPEAKER_00: See you next time on Science is not boring.
[14:45] SPEAKER_00: Bye everyone.

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