Are you pumped for today’s colorful Wonder of the Day? We hope so, because it’s a real life saver. What are we talking about? Blood, of course!

All parts of your body need blood. The movement of blood through your heart and around your body is called circulation.

When blood leaves your heart, it moves through tubes called blood vessels. There are three types of blood vessels: arteries, veins, and capillaries.

Arteries carry blood away from the heart to other parts of your body. As they get farther and farther away from the heart, arteries transfer blood to tiny blood vessels called capillaries. Capillaries transfer the oxygen and nutrients in the blood to cells that need it.

As cells use the oxygen and nutrients in blood, they also produce waste, including carbon dioxide. The capillaries take the carbon dioxide and other waste materials and transfer them to veins, which carry these items in the blood that returns back to the heart.

When the blood returns to the heart, the heart pumps it to the lungs. In the lungs, carbon dioxide is taken out of the blood and exhaled back into the air. With each new breath, oxygen enters the lungs and restores the blood before it is pumped back out to the body’s cells. Then the process starts all over again.

How long does this whole process take? Not long at all! Your heart can pump blood to all the cells in your body in less than 60 seconds. That’s under a minute!

Since arteries carry oxygen-rich blood away from the heart and veins carry oxygen-depleted blood back to the heart, some people believe that veins appear blue because blood without oxygen is blue. But it’s not true!

Blood is always red. Oxygen-rich blood is bright red as it leaves the heart. When it returns in veins without much oxygen, it’s still red, but it’s a deeper, darker red. So why do veins look blue?

It all has to do with the science of light. The colors we see are the result of which wavelengths of light are reflected back to our eyes. Veins appear blue because blue light is reflected back to our eyes.

This may seem odd, since veins contain deep, dark red blood. Scientists believe there are several factors that lead to our seeing veins as blue rather than red.

Blue light does not penetrate human tissue as deeply as red light does. As a result, veins that are close to the surface of the skin will be more likely to reflect blue light back to the eye.

The oxygen-depleted blood that veins carry also tends to absorb red light more than blue light. When the dark red blood absorbs more red light, blue light is more likely to be reflected back to our eyes.

Scientists also believe that blue light tends to scatter and reflect more easily when it passes through human tissue. This also contributes to veins appearing blue despite the dark red blood within them.

132 Join the Discussion

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  1. WOW! I had no clue that blood was always red! I have always thought that blood was blue because there was no oxygen in it. Now I know that it really is a darker red. Now I can tell people the real truth!:) I don’t really understand how the light reflects blue. Can you help me understand that? I think wonderopolis is awesome!

  2. Hi, wonderopolis, good video. Fyi, love your information. My class will go crazy because I commented again and I mean IT. SO GOT YOU, Mrs. Kennedy. #1 website, loved the video.

  3. That was pretty cool, I loved it…..In fact it’s funny that y’all did this article, because my class happens to be learning bout it now….

    • We think that’s a super COOL coincidence that you and your classmates are learning about blood in school, Lavell! Thanks for sharing your comment with us today! :-)

  4. OK, I don’t get it… I know that blood cells are blue and when oxygen gets in the blue blood cell then it turns red.. but I don’t know why our veins are blue that is like really weird… OK so can you explain it for me… thanks… bye…

    • We’re glad you stopped by this Wonder of the Day®, Kamryn! Thanks so much for sharing your comment with us to let us know you thought it was cool! :-)

    • Hi there, Wonder Friend! As we age, our skin gets more wrinkles and loses some of its elasticity, allowing for a more prominent view of our blood vessels and veins. That’s why the elderly lady’s hands in the still picture look the way they do, and why we chose to use the picture along with today’s Wonder of the Day® about veins. We think her hands tell a WONDERful story of her life! Think of all the awesome things she has experienced and all the WONDERS of the world around her she has seen! :-)

    • Hi, Alex! We think a lot of Wonder Friends would agree with you, but we encourage you to re-visit today’s Wonder to learn why it only “appears” that the blood in our veins is blue! Thanks so much for sharing your awesome comment with us today! :-)


    • Thanks for sharing all the things you know about blood and veins, Josh! We’re super sorry to hear that you get sick and nauseous sometimes. We do, too, and we think being sick is NOT our favorite thing! We can’t wait to see what tomorrow’s Wonder is about, and we hope you can’t, either! :-)

    • Hello, Jared! Thanks so much for checking out Wonderopolis! Did you know that blood cells are always red, not blue? The reason our veins “appear” blue is because of the light that is reflected off of them! We encourage you to re-explore today’s Wonder to learn more about this cool phenomenon! :-)

    • We appreciate you sharing what you think about blood and veins, Shirley! Thank you for visiting today’s Wonder of the Day®! :-)

  6. WHAT MAKES THE VEIN BLUE? I LIKE THE video, it was cool. I LEARNED THAT veins are not blue.
    See you soon.

    • We’re so glad you shared what you learned by visiting today’s Wonder, Eric! Thank you so much! We hope you have a WONDERful day! :-)

    • We’re glad you’re WONDERing even more about blood and veins today, Rylan! Thanks for sharing your comment with everyone in Wonderopolis! :-)

  7. Dear Wonderopolis,
    I think tomorrow’s wonder is about a cookie. That person’s hands look weird. Ewwwwwwwwwww!!!

    • Hi, Claire! Thanks so much for sharing what you thought about the still image for today’s Wonder! Our skin becomes less elastic as we age, which makes it look more wrinkly and lets us see our blood vessels and veins even better. That’s why we picked that picture…we think it goes GREAT with today’s Wonder! :-)

    • Hello, Alexis! We think it’s super neat that you can see your veins, but please be careful when you are bending and squeezing your finger! We care about all of our Wonder Friends, and we don’t want you to get hurt! :-)

    • That’s a WONDERful question, Selina! We think it would make a SUPER future Wonder of the Day®…thanks so much for suggesting it! :-)

    • You’re absolutely right, Shirley! Way to go! We appreciate your comment very much and are so happy you visited today’s Wonder! :-)

  8. Your blood is really blue, but when it comes out, it changes to red. How do elephants pick up peanuts with their trunks?

    • Hi, Taylor! You might be surprised to learn that blood cells are always red! We encourage you to re-visit this Wonder about blood to find out even more about why veins “look” blue! Thanks for your suggestion of a Wonder about how elephants pick up peanuts with their trunks…we think that’s AWESOME! :-)

  9. I think that veins are blue because your body wants to look cooler!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    • We like that way of thinking, Tyler! Thanks for sharing your views about veins with us today…we think your comment ROCKS! :-)

    • Hello, Jordan! We hope you will explore today’s Wonder a little further to find out why blood is always red and veins only “appear” blue! Have a WONDERful day! :-)

    • We know a lot of Wonder Friends think that about blood and veins, Tiarra, but did you know that blood is always red? It only “looks” blue when we see our veins under our skin! We hope you will re-visit today’s Wonder to find out why! :-)

  10. I don’t get how blue light reflects off of your veins. Why is your blood red when it hits air? Why is it blue when it’s in your body?

  11. I really do not get why veins are blue and blood is red. I would like to hear a little more so that way I can understand it a little more. This is a really good wonder, by the way.

  12. Hi there, Kaitlyn and Zoey! Thank you BOTH so much for your awesome comments! We appreciate them!

    Even though blood cells are always red, the blood in our veins “appears” blue because of the way our eyes “see” the light waves that bounce off of it. You can read more about how this works by visiting another Wonder that explains about light waves:

    Past Wonder #306 – Why Is the Sky Blue? :-)

    • That’s a clever way to think about it, Johnathan, but did you know that blood cells are always red? We invite you to re-explore today’s Wonder to learn more about why veins only “appear” blue to our eyes (hint: it has something to do with light waves reflecting). Thanks so much for your GREAT comment today! :-)

  13. Hi,
    The reason blood is red and veins are blue because in side your body blood is blue, but when it contacts air the chemicals turn it red.

    • That’s what a LOT of Wonder Friends think, Ben, but re-explore this Wonder of the Day® and learn why blood cells are always red…not blue! Thanks so much for your comment today! :-)

  14. How come there’s no video about this wonder of the day???? Also I agree with Taylor, I think you should do a wonder of the day about how elephants pick up peanuts with their trunks!!!!!!!! I also think that Wonderopolis is really cool!!!!!!!!!!!!!!!!!

    • Hi, Callie! Thanks for letting us know you think Wonderopolis is cool! We’re super sorry you aren’t able to see the video for today’s Wonder! Are you visiting Wonderopolis at school? If so, it might help you to know that some schools and school districts place an internet “block” on videos from certain websites because they are trying to protect their students. You might want to check with your teacher to ask if he or she might be able to get the block removed so you and your classmates can see all the videos on Wonderopolis. :-)

    • Hi, Brandie! Thanks for leaving us a comment to let us know you visited today’s Wonder of the Day®! We sure hope you liked learning in Wonderopolis today! :-)

  15. Wonderopolis,
    I think today’s wonderopolis quite interesting! I REALLY, REALLY, REALLY, REALLY want a wonderopolis about WOLVES. I love wolves!

    • Thanks so much for stopping by today’s Wonder and for leaving us this great comment to let us know you enjoyed exploring it, Aleia! :-)

    • Thanks for sharing your feelings about today’s Wonder, Caelah! We appreciate your opinion very much! We also like your idea for a future Wonder about wolves…thanks so much for suggesting it! :-)

  16. That old lady looks ewwwww! With blue puffy veins. I wish there was a wonder about Selena Gomez. I think it is cool how your body and blood work together so you can breathe. I liked learning about how the oxygen hits the red blood so it turns blue and it becomes a vein. The video was awesome. I have not looked at all the things on Wonderopolis, but is there any wonders about the future? I love thinking about what it would be like. I think that by the time I am a teenager, there will be flying cars. Did you know that someone has invented a robot that can walk and talk and is programed to do stuff, like if you want a glass of milk it will go and get it for you. I loved this wonder of the day! BBBBBBBBBBBBBBBBBYYYYYYYYEEEEEEE!!!!

  17. One day, all of our hands are going to look like that, so we should not make fun of the woman in the picture. Her hands remind me of my grandma’s hands.

    • You’re absolutely right, Eric! We think they are WONDERful hands, because they are the hands of someone who has lived a long life and seen many amazing things! They are the hands of someone who has loved, laughed, and learned! Thank you for sharing your point of view about the hands in the picture for today’s Wonder…we think you ROCK (and we know your grandma thinks that, too)! :-)

  18. WOW! I always thought that the cause was that when you look at your blood through your skin it was blue but when you cut your hand it would come out red because of the oxygen that is hitting it. Thanks for this wonder! See ya soon!

    • Thanks so much for letting us know you learned something new by visiting today’s Wonder, Grant! We think that’s AWESOME! :-)

    • WOW, Leah, that’s really COOL! We’re glad you had such great background knowledge about blood and veins before you stopped by this Wonder today! Please tell your dad that we appreciate his service to our country! :-)

  19. Absolutely terrific site you have here. Thank you for taking the time to respond to all the great student comments. I WONDER how you are able to do it so fast!!!!!!

  20. Hi!
    I asked my nurse that when I was in the hospital and she had no idea why. My blood is blue. Is that normal?
    Food Allergy Girl

    • Hi, Food Allergy Girl! Thanks so much for commenting on today’s Wonder and sharing about your experience! We’re sorry to hear that you were in the hospital. We hope you are feeling better! :-)

  21. I appreciate your Dad’s service to our country, Leah. God be willing he shall remain safe. I am praying for you along with your family. My Grandfather (Grandpa Chuck) died along with many others in the Vietnam war. May he R.I.P. I love you<3

  22. wow! I did not know that veins were red and not blue. I love wonderopolis because it always informs me of cool facts. Maybe I can use one on a test some day. Thanks, Wonderopolis.

  23. Hi Wonderopolis!
    This is so cool! I never knew that blood was always red! I though that it was blue inside your body but when it reaches oxygen then it immediately turns red!
    Thanks so much! I LOVE learning new things from Wonderopolis!!!

  24. I am learning about the body at the moment, so it sort of gave it away, but it’s interesting anyway. Thanks, Wonderopolis!

    • Hi, Rusty! We’re glad to hear that you are studying about the human body! It’s WONDERful FUN to learn all the cool ways our bodies work, isn’t it? :-)

    • You’re welcome, Ryco! Thanks so much for visiting Wonderopolis today and leaving us this GREAT comment to let us know you learned something new! :-)

  25. Dear wonderopolis,
    Awesome wonder! It has something to do with the air you breathe. I think tomorrow’s wonder is about bugs.
    Paige 😉

    • We hope you had a GREAT day, Paige! Thanks for making ours a little more AWESOME by leaving us this comment! :-)

    • That’s really AWESOME, Kaya! Thank you for sharing your personal connection to this Wonder of the Day®! Please tell your dad that we said, “hello!” :-)

  26. I understood that veins look blue because they reflect blue wavelengths. But, please tell me what would be the color of veins without blood?

    • Thanks for WONDERing even MORE about veins, Amoolya! We’re not sure what color veins would be without blood. We’ll BOTH have to do some more thinking and searching to find the answer to that question! :-)

    • Thank you for sharing how you feel about the still picture for this Wonder, Emily! We appreciate and respect your opinion! :-)

    • Thanks for sharing what you think about the picture for this Wonder of the Day®, Lauren! As we get older, our skin becomes less elastic and lets the veins in our body show a bit more. That’s why we chose the photo…we think it’s a GREAT one to accompany this Wonder about veins! :-)

  27. I think this was a GREAT wonder!!! But I want people to stop being so mean to that lady’s hands! Our hands might look like that someday!!! Just think about the wonderful places she’s been and all the interesting things she’s touched! Blood is blue and when you get cut it meets the air and turns red. Lobsters blood is blue.

    • We really appreciate your comment, Anne! Thanks for imagining the life the lady in the picture has lived and all the WONDERful things she has experienced in this world! :-)

    • We’re sorry you can’t visit Wonderopolis at school, Jaidyn, but we’re REALLY happy that you visited us from home! We hope you learned some interesting new facts about veins and blood today! :-)

    • Happy Friday, “Mystery Person!” We’re so glad you left us this comment today and we hope you have an AWESOME weekend! :-)

    • Hello, Wonder Friend Class! Your awesome question is what this Wonder of the Day® is all about! We invite you to re-explore this Wonder to learn what color veins are! :-)

    • We think YOU are COOL for checking out this Wonder of the Day®, Mya! Thank you! We hope you learned some fun facts! :-)

  28. My Classmates Told Me That Our Blood Is Red When School Was Still Going On. THIS WONDER HAS PROVEN THEM WRONG.

    • We’re so glad that you were able to find information that supported your opinion, Paul! Way to go! Virtual high fives being sent your way! Thanks for WONDERing with us today! :)

  29. So is that why in the video game Halo the aliens blood is blue maybe because they don’t breathe oxygen or just the light reflecting to make it look blue?

    • Hi soccer news transfers! Thanks for WONDERing with us today! We are glad you enjoyed visiting WONDERopolis. There is a new WONDER posted everyday, so be sure to check back very soon! :)

    • We’re glad you liked this WONDER, jaidyn brooks! Thanks for WONDERing with us on this WONDERful Wednesday! :)

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Have you ever wondered…

  • If blood is red, why are veins blue?
  • How long does it take your heart to pump blood throughout your entire body?
  • What are the three types of blood vessels?

Wonder Gallery

dreamstime_xl_792536 customdreamstime_xl_19337291 customdreamstime_xl_20467239 customhands-of-old-woman_shutterstock_72428473Vimeo Video

Try It Out

Want to learn more about the circulatory system? Invite a friend or family member to help you check out the following fun activities:

  • Watch this interesting online presentation about the circulatory system to see exactly how the heart and its many parts work to pump blood throughout your body.
  • If you haven’t been to the doctor for a check-up in a while, ask your parents to schedule a check-up for you. Make sure you have the doctor or nurse help you understand how your blood pressure is measured. Is it low, high or just right? Alternatively, you may be able to take a field trip to a local pharmacy that has a blood pressure measurement machine. Ask an adult to help you use the machine to check your blood pressure. Keeping an eye on your blood pressure in the future will help you stay healthy!
  • Talk to your parents about your medical history. Has anyone in your family ever had heart problems? To avoid problems of your own in the future, you should always do what you can to keep your heart healthy and working like it should. Using the pointers below, create your own “Happy, Healthy Heart Plan” to follow in the future. Here are some things you can do to keep your heart happy:
    • Your heart is a muscle. Like your other muscles, it’s important to exercise it. But how do you exercise your heart? You do cardiovascular exercises, like running, biking or jumping, that make your heart beat faster.
    • Eat heart-healthy foods, like fresh fruits and vegetables. Avoid unhealthy foods that are high in saturated or trans fats.
    • Don’t smoke! Research has shown that smoking damages the heart and your blood vessels, so don’t ever start smoking!

Still Wondering

Children know about many different parts of the body — from organs to bones to blood. In Science NetLinks’ All Systems Go! online interactive activity, children learn about the concept of separate body parts working together to build a body system.


Test Your Knowledge

Wonder What’s Next?

Gulp! Tomorrow’s Wonder of the Day will really give you something to chew on.

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