Hi! My name is Ruby, and I am a red blood cell! I travel through the human body doing a very important job: delivering oxygen to cells that need it.

Today. I'm getting ready for another exciting adventure! My first stop? The lungs, where I'll pick up my good friend, Orion the Oxygen!

Come along and see where our journey takes us!

WHEEEE! THIS IS SO MUCH FUN!. I'M ON MY WAY, ORION!

I take the fastest way to Orion. The Pulmonary Artery Express! It's just like a big waterslide! Right now, I'm carrying very little oxygen, so I'm traveling in low-oxygen blood. But don't worry! I'm heading to the lungs to pick up Orion! 

Ruby! I've been waiting for you!

READY FOR OUR NEXT ADVENTURE, ORION?

I finally made it to the lungs! Here in the lungs are millions of tiny air sacs called alveoli. This is where something AMAZING happens! Oxygen moves from the alveoli into tiny blood vessels called capillaries. 

Orion hops aboard, and now I'm carrying oxygen! Let's head back to the heart!

Yes! I can't wait!

ORION! I'M HERE! LET'S GO! 

We travel through the pulmonary veins, carrying oxygen. That means we're traveling in oxygen-rich blood! "Look, Orion! We've made it to the left atrium!" I say. "The left atrium? What's that?" Orion asks. I"It's one of the heart's four chambers! But we're not done yet. We still have more adventures ahead of us!

We made it to the left atrium, Orion!

What's an atrium?