Two loops, one heart
Your blood travels in two big loops, like a figure eight with the heart in the middle.
The short loop goes from the heart to the lungs and back. This is the lung loop. Its science name is pulmonary circulation. Pulmonary means having to do with the lungs.
The long loop goes from the heart out to the whole body and back. Its science name is systemic circulation (sis-TEM-ik), because it reaches every system in your body.
Blood goes around the lung loop, then the body loop, then the lung loop again, over and over.
The trip to the lungs
Blood comes back from the body low on oxygen. It flows into the right side of the heart.
The right ventricle squeezes and sends it into the pulmonary artery. That artery splits in two, one branch for each lung.
This artery carries oxygen-poor blood, unlike most arteries. It is still an artery, because it carries blood away from the heart.
The big swap
Inside your lungs are hundreds of millions of tiny air sacs called alveoli (al-VEE-uh-lie). They look like tiny bunches of grapes, and each one is wrapped in a net of capillaries.
When you breathe in, air fills the sacs. Oxygen slips out of the air, through the thin walls, and into your blood.
At the same time, carbon dioxide, a waste gas your cells make, slips out of your blood and into the air sacs. You breathe it out.
Breathe in oxygen, breathe out carbon dioxide. That swap is the whole point of the lung loop.
Dark red to bright red
Blood going to the lungs is dark red, because it has given away most of its oxygen. Blood coming back from the lungs is bright red, full of fresh oxygen.
The color comes from red blood cells. When they hold lots of oxygen, they turn bright cherry red. When they have less, they are a deep, dark red.
Remember: blood is never blue. It just changes from one red to another.
Back to the left side
Fresh, bright red blood flows back to the heart through the pulmonary veins. They carry oxygen-rich blood, the opposite of most veins.
They empty into the left atrium. From there, the blood drops into the left ventricle, which pumps it out into the body loop.
The lung loop is short, so the right side of the heart does not have to push very hard. That is why its wall is thinner than the left side's.
Try it: take a big, slow breath in, hold it for a moment, and breathe out. With that one breath, your lung loop loaded oxygen into millions and millions of red blood cells.