Pulmonary Circulation

This is a letter I received from a fourth-grader named C. , asking a question that has plagued legions of scholars through the years and was only recently made clear:

My name is Carmella.I would like to ask you if you could tell me why the pulmonary artery is blue and the pulmonary veins are red . When the veins are blue they take the blood back to the heart and the red arterys take the blood to all the parts of the body.

And my response:

Dear Carmella,

This is a great question! To understand the answer, it is important to know that when you see real arteries and veins, they don't look red and blue like they do in books. The actually all look sort of purple, and it can be hard to tell which ones are arteries and which ones are veins. The way we tell in the operating room is by putting our finger on them...the arteries are firmer, and you can feel a pulse in them, while the veins are softer and don't pulse. Why do you suppose that is? I'll give you a hint: it's because of all the capillaries.

Anyway, you're totally right that arteries take blood away from the heart, and veins take blood back to the heart. What the red and blue colors on your picture tell you is not whether something is an artery or a vein, but whether that blood is carrying oxygen or not. When the blood goes from the heart to the rest of the body through the arteries, it carries oxygen to the tissues, so those arteries are colored red. And when it comes back to the heart from the body, the tissues have taken the oxygen, so the blood doesn't have oxygen in it and we color them blue. So how does the blood get oxygen again, to take back to the tissues? I bet you know the answer...it goes to the lungs.

So when the blood leaves the heart to go to the lungs, it is going away from the heart, which means it is going in an artery - the pulmonary artery - but it doesn't have oxygen in it - so the pulmonary artery is colored blue in diagrams. Then, when it returns from the lungs through the pulmonary veins, it DOES have oxygen in it, so it is colored red. It's exactly the opposite of the way the blood flows in the rest of the body, which is why we talk about there being two different circulations, the pulmonary and the systemic, and why there are two different pumps within the heart, the right side pump that pumps blood from the body to the lungs, and the left side pump that pumps blood from the lungs to the body.

It's complicated, isn't it! I think the best way to understand it is to draw a picture or make a model, and trace the route of a blood cell from the time it gets oxygen, through the tissues where it delivers its oxygen, to when it returns to the lungs to get more oxygen. That's how I figured it out!