Sunday, November 27, 2005

The Color of Nerves

This is a letter I received from N., whose national identity clearly weighs heavily, asking a great question that reminded me that there are lots of people who do NOT see parts of the human body exposed all day long.

My name is N. and I come from Austria not Australia! I would like to learn what colour nerves are.I am studying the nervous system and I know that we have trillions and trillions of nerves in our body.I hope you have an answer to my question.

So I told her:

Thanks for clarifying where you're from. People must get that confused a lot, and expect you to know a lot about kangaroos and things. How bothersome.

We certainly do have a lot of nerves in our bodies. Far too many for anyone to count. Most of them are so tiny that you can't see them at all, but the ones that you can see are a very pale yellow, almost white! I wish that things in the body were as clear in the operating room as they are in the books, but in reality it can be really hard to tell what's what, since most things are sort of pink/red and they are all stuck together. We have to be very careful to make sure that we don't cut things we're not supposed to, obviously, so we dissect very carefully to find all the nerves, arteries, and veins. The nerves look and feel a lot like rubber bands, while the arteries feel like little garden hoses, and the veins feel like floppy tubes.

Sometimes we have to cut nerves in order to be able to do the surgery properly and make the patient better. What do you suppose patients with cut nerves feel when they wake up? Actually, most of the time, they never even notice! There are so many nerves that even if you cut some of them, the skin is still covered with plenty of others. But sometimes, people have a little bit of numbness over the skin where we cut the nerve. Even then, other nerves will sometimes grow into that area and the numb feeling will go away with time. Pretty amazing, huh?

I hope that helps to answer your question. Please let me know if you have any more questions, and I'll do my best to answer them too!

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 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!

Heart-lung Machines

This is an excellent question from G., in Germany:

I have a question if you would have the time to answer it I would
really appreciate it.If you take out your heart in a operation how are
you going to live ? Are they putting a fake heart inside your body?
When they are changing it how does your blood get oxygen?

To which I say:

Dear G.,

What an excellent question! We take hearts out when we transplant them, which means that we put another one back in to replace the heart that isn't working. That happens pretty fast, so people don't go without a heart for very long. But for the short time (usually about three hours) that their heart is disconnected from their veins and arteries, and isn't pumping blood around their bodies anymore, we have a machine called a heart-lung machine that does the work of the heart for them. It's really cool.

The machine connects to the biggest artery that comes out of the heart, which is called the aorta, and to the veins that come back to the heart, which are called the vena cavas. It takes the blood from the veins, puts oxygen into it, just like the lungs do, and then puts the blood with the oxygen in it back into the aorta, where it gets pumped back to the rest of the body. So it does the same job as the heart and the lungs while the patient's heart is being operated on.

You couldn't have the machine on when you're awake and walking around, though, because it is very big and would hard to move around with. Here's a picture of what it looks like in the operating room:

Can you imagine having to have that whole machine and the guy to run it following you around when you went out to the grocery store?! So for the moment, you still need to have a heart in your body most of the time. We are working on making artificial hearts for people whose hearts don't work and who can't get a transplanted heart from another person, but so far they don't work nearly as well as the real thing!

I hope that helps to answer your question! Let me know if you have more questions. It would be a pleasure to do my best to answer them.