The idea for this week's experiment came to me while I was moving our stuff into the hotel where we will be staying for the next 3 weeks. The room is on the second floor and we do not travel light. Basically we take everything except the kitchen sink. We would probably take that as well, but it is too hard to disconnect. After the 14th trip up the stairs, I started to think about muscles and how they get tired. One thing lead to another, and soon I had this week's experiment written. Why do your muscles get tired? And more important, why does your heart, which is also a muscle, not get tired? To find out, you will need:

- you

Your heart is a group of muscles which pump the blood through your body. To get an idea of its size, make a fist. That is about the same size as your heart. Pretending for a minute that your fist is your heart, lets take the idea a little farther. Open you hand about half way and then close it again. If you do that over and over, you can imagine that it is your heart beating. You can even make heart sounds (bump-bump) if you (bump-bump) want to. (bump-bump)

Keep this pretend heart beating as you read this. Soon your hand will begin to get tired. If you keep opening and closing your hand even after you are tired, it will begin to hurt. Why?

When you move your muscles, a chemical reaction takes place. Normally, this chemical reaction needs oxygen. We get this oxygen when we breath. The air moves into your lungs and the oxygen is absorbed by your blood. Your blood carries the oxygen to your muscles. As long as the muscle has plenty of oxygen, everything is fine and it can keep on moving.

If the muscle uses up oxygen faster than the blood can deliver it, then what happens? The muscle does not instantly shut down when the oxygen runs out. Instead, a different chemical reaction takes over. It lets your muscles move even if they do not have enough oxygen. The problem with this backup system is that the reaction makes a chemical called lactic acid. This acid irritates the muscle, making it hurt. If you overdo it, your muscles will be sore the next day. Keep overdoing it and you can damage the muscle.

If your heart is made of muscle, why doesn't it get tired? After all, your heart beats all day and all night, for your entire life. A large part of the answer has to do with blood. Your heart is between your lungs. Blood picks up oxygen from the lungs and flows directly to the heart. This insures that the heart always has plenty of oxygen, so it does not get tired. The one exception is if the blood vessels that lead to your heart get blocked. Then the heart muscles run low on oxygen and get tired. The pain that you feel is what tells you that you are having a heart attack.

Athletes exercise regularly to increase the blood flowing to their muscles. If the muscles get more blood, they get more oxygen. Then they can work harder and longer before they get tired. Right now, I could use some extra oxygen, so I could unload the rest of our stuff.

This is Members Only content.

Become a Member Today at the Introductory Price of
Only $20 for an Entire Year, and get full access to this site.

Finger Feedback

An experiment in biofeedback.

This is Members Only content.

Become a Member Today at the Introductory Price of
Only $20 for an Entire Year, and get full access to this site.

Looking into Your Eyes

Can you really see the inside of your own eye? Of course you can.

This is Members Only content.

Become a Member Today at the Introductory Price of
Only $20 for an Entire Year, and get full access to this site.

Seeing Spots

Look at blood cells without a microscope.

This is Members Only content.

Become a Member Today at the Introductory Price of
Only $20 for an Entire Year, and get full access to this site.

Scratch Science

Are you just itching to learn about histamines?