Key concepts

Cardiovascular system
Energy and metabolism

From National Science Education Standards: Personal health

When you exercise, do you notice that you get out of breath? What about feeling your heart rate—your pulse—increasing? These two changes are not coincidental—they are both important, and natural, reactions of your cardiovascular system to exercise.

From your brain down to your fingers and toes, your body needs plenty of oxygen to keep going. That oxygen is carried through your body in the bloodstream. Blood is pumped through the heart and picks up oxygen as it passes by the lungs. 

Let's track your heart kicking it into high gear during exercise. But don't get moving just yet; first we need to count your resting heart rate.

When you are exercising, your muscles need extra oxygen—some three times as much as resting muscles. This need means that your heart starts pumping faster, which makes for a quicker pulse. Meanwhile, your lungs are also taking in more air, hence the harder breathing.

So, getting out of breath while exercising is just a sign that your muscles are working. The more you exercise, the more efficient your body will be at getting oxygen to your muscles, so you can exercise more without getting out of breath. Of course, pushing exercise too hard can be dangerous and, if you feel faint, you should stop the activity.

•    Stopwatch or timer with a second hand
•    A person who has been relaxing for at least 15 minutes
•    Room to do jumping jacks
•    Pencil and paper

•    Start this activity well rested (sitting down to read for 15 minutes or so should do the trick).
•    Have a stopwatch handy.
•    Note: Be sure to drink plenty of water when you exercise. All of that work makes your body lose water through sweat—as well as moisture that is exhaled when you're breathing quickly.

•    While you are still sitting, put two fingers (not your thumb, which has its own strong pulse) on the underside of your wrist. Can you find your pulse?
•    Count the number of heartbeats you feel for 30 seconds. Write that number down and multiply it by two. That's your resting heart rate: the number of times your heart beats every minute when you are not moving much.
•    Notice your breathing. How many breaths are you taking every minute?
•    Now, get ready to get moving! Make sure you have room enough for jumping jacks, and keep that stopwatch handy.
•    Do 20 jumping jacks (or as many as it takes to get out of breath).
•    Without resting, count the number of heartbeats you feel in 30 seconds. Write that number down and multiply it by two.
•    How much did your heart rate increase after the jumping jacks?
•    How many breaths did you take in a minute after the jumping jacks?
•    How did your breathing change?
•    Try other activities and see how they affect your heart rate and breathing. What does that mean about how much oxygen each one requires—and how much your muscles are moving?
•    After you exercise, try seeing how long it takes for your heart rate to return to its resting rate.

Read on for observations, results and more resources.

Observations and results
What were your resting and exercising heart rates? How long did it take your heart rate to go back down to normal? Was it before or after you had caught your breath?

As you exercise more, your body gets more efficient and does not require as much heavy breathing or quick heart pumping. People who exercise regularly can do so longer without getting out of breath as quickly. They also tend to have heart rates that return to their resting levels more quickly after physical activity.

But of course, oxygen is not the only substance the body needs. We also need food for fuel. When we eat food, some of it is broken down by the body and transformed into the energy that gets us moving (these energy units are known as "calories"). More food does not always mean more energy. It depends on the type of food you are eating and how your body breaks the food down. For instance, the body can break down sugar and other processed carbohydrates, such as white bread, quickly. But sustained energy is better gained from foods that are harder to break down, such as lean protein and whole grains.

While breathing at different rates helps control the amount of air the body uses, the amount of energy from food the body uses is controlled much differently. If the body gets way more energy (or calories) than it can burn off, it will often store it away as fat.

What are some ways you and your friends and family can get more physical activity every day?

Share your jumping heart rate observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.

More to explore
"If a Person's Lung Size Cannot Increase, How Does Exercise Serve to Improve Lung Function?" from Scientific American
"Does Exercise Really Make You Healthier?" from Scientific American
"Your Heart & Circulatory System" overview from KidsHealth
"Target Heart Rate for Children" table from Horizon and Blue Cross Blue Shield
Wallie Exercises by Steve Ettinger, ages 4–8
The Amazing Circulatory System: How Does My Heart Work? by John Burstein, ages 9–12

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