Key Concepts

Circulatory system


Why is it important to eat healthily and get exercise? Doing these things keeps your blood flowing well through your circulatory system, which is key to staying healthy. The job of your circulation is to transport blood through your heart, veins and arteries to provide oxygen and nutrients to your body. If this blood flow doesn't work properly, it can negatively impact health. In this activity you will find out what happens to the blood flow of people whose circulatory systems are impacted by a condition called coronary heart disease.


Every part of the human body needs to be supplied with oxygen and nutrients. This task is accomplished by the body's circulatory system, which is also called the cardiovascular system (and includes the heart, veins and arteries). Blood transports oxygen and nutrients by traveling through the arteries and delivering them to the other parts of the body. The heart acts as a powerful pump that generates the force necessary to move the blood around the circulatory system. The arteries carry blood from the heart to the lungs, where it is oxygenated, and then to the organs. Veins return the blood to the heart. When something goes wrong with the body's circulatory system it can lead to serious health consequences.

The most common problem with the circulatory system today is coronary heart disease (also called coronary artery disease). Coronary heart disease is caused by the buildup of cholesterol and other material, called plaque, in the arteries. Over time the plaque grows thicker, and the arteries become narrower. As the arteries narrow they cannot carry as much blood to the organs. The decrease in oxygenated blood can lead to many health problems.

In this activity you will model what happens to blood flow when coronary heart disease narrows a person's arteries. You will do this by comparing the blood flow through straws (arteries) of different diameters. Of course you won't use real blood—but colored water instead!


  • Two 16-ounce disposable plastic cups
  • Two straws with different diameters
  • Modeling clay
  • Stopwatch
  • Tray or shallow tub (large enough to hold more than 16 ounces of water)
  • Water
  • Red food coloring
  • Measuring cup (with milliliters)
  • Scissors
  • Permanent marker
  • Paper
  • A workspace that can tolerate spills
  • Adult helper


  • Measure 400 milliliters of water, and pour it into the first plastic cup. Mark the water level on the outside of the cup using a permanent marker. Then make a mark on the other 16 oz. cup at the same height. Now pour the water out of the cup so it is empty.
  • Have an adult help to carefully use the scissors to poke a hole into the side of each cup about one inch from the bottom. Widen the hole enough to fit each straw.
  • Cut a two-inch length from each straw. What do the straws represent in your blood flow model?
  • Insert the straws into the holes. Make sure they are not squeezed. The straws should point down on the outside of the cup so water can flow out easily.
  • Use modeling clay to seal the inside and outside of the hole around the straw.
  • Prepare one liter of water. Add red food coloring to the water. What does the red water represent in your blood-flow model?


  • Start by placing the cup with the wider straw into the tray. What condition do you think the cup with the wide straw represents—a healthy or unhealthy cardiovascular system? Can you explain why?
  • Seal the end of the straw with a piece of modeling clay.
  • Fill the cup up to the mark with red water. (Improve the clay seal if needed, and make sure you are starting with water up to the line.)
  • Get your stopwatch ready! Remove the clay seal from the end of the straw, and immediately start the stopwatch. Stop the stopwatch when the water stops flowing through the straw. How long does it take until the water stops flowing out of the cup? Record the time on a piece of paper.
  • Place the cup with the narrower straw into the tray. What condition does the cup with the narrower straw represent? Why do you think this is the case?
  • Seal the end of the narrow straw with a piece of modeling clay.
  • Fill the cup with the narrow straw up to the mark with red water. (Improve the clay seal, and add more water if needed.)
  • Remove the clay seal from the end of the straw, and immediately start the stopwatch. Stop the stopwatch when the water stops flowing. Record the time on a piece of paper. How long did it take this time until the water stopped flowing?
  • Compare both times that you recorded on your paper. Which condition (wide or narrow straw) resulted in a faster and better blood flow? What does your result say about the blood flow in people with coronary heart disease?
  • Extra: Calculate the flow rates of the "blood" in both of your setups (wide and narrow straw) by dividing the water volume in milliliters at the beginning by the time it took for the water to flow out of the cup. How do both numbers compare?
  • Extra: Instead of using two different-sized straws use two identical straws. Then use modeling clay to "clog" one of the straws to model the plaque buildup inside an artery. What happens to the speed of the water exiting the straws?


You probably guessed correctly that the straws in your blood flow model represent the arteries in your body that transport blood from your heart to your organs. The red water in your model is the blood that flows through the arteries. Testing the setup with the wide straw simulates a healthy cardiovascular system in which the arteries are free from any plaque. As you likely noticed the "blood flow" out of the cup should have been relatively fast. The second setup with the narrow straw represents an unhealthy cardiovascular system. The artery (straw) is starting to get clogged with plaque and thus the diameter of the artery gets smaller and smaller. This also means that less blood can flow within the artery, which you should have observed in the activity. With the narrower straw the water takes much longer to flow out of the cup. In the worst case during coronary artery disease, the whole artery can get clogged and no blood can flow to your organs anymore. This is why it's important for people to get exercise and eat healthily throughout life, which helps keep arteries open to good blood flow.


Pour the red water into the sink. Recycle cups if possible, and dispose of the straws in the trash. You can dry your modeling clay and reuse it.

More to Explore

Heart and Circulatory System, TeensHealth
Video: Heart and Circulatory System, from Mayo Clinic
Sweaty Science: How Does Heart Rate Change with Exercise?, from Scientific American|
Preventing Heart Disease, from the Harvard T.H. Chan School of Public Health
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies