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3.1: The Heart

Difficulty Level: At Grade Created by: CK-12
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How does the heart pump blood?

People have had many different ideas about what the heart does. More than \begin{align*}3000\end{align*} years ago, the ancient Egyptians believed the heart held a person's thoughts and feelings. But many centuries later, in the year \begin{align*}200\end{align*}, a Greek doctor named Galen proposed that the heart worked like an oven to warm blood. He also thought that blood flowed in the body like tides. Finally, he suggested that it was the liver, not the heart that controlled the circulatory system. Of course, you know all of those ideas are incorrect. But his incorrect ideas actually lasted almost \begin{align*}1500\end{align*} years! Then, in \begin{align*}1628\end{align*}, an English doctor named William Harvey showed that the heart pumped blood around the body. But what's interesting is that the same idea was described in an ancient Chinese text more than \begin{align*}4000\end{align*} years earlier. In this section you will investigate how the heart functions and learn some more about why your heart is such an important part of your body.

The Heart

Figure 2.1 Your heart is located in the middle of your chest just above your stomach. It is about \begin{align*}12\;\mathrm{centimeters}\end{align*} long, \begin{align*}9\;\mathrm{centimeters}\end{align*} wide, and \begin{align*}6\;\mathrm{centimeters}\end{align*}

Your heart is the pump for your circulatory system. It is about the size of your two fists. Your heart is located in your chest behind your breastbone (sternum) and it tilts towards the left. It weighs about \begin{align*}300\;\mathrm{grams}\end{align*}, which is a little less than a can of soda. Your body pumps about \begin{align*}15\;\mathrm{times}\end{align*} its own weight in blood each minute, even while you're resting. This work goes on every minute of every day of every month of every year of your life.

Activity 2-1: Exploring the Heart


Have you ever seen or touched a real heart? What does the heart look like? Your heart is a double pump about the size of your two hands clenched together. In this activity you explore me parts of me heart and how they work. There are questions to help you mink about what you're learning. Discuss the questions and answers with your group to help you work through this activity. Write your answers and important discussion points on your Activity Report. Together your group should choose one person to be me recorder.


  • Animal heart (sheep, cow, or pig)
  • Tweezers
  • Scalpel
  • Scissors
  • Probes
  • Apron or smock
  • Plastic or latex disposable gloves
  • Dissection pan
  • Paper towels
  • Plastic bag
  • Activity Reports 1, 2, and 3


Step 1 Put on plastic gloves to work with the heart. Why do you mink this precaution is important?

Step 2 Place the heart specimen in me dissection pan and place me pan with the heart in front of you. Clench your two hands together to represent your heart. Compare me sizes by putting your clenched hands near the animal heart. How does the size of your clenched hands compare with the size of me heart specimen?

Step 3 Touch me heart. Describe how the heart muscle feels when you touch it.

Step 4 Observe me vessels that carry blood to and from me heart. There are two kinds of vessels. The arteries are the vessels that carry blood away from me heart. The veins are vessels mat carry blood to me heart. How can you tell me difference between arteries and veins?

Figure 2.2 The surface of a sheep heart.

Step 5 A membrane sac that covers the heart has been removed. This sac is called the pericardium (payr-ih-KAR-dee-um). Pericardium means “around the heart.”

Step 6 With your teacher's help, put your heart specimen in its correct anatomical position with the front of the heart facing you.

The Right Side of the Heart

Step 7 Find a wavy-edged flap that has an opening on the top. This flap is the chamber of the heart called the atrium (AY-treeuhm). There is a right atrium and a left atrium. The vessels entering the right atrium are the superior vena cava and the inferior vena cava. These veins bring oxygen-poor blood back to the heart from the rest of the body. Each atrium is a receiving chamber for the heart. Each atrium receives blood. Find the right atrium. Describe the atrium so the recorder can write the description on the Activity Report.

Step 8 Use your finger to find the opening into the right atrium. Push down into the right atrium with your finger. If your finger goes completely into the heart, then you have reached the right ventricle (VE -trih-kuhl). Take your finger out of the heart. Then push a straw back into where your finger was. If you push the straw all the way down, it will be in both the right atrium and the right ventricle.

Figure 2.3 The right atrium and right ventricle of a sheep heart.

Step 9 Push on the straw until you can see it stretching the wall of the heart. Stick the point of the scissors through the heart wall to meet the tip of the straw. Cut all the way up to the top following the straw. Now you can observe the inside of the heart. Find the inside of the atrium and the ventricle. What can you observe and describe inside the heart? Write your responses on your Activity Report.

Step 10 Look between the right atrium and the right ventricle. You should be able to see thin, transparent membranes. Those membranes form the tricuspid (try-KUHspihd) valve. The tricuspid valve stays open when the right ventricle fills with blood. Describe the valve on your Activity Report. So the blood entering the right ventricle through the right atrium passes through the tricuspid valve. Then the blood leaves the right ventricle through the pulmonary (PUHL-muhn-ayr-ee) artery. When the heart pumps the blood out of the ventricle, the tricuspid valve closes making a "lub" sound. And the pulmonary (or semi-lunar) valve opens to let the blood into the pulmonary artery. The pulmonary valve is between the right ventricle and the pulmonary artery.

Step 11 Look at the open side of the heart. Poke the second finger of your right hand into the back of the lower chamber. After your finger comes out on top, put a straw in the opening. The straw marks the pulmonary artery, which carries oxygen-poor blood to the lungs. Remember that the blood gives off carbon dioxide and picks up oxygen in the lungs, and that arteries carry blood away from the heart. But the pulmonary artery is different from all the other arteries. The pulmonary artery is the only artery in the body that carries blood low in oxygen. All other arteries carry blood rich in oxygen to all parts of the body. Why do you think the blood in the pulmonary artery is low in oxygen?

The Left Side of the Heart

Step 12 Find the opening on the top of the left side of the heart. The opening leads to the left atrium. Put a straw through the opening. Push it all the way down. After the blood leaves the lungs it returns to the heart through the pulmonary vein. Remember that all veins carry blood to the heart. Pulmonary veins are the only veins in the body that carry oxygen-rich instead of oxygen-poor blood. All of the other veins carry oxygen-poor blood back from the body to the heart. The pulmonary veins bring oxygen-rich blood back from the lungs into the left atrium.

Figure 2.4 This is an interior view of the right atrium and the right ventricle showing the tricuspid. pulmonary, and aortic valves.

Step 13 Poke your scissors through the point where you think the end of the straw would be. Cut until you see the end of the straw. The end of the straw should be in the left ventricle. The straw has passed through the mitral or bicuspid valve. You just cut through the wall of the left ventricle. How is the wall of the left ventricle different from the wall of the right ventricle?

Step 14 Find the opening from the lower left ventricle near the middle of the heart that leads to the outside. This opening leads to a large artery called the aorta (ay-OHR-tuh). Put a straw in the aorta. Which kind of blood, oxygen-rich or oxygen-poor, do you think flows through the aorta?

Step 15 Cut down the aorta toward the heart and observe the valve. This is the aortic semi-lunar valve or aortic valve. Describe what the aortic valve looks like on your Activity Report.

Step 16 Look for the two coronary arteries in the walls of the ventricles. Locate two tiny holes just above the aortic valve. This is were the coronary arteries leave the aorta to transport food and oxygen to the heart.

Step 17 What other observations can you make as you examine the heart? Describe any new observations on your Activity Report. Make sure you discuss and answer all of the questions on the Activity Report.

Step 18 Follow directions from your teacher to complete your dissection and clean up.

Did You Know? When you were a fetus the heart was one of the first organs to develop. After only four weeks of development your tiny heart began to form in the shape of a tube. This tube became an atrium and a ventricle in a two-chambered heart. A two-chambered heart is similar to the kind of heart a fish has. Later, as you continued developing, your heart developed a third chamber. A three-chambered heart is similar to a frog's heart. Then long before you were born, the fourth chamber developed. That tiny heart was a miniature, fully functioning heart.

How Do You Know You're Alive? Your vital signs provide health care workers with a good idea of what's going on inside your body at a given moment. Your vital signs include your temperature, heart rate, breathing rate, and blood pressure. You can measure some of these vital signs yourself. You can take your temperature with a thermometer. Also, you can feel your pulse on your wrist. Look at the drawing in Figure 2.5.

When you feel your pulse, the blood in your arteries is flowing beneath your fingers and pushing on them. The up-and-down movement of your pulse tells you there are two parts to your heart's pumping cycle. The pulse that pushes up against your fingers is the squirting phase of the heart cycle. The squirting phase is when your heart squeezes blood into your arteries under high pressure. This squirting phase is called systole (SIS-toe-Iee). Systole is a Greek word that means contracting. That correctly describes the contracting of the heart muscle when it squeezes and squirts out the blood.

Figure 2.5 You can feel your pulse on your wrist, near the base of your thumb. Calculate your heart rate by counting the number of beats per minute. A normal resting heart rate for an adult is \begin{align*}60\end{align*} to \begin{align*}100\end{align*} beats per minute.

Figure 2.6 Your heartbeat is the rhythm of the heart muscle relaxing and contracting. When the heart muscle relaxes, the heart fills with blood. When the heart muscle contracts, it sends blood flowing out of the heart and to the lungs or body.

Is Pumping Hard Work? How hard does your heart work? Put a tennis ball in your hand and squeeze it until it dents. That's how hard your heart muscles squeeze each time to send blood around your body. Do you think that's not too hard? Well, try to do it \begin{align*}70\;\mathrm{times}\end{align*} in a minute. Your heart pumps about \begin{align*}70\;\mathrm{times}\end{align*} per minute all day every day of your life. Your heart works even harder when you exercise.

Heartbeats Determine your own heart rate by taking your pulse. Look at Figure 2.5 again. Place your fingers lightly on your wrist at the same place shown in the drawing. Find your pulse. Count the number of beats in one minute. Then use your heart rate to calculate the following:

  • How many times does your heart beat in a day? In a week? In a year?
  • How many times might your heart beat in your lifetime? Assume you will live to age \begin{align*}80\end{align*}.
  • Do you think your estimates are high or low? Why?

Did You Know? A fretful newborn baby will often calm down and sleep if a ticking clock is placed nearby. Perhaps the ticking mimics the sound of its mother's heartbeat from its time in the womb.

When the pulse you feel with your fingers falls away, your heart is not squirting blood. That is when the heart is filling with blood. This filling phase of the heart cycle is called diastole (dy-AS-toe-lee). Diastole is also a Greek word that means expand. Expanding describes how the heart muscle relaxes so the heart can fill with blood. Your heart fills and squirts all your life. The sequence of fill (diastole) then squirt (systole) repeats over and over throughout your entire life. This sequence is called the cardiac (heart) cycle because it repeats over and over.

Word Origins Use a dictionary to find the word origins of the following terms.

  1. atrium
  2. ventricle
  3. pulmonary
  4. coronary
  5. cardiac

Figure 2.7 A four-chambered heart.

Your friend tells you that she has a “heart murmur.” What do you think this means?

A Closer look at the Heart

Let's take a closer look at how the heart works. The heart's job is to pump blood throughout the body. The blood delivers oxygen and nutrients to cells. The blood also picks up waste materials like carbon dioxide gas. You couldn't live without this constant circulation of gases and nutrients.

Remember the model you built in the beginning of this unit. Your heart is actually two pumps in one. The right pump moves blood from the heart to the lungs to pick up oxygen and release carbon dioxide, then back to the heart. The left pump moves blood around the body. In each pump there is an atrium and a ventricle. So a human heart has four chambers: two atria and two ventricles.

The Heart Pump

Your heart is a pressure pump. The heart squeezes the blood and creates blood pressure. Blood pressure is the force that moves blood through your body and keeps the blood moving in one direction. A good way to begin learning how the heart works is to explore how a siphon pump works.

Have you ever used a siphon? You may have used a siphon to empty a fish tank, as shown in Figure 2.8. The siphon is the tube that allows water from the fish tank to flow into the bucket. You put one end of the siphon in the tank and suck on the other end until the tube fills with water. Then you quickly put the free end of the tube in the bucket. Gravity is the force that keeps the water moving from the tank to the bucket. As long as the free end of the tube is below the water level in the tank, gravity will pull the water through the siphon. Can you explain how this works?

Figure 2.8 A siphon moves water downhill using gravity.

Figure 2.9 A siphon pump moves water uphill aganist the force of gravity.

How could you get the water to flow from the bucket into the tank? One way, of course, is to lift the bucket and pour the water into the tank. But, suppose that the bucket is too heavy for you. Instead, you could use a siphon pump like the one shown in Figure 2.9.

In a siphon pump, a squeeze puts pressure on the liquid in the bulb or chamber. The pressure of the liquid closes the inlet valve and opens the outlet valve. The closed inlet valve keeps liquid from going into the inlet hose. So all the liquid is forced out the outlet hose.

Figure 2.10 A siphon pump is a squeeze bulb with a hose on each end. One hose is an inlet hose, and the other hose is an outlet hose. There is a valve where each hose connects to the squeeze bulb.

The valves keep fluid moving in just one direction. Each valve works like a trapdoor that opens in one direction. Pressing against the door from the “wrong” side keeps the door closed. Pushing on the door from the “correct” side lets the door open.

Figure 2.11 When using a siphon pump, you squeeze the bump chamber, or squeeze bulb, so it empties. Squeezing closes the inlet valve but opens the outlet valve. How is his action similar to the action in the heart?

Figure 2.12 The bulb expands when the bulb of a siphon pump is not being squeezed. The outlet valve closes and the inlet valve opens. Liquid enters the bulb through the inlet hose. The expanding bulb wall lets the pump draw liquid in and get ready for the next squeeze.

Activity 2-2: Siphon Pump


Your heart is really two pumps in one. Each pump works like a siphon pump. In this activity you explore how siphons and siphon pumps work. As you learn about siphon pumps think about how the heart is like two siphon pumps working together.


  • Rubber or plastic tube (about \begin{align*}1\;\mathrm{cm}\end{align*} diameter and \begin{align*}1.5\;\mathrm{m}\end{align*} long)
  • Siphon pump with inlet and outlet tubes
  • \begin{align*}2\end{align*} large containers (such as buckets or dishpans)
  • Water
  • Activity Report


Part A: How Does A Siphon Work?

Step 1 Place one of the containers near the edge of a stool or table. Fill the container about three-quarters full with water. Place the other container (empty) on the floor. Completely submerge the siphon tube in the water-filled container and remove the bubbles from the tube.

Step 2 Now plug both ends of the tube with your fingers. Hold one end of the tube full of water in the top container and place the other end in the container on the floor so it will empty into the floor container.

Step 3 Raise the lower end of the tube to the level of the upper end of the tube. What happens to the rate of flow?

Step 4 Find out what determines how fast the water flows through the siphon. Experiment by moving the bottom end of the water-filled tube below the water level in the floor container. What happens when you raise the bottom end of the tube to the level of the water's surface in the top container?

Step 5 Siphon all the water from the top container into the bottom container.

Part B: How Does a Siphon Pump Work?

Step 1 Immerse the siphon pump and hose in the bottom container of water.

Step 2 Squeeze the bulb several times to remove bubbles. Continue to squeeze the bulb until water comes out.

Step 3 Plug the outlet hose with your finger and pump the bulb. Watch what happens.

Step 4 Use the siphon pump to pump water from the lower container to the higher one as shown in Figure 2.13.

Part C: How Is Your Heart Like a Siphon Pump?

Step 1 Imagine that the siphon pump represents the right side of your heart. If this pump is your heart, where is the blood coming from? Where is the blood going?

Step 2 Now imagine that the siphon pump represents the left side of your heart. Where is the blood coming from? Where is it going?

Step 3 The heart of a fish consists of a single pump. You, other mammals, birds, and crocodiles all have a double pump. Two siphon pumps working together represent both sides of a double pump. What advantage(s) does the double-pump arrangement have?

Figure 2.13 A siphon pump moves water from the lower container to the higher one.

Did You Know? A heart murmur is an extra sound the heart makes between the lub-dupp sounds. Poorly formed valves that allow some leakage between the heart's chambers cause the extra sound. Most people with heart murmurs don't have medical problems due to the leakage. But in some cases the leakage keeps the heart from working properly and medical treatment may be needed.

Figure 2.14 Your heart works like two siphon pumps.

Did You Know? Your heart rate slows down as you get older. At birth it was probably around \begin{align*}120\end{align*} beats per minute. During elementary school it was probably between \begin{align*}80-100\end{align*} beats per minute. By adulthood it will be about \begin{align*}70\end{align*} beats per minute. Compare this to the heart rates of some animals.

Animal Heart rate Animal (beats per minute)
Mouse \begin{align*}650\end{align*}
Iguana \begin{align*}150\end{align*}
Cat \begin{align*}150\end{align*}
Hamster (hibernating) \begin{align*}4\end{align*}

Each of the two heart pumps works like a siphon pump. Figure 2.14 compares the pumps. In both pumps the squeeze/fill cycle moves liquid through from the inlet to the outlet tube. Notice that the inlet “tubes” of the heart are swollen just in front of the inlet valves. These swollen areas represent the atria.

The two pumps squeeze together and relax together. Both pumps beat as one heart. The right side of the heart pushes blood through your lungs and back to the left side of the heart. The left side pushes blood through the rest of your body and back to the right side of the heart.

The heart cycle is like the pump cycle-fill (diastole), squeeze (systole). During systole both heart pumps squeeze together. When they start to squeeze the inlet valves slam shut, making the first heart sound-“lub.” The outlet valves are open. The ventricles squirt blood into the arteries. The inlet valves are the tricuspid valves on the right side of the heart and the mitral or bicuspid valve on the left side. The outlet valves are the pulmonary valve on the right side of the heart and the aortic valve on the left side. When the heart pumps stop squeezing, the outlet valves slam shut making the second heart sound-“dub.” The two heart sounds mark the heart cycle. The “lub” sound occurs at the beginning of systole. The “dub” sound occurs at the beginning of diastole.

During diastole, both heart pumps fill together, as shown in Figure 2.16. The ventricles are relaxed. Blood flows from atria to ventricles in both sides of the heart. During diastole (filling) the heart works like a filling siphon pump. The inlet valves (tricuspid and mitral) are open but the outlet valves (pulmonary and aortic) are closed.

Figure 2.15 Your two heart pumps during systole.

Figure 2.16 Your two heart pumps during diastole.

What's Your Cardiac Output Today?

What is your body's cardiac output for one day? You can figure it out.

  • Take your pulse to determine your heart rate.
  • Multiply your heart rate by atypical stroke volume, which is about \begin{align*}70\;\mathrm{milliliters.}\end{align*} This calculation gives you your cardiac output for one minute.
  • Multiply your cardiac output for one minute by the number of minutes in one day. How might your cardiac output change if you start exercising?

The blood your heart puts out with each squeeze is called your stroke volume. Your heart rate is the number of times your heart beats in one minute. The amount of blood your heart pumps out in a certain period of time is called your cardiac output.

Let's summarize what you have learned so far about the heart. You learned about:

  • the parts of the heart,
  • how the two sides or pumps of the heart beat and fill together,
  • that the heart cycle is systole (squeezing) and diastole(filling), and
  • how your heart works like two siphon pumps connected together.

  • When exercising your working muscles help return blood to the heart more quickly. Remember that the stroke volume is the amount of blood the heart pumps out with each squeeze. Does exercise increase or decrease the heart stroke volume? Explain. Remember, what goes into the heart must come out!
  • Why do athletes often have lower resting heart rates than non-athletes? If their hearts don't beat as frequently why don't they faint from a shortage of blood in circulation?

Figure 2.17 If you cut open a heart, it would look like a mass of twisted tubes and muscle tissue.

Now you've read that the heart works like two siphon pumps. But the heart you used in Activity 2-1 didn't look like two siphon pumps did it? The picture of the heart shown in Figure 2.17 doesn't really look like a picture of two siphon pumps either. If you cut open a heart as you did in Activity 2-1 and look inside, all the compartments seem to be twisted together. It all looks pretty complicated. Actually, the heart looks complicated because the two pumps twisted together as the heart developed.

Figure 2.18 A fish heart has one pump with two chambers, an atrium and a ventricle.

To understand the structure of a heart a little better, it helps to look at a less complicated heart such as a fish's heart. So, look at Figure 2.18. A fish heart is similar in structure to ours. Except the fish heart has only one pump. The fish heart's single pump has two chambers. It has an atrium and a ventricle. The pump structure of the fish's heart is straight.

The bending and folding of the two pumps inside a human heart make its structure a little harder to understand. Although the pictures may be hard to follow, remember that the human heart is simply two pumps. One pump moves blood from the body to the lungs. The other pump moves blood from the lungs to the body.

To see the two pumps that make up the human heart look at Figure 2.19. Follow the heavy black arrows to see how the pumps are bent or folded at the ventricles. Notice that the pumps are like mirror images of each other. This way the flow of blood moves in the opposite direction.

Figure 2.19 Here are the two pumps in the human heart separated. The arrows show the directions of blood flow.

Figure 2.20 The heart's two pumps work together side by side. Can you trace the flow of blood through the heart?

Did You Know? When you exercise your heart can pump \begin{align*}5\end{align*} to \begin{align*}7\;\mathrm{times}\end{align*} as much blood as when you are at rest. What would be your cardiac output increase if during exercise your heart rate goes from 50 (resting rate) to \begin{align*}120\end{align*} and your stroke volume goes from \begin{align*}70\;\mathrm{ml}\end{align*} to \begin{align*}85\;\mathrm{m}\end{align*}? What percentage increase is this?

Now imagine that the two pumps moved together and attached. If that happened, they would look like the drawing of the heart in Figure 2.20. The left side of the heart is shown on the right side of the drawing. The right side of the heart is shown on the left side of the drawing. Now that may not make sense. So look at the drawing. Remember that this is just a drawing. In your body, the left pump sits to the left in your chest nearer to your left arm. Hold this picture up to your chest and look at it in a mirror. Study the arrows to follow the path that the blood takes.

Figure 2.21 One pump brings blood from the body to the lungs to get oxygen. The other pump brings the oxygen-rich blood from the lungs and pumps it throughout the body.

Why do you have two pumps in your heart? Isn't one enough? Figure 2.21 shows why you need two pumps. The right side of the heart brings in oxygen-poor blood from the body and sends that blood out to the lungs. There an exchange of carbon dioxide and oxygen occurs. The lungs get rid of the carbon dioxide lie cells produced and replace the oxygen the body cells will use. Then the oxygen-rich blood returns to lie left side of lie heart. The left side of the heart receives lie oxygen-rich blood from lie lungs. Then it pumps that oxygen-rich blood out to lie body. You need two pumps for the whole process to take place.One pump can't do the job alone.

Why can a fish get by with only one pump in its heart when you need two?


What makes your heart beat? How does your heart know when to beat and how fast to beat? In the wall of the right atrium there are specialized muscle cells. These special cells can produce an electrical signal. That electrical signal travels through the heart muscle. The rhythm of the electrical signal makes the heart beat. The specialized muscle cells in the right atrium are called lie heart's pacemaker. The pacemaker makes the heart beat faster or slower when it receives messages from the nervous system of from chemical “messengers” called hormones.

Figure 2.22 The heart's pacemaker is sometimes called the SA node. The pacemaker initiates your heartbeat, signaling your atria to contract. The signal is delayed at the AV node. The AV node then tells your ventricles to contract.

Imagine you are a drop of blood in the left atrium of the heart. Describe your voyage through the heart and body and back to the heart. Include labeled diagrams to illustrate your story.

There is another group of specialized muscle cells that sits between the atria and the ventricles. This group of special cells is called the AV node. The AV node causes a slight delay in the electrical signal going to the ventricles. This delay allows the atria to contract a little before the ventricles contract. This early contraction of the atria helps to fill the ventricles to their full capacity before they squeeze the blood out again.

Review Questions

  1. What is the heart cycle?
  2. Describe the four steps in which blood flows through the heart. Draw a picture showing the path of blood from when it enters the right atrium until it leaves the left ventricle.
  3. What do heart valves do?
  4. How is a siphon pump most similar to a heart?
  5. When you hear your heart beat, what exactly are you hearing?
  6. What two properties of your heart can change to increase your cardiac output?

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