BioEd Online

Teacher Background

The heart is a sophisticated mechanical pump made of strong muscle. Thus, to understand how the heart works, it is helpful to know a little about pumps.

A pump is a mechanical device that moves fluid or gas by pressure or suction. Consider, for example, a simple bicycle pump. When you pull the handle up, you create a vacuum inside the metal tube, which fills with air through a hole in the side. When you push the handle down, a one-way valve in the hole closes and air moves through the rubber tube, into the bike tire. What keeps the air from coming out of the tire and back into the pump? Another one-way valve at the end of the rubber tube prevents the air from moving backward.

A lotion dispenser illustrates the same principle. A plastic tube goes down from the top of the dispenser into the lotion. When you push down on the dispenser, the lotion already in the top of the tube (above the pump) squirts out into your hand. It does not flow back down into the pump mechanism because a one-way valve closes behind it when you push down. When you let go of the dispenser, a spring-driven pump pushes the top back up, sucking more lotion up into the top of the tube and pulling more lotion from the bottle to fill the tube below the pump.

Note that both a pumping mechanism and a one-way valve are required to make a pump work. The lotion bottle has two chambers (in the tube below the pump and in the dispenser above the pump). The lower chamber of the dispenser holds a portion of lotion, ready to move up into the pump.

Like the lotion pump, some animals, such as fish, have a two-chambered heart. The first chamber (atrium) fills with blood returning from the body and then passes it to the second, more muscular chamber (ventricle). The ventricle contracts, pushing the blood out into the vessels that carry it through the gills for oxygenation and on to the body. A one-way valve prevents the blood from flowing backward into the atrium. Other animals, such as reptiles and amphibians, have three-chambered hearts.

Birds and mammals, including humans, have four-chambered hearts. Two chambers receive blood and the other two pump it out. The receiving chambers are known as atria (the singular form is atrium). The right atrium receives oxygen-depleted blood from the body’s major veins (vessels that bring blood to the heart), and the left atrium receives oxygen-rich blood from the lungs. The atria transfer their blood, through one-way valves, into the two different pumping chambers, called ventricles. The right ventricle pumps oxygen-depleted blood via smaller blood vessels through the lungs, where it is replenished with oxygen, and cleansed of carbon dioxide. The left ventricle squeezes (contracts) to pump oxygenated blood out into the rest of the body through large arteries (vessels that carry blood away from the heart).

So ultimately, animals with four- chambered hearts have two circulation loops. The first loop travels to and from the lungs (pulmonary circulation). Blood filled with carbon dioxide enters the lungs, where carbon dioxide is replaced with oxygen, and then carried from the lungs back to the heart for pumping to the rest of the body. The second loop carries blood to all parts of the body, delivering oxygen and nutrients and gathering wastes for proper disposal (systemic circulation). This very efficient system keeps blood moving in the right direction, and to the right parts of the body, 24 hours a day.

Why doesn’t the blood get pushed back into the atria when the ventricles contract? Valves! Remember the one-way valves in the mechanical pumps? Similar one-way valves between each chamber in our hearts ensure that blood moves in only one direction. The heart also has valves at the exits to the ventricles, so blood can’t get sucked back in. Thanks to valves, the blood in our bodies always moves forward, never backward.

Objectives and Standards

Life Science

• Living systems at all levels of organization demonstrate the complementary nature of structure and function. Important levels of organization for structure and function include cells, organs, tissues, organ systems, whole organisms and ecosystems.

• Specialized cells perform specialized functions in multi-cellular organisms. Groups of specialized cells cooperate to form a tissue, such as a muscle.

• Different tissues are, in turn, grouped together to form larger functional units, called organs. Each type of cell, tissue and organ has a distinct structure and set of functions that serve the organism as a whole.

• The human organism has systems for digestion, respiration, reproduction, circulation, excretion, movement, control and coordination, and for protection from diseases. These systems interact with one another.

Science, Health and Math Skills

• Communicating

• Using information

• Interpreting information

• Applying knowledge

Materials and Setup

Teacher Materials (see Setup, below)

• Pump dispenser of lotion or soap

Materials per Student

• Copy of the student sheet (see Lesson pdf)

Setup

1. Begin with a class demonstration and discussion. Follow with students working in groups.

2. At the end of the activity (see Procedure, Item 7), the class will view a BioEd Online video, “A Look at the Heart, Part 2.” To access the file, go to www.bioedonline.org/, look under the "Resources" tab, and click on the "Videos" link.

Procedure and Extensions

1. Show students the pump dispenser and demonstrate its use. Ask, What does this dispenser do? Allow students to provide a variety of answers. When someone mentions, “pump,” ask, What is the job of a pump? Help students understand that many kinds of pumps use compression and suction to move a fluid or gas. Humans use suction, for example, when drinking from a straw. The lotion pump uses suction to draw lotion up into a tube. It then releases the lotion when pressure is applied to the top of the dispenser. Mention that a one-way valve keeps the liquid from running out of the bottom of the tube when the top is pressed.

2. Ask, How is the lotion dispenser like a heart? [both are pumps] Explain that like a lotion pump, the heart relies on suction, pressure and compression, which allow it to initiate the movement of blood through the lungs and the rest of the body.

3. Give each student a copy of the student sheet, which provides a labeled diagram and an unlabeled photograph showing the inside of the heart. Direct students to identify on the diagram the receiving areas (atria) and pumping areas (ventricles) of the heart. Help students find the same structures on the photograph. Ask, Which chambers receive blood from the body or lungs? [atria] Which chambers pump blood away from the heart? [ventricles]

4. Point out the valves in the heart diagram. Ask, What might the valves do? [prevent blood from flowing backward] Have students find and circle all of the valves in the heart diagram.

5. Now, have students locate and label on the photograph each part that is identified on the diagram. When students are finished labeling their heart photographs, let them share their work within their groups to check answers and discuss any discrepancies or questions.

6. Conduct a class discussion about the internal structures of the heart. Ask, Which chambers have thicker walls? [ventricles] Why might the ventricle walls be thicker? [they work harder to squeeze blood out through the arteries] Are the muscular walls of the two ventricles equally thick? Why or why not? [No. One ventricle pumps blood to more distant parts of the body.] What would happen if a valve stopped working? [blood might leak back into the atrium and pumping might be less efficient]

7. As a class, view “A Look at the Heart, Part 2” (see Setup). Lead a discussion about the similarities and differences between the sheep’s heart shown in the video and the diagram of the heart that students used for this activity. Or, use a model of the human heart to demonstrate the internal parts that students identified in the photograph. If you will be conducting the activity, "Examining the Heart," tell students they will have an opportunity to observe these structures on a animal specimen.

8. Have students add any new information to their concept maps.

Handouts and Downloads

Download color images from The Science of the Heart and Circulation Teacher's Guide.