In this Investigation, we will continue our study of the human body by focusing on another of the body’s systems, the circulatory system.

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Begin the Investigation by reviewing some of the concepts that the students have encountered in the CELL thus far.  

• What are the body systems that you have investigated? Students have investigated the nervous, skeletal, muscular, and respiratory systems.
• Diagrams of the four body systems are presented on the slide. 
• What is the function of the nervous system?  Skeletal system?  Muscular system?  Respiratory system? Students should indicate that the nervous system controls the actions of the body, helps us to interpret and understand the world around us, and helps us to think about and remember information.  The skeletal system provides structure, protection, and support for the body.  The muscular system enables the body to move.  The respiratory system moves fresh air containing oxygen into the body and exhales carbon dioxide from the body.
• Ask students: What are the parts of the respiratory system? The respiratory system consists of the nose, mouth, trachea, bronchi, and lungs. 

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In this Investigation, we will study the circulatory system. The circulatory system moves blood throughout the body. 

• The importance of moving blood is that blood supplies all of the billions of body cells with oxygen and nutrients and removes waste materials released from cells as they perform all of their functions.
• The lungs are responsible for bringing oxygen into close contact with circulating blood. It is also in the lungs that carbon dioxide gas is removed from the blood and exhaled from the body.
• Therefore, the circulatory and respiratory systems work very closely together. In fact, the system that contains the components of both the circulatory system and the respiratory system is often called the cardiopulmonary system.

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A major purpose of the circulatory system is to transport oxygen (O₂) throughout the body. At the same time, it collects a waste product from the cells called carbon dioxide (CO₂). The specific type of blood cells that carries oxygen and carbon dioxide are the erythrocytes also called red blood cells (RBCs). 

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There are a number of different types of blood cells. White blood cells (leukocytes) are important for fighting infections. Platelets circulate in the blood and are important in forming blood clots. 

In terms of our discussion of the blood carrying oxygen throughout the body, however, the most abundant blood cell is the red blood cell, also called an erythrocyte. The video above illustrates red blood cells rushing through a blood vessel.

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RBCs carrying CO₂ waste from the body’s cells enter the lungs after circulating throughout the body. In the lung, the CO₂ is released by the RBCs and is exhaled by the lung. Next, the RBC binds to an oxygen molecule (O₂) breathed in through inhalation by the lungs.

This process is repeated by the RBCs over and over until they are worn out and replaced. Normal RBCs last about 121 days before they are replaced.

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While this slide is up, ask the questions that follow to prompt students to Recall and share their previous knowledge on this topic.

1. Ask students: What do you know about the heart? Student answers may vary. Allow time for students to share their ideas.

2. Tell students that the heart is the size of a fist. Encourage students to make a fist and place it on their chest, over their heart. The heart is located just to the left of the center of the chest.

3. Ask students: What do you think the heart does? Student answers may vary. Encourage students to realize that the heart constantly pumps blood throughout the body.

4. Ask students: How does the blood get from the heart to the rest of the body? Student answers may vary. This question leads to the next slide about arteries and veins.

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Point out to students that blood enters the heart from the body through a major blood vessel called the Superior Vena Cava. After returning to the heart from the lungs, oxygenated blood leaves the heart through the Aorta and circulates through the body before returning to the heart through the Superior Vena Cava once again. 

The heart beats close to 100,000 times a day and about 35 million times per year. That is well over 2.4 billion beats in a typical lifetime! 

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In the lower-left of this slide is a section of heart tissue. It is a form of muscle tissue unique to the heart and called cardiac muscle. 

The background of the slide shows a video of heart cells grown in a lab as tissue culture. When heart cells are grown in the lab, they spontaneously contract in a very regular, rhythmic manner. It is awesome to observe!

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1. Explain to students that the blood moves away from the heart to the rest of the body through blood vessels called arteries. Once the blood reaches the other parts of the body, it moves back toward the heart through blood vessels called veins.

2. When in the lab, use the human torso model to point out the heart, veins, and arteries to the students.

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This summary slide reviews several key points. It is important to remember that the entire function of the circulatory system is to assure that every cell in the body receives an adequate supply of blood.