Alveoli: Function, Definition & Sacs

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  • 0:01 Alveoli Defined
  • 0:40 The Respiratory System
  • 1:18 The Bronchial Tree
  • 2:27 Alveoli In our Anatomy
  • 3:03 Function of Alveoli
  • 4:54 Lesson Summary
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Lesson Transcript
Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Expert Contributor
Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

Alveoli are tiny sacs within our lungs that allow oxygen and carbon dioxide to move between the lungs and bloodstream. Learn more about how they function and quiz your knowledge at the end.

Alveoli Defined

Our bodies perform certain functions every second of the day and night without our conscious awareness. For example, breathing is a job that our body does for us, whether we are asleep or awake, conscious or unconscious. But what is the actual purpose of breathing, other than merely keeping us alive? You probably already know that it has to do with taking in oxygen and getting rid of carbon dioxide. In this lesson, we will learn about tiny organs that help our body parts get the oxygen that we breathe in and get rid of the carbon dioxide we don't need. These organs are called alveoli.

The Respiratory System

Our bodies need oxygen in order to live. We get our oxygen from the air we breathe. However, in order for our bodies to use this oxygen, it must get from our lungs into our bloodstream. This will eventually happen in the alveoli; but we will discuss that a little later. To understand alveoli, we first need to examine the major parts of the respiratory system.

Respiratory System
Diagram of Respiratory System

Our respiratory system includes structures involved in our breathing. When you take a breath, air is drawn into your mouth and nose and into a tube called the trachea, or windpipe. Let's follow the path of the air as it travels through the trachea and into your lungs.

The Bronchial Tree

As we head into the lungs, the trachea branches into two main sections, each called a bronchus. There is a right primary bronchus that goes into the right lung, and a left primary bronchus that goes into the left lung. Each of these bronchi (plural for bronchus) then branch into more bronchi. Those, in turn, branch into smaller tubes called bronchioles. All of this branching eventually results in a structure that truly resembles an upside-down tree. The trachea is the trunk, with all of the branches coming from it. For that reason, it is known as the bronchial tree.

Although this branching does not continue forever, it does happen about 25 times after the first branching of the trachea. The last bronchioles divide into what are called respiratory bronchioles, each of which divide into tiny openings called alveolar ducts. You can imagine how each tube has gotten smaller and smaller as it has branched. By the time we reach the alveoli, the tubes are microscopic - and there are millions of them!

Alveoli in Our Anatomy

At the end of each of the many tiny branches of our bronchial tree, we find openings to microscopic sacs. Each little sac is an alveolus, singular for alveoli. There may be several alveoli coming from one duct, forming a little clump. These groups of alveoli somewhat resemble a cluster of grapes that are all attached. It is in the alveoli that one of the most important transfers in our entire body takes place. It is here that the respiratory system comes into direct contact with the circulatory system, or blood vessels.

Alveoli and Capillaries
Capillaries and Alveoli

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Additional Activities

Alveoli Model

In this activity, students will be creating an alveoli model out of clay. To complete this activity, students will need access to different colored modeling clay, such as pink, blue, red, or white. Creating a model out of clay will help students understand the anatomy of the alveoli and its role in diffusion of gases. For example, students might use pink clay to construct small, hollow spheres that will be the alveoli. They can use red clay tubes to represent the oxygen rich capillaries and blue clay tubes to represent the oxygen poor capillaries. Students could use white clay to represent the bronchioles that connect to the alveoli.

Student Instructions

You're familiar with the anatomy of the alveoli from reading about them in the lesson. In this activity, you'll be using that knowledge to create a model of the alveoli out of clay. Your model should include the alveoli sacs as well as the blood vessels that transport blood to and from the alveoli. You can use the images in the lesson as a model. To make sure that your clay model has all of the necessary components, check out the criteria for success below.

Criteria For Success

  • Model accurately represents the relative size of the alveoli and blood vessels
  • Model uses different colors to represent different anatomical parts of the model including the bronchioles, alveoli, and capillaries (both oxygen rich and oxygen poor)
  • Model is at least six inches tall
  • Students can explain the different parts of their model

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