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Alveoli: Function, Definition & Sacs

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Wendy McDougal

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

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.

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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, they 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

Function of Alveoli

We breathe in oxygen so that parts of our body can use it for many different cellular functions. But we must somehow get the oxygen from our lungs into our bloodstream, so it can be transported to the many places it is needed. Likewise, our bodies produce carbon dioxide as a waste product. It must be exhaled from our body, but must get from the bloodstream to our lungs. It is the alveoli that are responsible for this transfer of gases. But how, exactly, does this work?

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