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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.

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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

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?Attached to the outer walls of alveoli are many tiny blood vessels called capillaries. Essentially, the capillaries are like a netting covering the alveolar sacs.

The place where the capillary tissue meets the alveolar tissue creates an extremely thin layer that molecules can pass through. It is called the respiratory membrane. Think of a cartoon where a ghost passes right through the walls.

That is how we can imagine oxygen and carbon dioxide traveling through the membrane.Now let’s break it down from start to finish. Air is inhaled and travels through into the lungs, bronchi, bronchioles, and finally into the alveoli. Here, O2 passes through the respiratory membrane and into a capillary.

It is now in the bloodstream, where it binds to a red blood cell and travels through the blood to an area where it is needed.Likewise, CO2 has been produced as a waste product in the cells and is now traveling through the bloodstream. It arrives via a capillary to an alveolus, and passes through the respiratory membrane. It now takes the opposite path of the O2, traveling through the bronchial tree until it reaches the lungs, where it is exhaled. This demonstrates the vital importance of the alveoli as they provide an interface between the respiratory system and the circulatory system.

Lesson Summary

Alveoli are integral parts of our respiratory system. They are the tiny sacs at the very end of the last bronchioles of the bronchial tree.

Alveoli are covered with capillaries, and a respiratory membrane is created where alveoli and capillaries meet. O2 and CO2 pass through this membrane in and out of the bloodstream.

Learning Outcomes

By the end of this lesson you should be able to:

  • Summarize the function and structure of the respiratory system
  • Describe the bronchial tree
  • Discuss the alveoli’s part in the respiratory process

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