Video: Bronchioles and alveoli

This is a sponge – a soft substance that is full of small holes and can absorb a lot of liquid. Okay, so you’ve already knew that. If we were to look at the texture of a lung, we'd actually find it has an internal texture not unlike that of a sponge. It, too, is filled up of tiny pockets or air sacs known as alveoli. Who knew? But did you know that combined, our two lungs contain upwards of four hundred million of these tiny alveoli? That's a lot, right? Even more, if you were to lay them all out flat, they would cover one hundred square meters – enough to cover an entire tennis court.

And there's more. Wrapped around each of these alveoli are tiny microscopic capillaries, which carry blood to and from the alveolus for gaseous exchange. If we were to unwind these vessels and place them end to end, they would cover almost one thousand kilometers. Who knew that there was so much anatomy packed within your ribcage? Intrigued to know more? Let's dive into the anatomy of the smallest part of the lungs and learn about the bronchioles and the alveoli?

To start with in this tutorial, we're going to first begin by looking at the overall structure of the lungs. We'll look at structures of the lower respiratory tract that act as a passageway for air into and out of the lungs, first talking about the main bronchi and then going on to investigate the subdivisions of these airways. Then we'll follow the path of an oxygen molecule through the smaller airways into our bloodstream by zooming in on the bronchioles and the alveoli, and we'll look at their specific parts and features.

From there, we'll move on to look at nervous structures associated with the bronchioles and alveoli followed by any vasculature in the area. And to wrap up, we’ll visit some relevant clinical scenarios to put it all into real-life context. Let's get to it.

Can you spot the lungs? Of course, you can. In this image, we can see both the right and the left lungs in situ in this woman. Now, how does air actually get into the lungs? What path does it take? When we inhale air, our body is looking to take in oxygen. Here, we can see the wee oxygen molecule entering the oral cavity of the woman and traveling towards her lungs as she inhales. The oxygen travels into the oral and nasal cavities, into the pharynx, into the larynx, and then into the trachea.

At approximately the level of the sternal angle, the trachea bifurcates into the right main bronchus and the left main bronchus, and at this point, our wee little oxygen pal can take many different paths, but ultimately, will end up in an alveolus. But, where are these alveoli, I hear you ask?

Well, in this image, we can see both the right and left main bronchi highlighted in green, which supply oxygen to the right and left lungs, respectively. Let's move in a little closer now and look at this image here, and we're going to focus on this image for much of this tutorial and investigate the parts of the lung, specifically, in the right lung which we can see in this man's chest.

As I mentioned, the main bronchi are the first branches of the trachea and are the first components of this system destined for a specific lung. As the first branches, they are also called primary bronchi. Each main bronchus splits into lobar or secondary bronchi; however, this differs between the left and right bronchi.

In both illustrations, we see the right lobar bronchi of which there are three highlighted in green now. Each lobar bronchi travels to a different lobe of the right lung. This differs from the left side. The left primary bronchus only splits into two lobar bronchi, and this is because the left lung has two lobes instead of the three lobes the right lung has. The lobar branch further divide into even smaller air passageways – tertiary bronchi – which we will look at next.

And here are the tertiary bronchi. These are smaller and more numerous than the lobar or secondary bronchi and travel to the segments of each lobe of the lungs. Due to this, they’re also called segmental bronchi, and if we look more closely at this zoomed in part of the image, you can see segmental bronchi in the right lung highlighted in green.

Beyond the tertiary bronchi, branches are called bronchioles. Bronchioles can continue to branch and branch and branch twenty to twenty-five times before we reach the end. They continue to be a conduit for air to travel into and out of the body and there are a few different types of bronchioles. One that terminate as smaller bronchioles and just help to transport air are called conducting bronchioles and bronchioles that terminate with alveoli and also transport air are called respiratory bronchioles.

Terminal bronchioles – one of which we can now see highlighted in green – are a type of conducting bronchial. In fact, as their name suggests, they are the last, and therefore, smallest conducting bronchioles in the tree before we get to the alveoli. We can see that distal to the terminal bronchiole, there are other bronchioles that have some sacs on them, and we'll get to those next, but these are no longer conducting bronchioles.

Remember the other type of bronchiole? That's right – respiratory bronchioles. These are the bronchioles distal to the terminal bronchiole and now highlighted in green. Respiratory bronchioles, which measure about 0.5 millimeters in diameter, transport air and have these little sacs that facilitate gas exchange. So, these little sacs, what are they?

Well, finally we come to the end of the pathway for oxygen molecules in the lungs – the alveoli. Highlighted in green are many alveoli which is plural for alveolus, and an alveolus which comes from the Latin meaning “little cavity” is
the basic structural unit facilitating gas exchange in the lungs. This is why the alveoli are surrounded by a rich network of blood vessels, but we’ll get to that a little later.

Many alveoli open up internally to a space called the alveolar sac, and this is the space we can now see highlighted in green. The alveoli and alveolar sacs mark the end of the respiratory tract. Remember, the average adult human lung contains over four hundred million alveoli. I mean, that's a lot of gases exchange going on, wouldn't you say?

The final structure related to the bronchial tree are the pores of Kohn, also known as interalveolar connections, which we can now see highlighted in green. These are openings situated between adjacent alveoli that allow for air movements between the adjoining alveoli. In other words, the pores of Kohn allow for direct communication between adjacent alveoli to occur.

There are a few different cell types that make up the walls of the alveoli. Remember that the alveoli is where gaseous exchange occurs, so the cells that make up the alveolar walls are there to facilitate that process. What we can see highlighted in this image now is the pneumocyte. There are two types of pneumocytes – type I and type II. Type I pneumocytes are the ones we can see highlighted now. These cells form about ninety percent of the surface area lining the alveoli, but numbers-wise, account for about forty percent of the cells. Type I pneumocytes are very thin. Being so thin is super for gas exchange.

The type II pneumocytes account for the other sixty percent of the cells, but make up only about ten percent of the surface area. These pneumocytes have a round shape and possess a cytoplast rich in mitochondria as well as both the rough and smooth endoplasmic reticulum. Which brings me to the next characteristic of type two pneumocytes. Type two cells secrete a substance called surfactant. Surfactant lines the alveolar walls to lower the surface tension. This prevents the alveoli from collapsing during expiration and helps the walls to expand during inspiration.

The third cell type found in alveoli are the alveolar macrophages. These macrophages are the same as other macrophages just located in the alveoli. They patrol the area looking for any invasive bacteria, toxic particles, or other foreign bodies to phagocytose and get rid of. Alveolar macrophages are an important line of defense in preventing infection of the lungs.

Now you're all up-to-date with the structure of the bronchial tree. There's more, though. Let's look at a nervous structure that is found in the depths of the lungs.

The structure we can see highlighted in green in this image is one of many bronchial nerves. The lungs are innervated by different parts of the nervous system. These nerves carry fibers from the parasympathetic nervous system, which causes the bronchial tree to contract, among other things. There are also sympathetic nervous system fibers, which lead to bronchodilation.

The final group of structures that we're going to identify are vascular structures. So, of course, some arteries and veins. Let's see what we can find.

The first structure we'll look at is the pulmonary artery. Now, this artery is a little bit different than most. Although it's still carrying blood away from the heart as all arteries do, the blood that it carries is deoxygenated. This deoxygenated blood is being transported to the alveoli where it will get rid of any carbon dioxide and pick up oxygen. Before it gets to the alveoli, the pulmonary artery branches become smaller pulmonary arterioles, which we can now see highlighted in green. Arterioles are smaller, thinner arteries still carrying blood away from the heart for gas exchange.

And now we're at the level of the circulatory system, where gas exchange in the lung occurs. Capillaries are the smallest and most abundant type of blood vessel in the body connecting arterioles to the venous system. Because the capillary walls are so thin, it's easy for molecules of gas to pass through via passive diffusion. Therefore, oxygen is able to diffuse from the alveoli into the capillaries and carbon dioxide is able to diffuse from the blood in the capillaries to the alveoli destined for exhalation.

After passing through the capillary bed, the now oxygenated blood travels into the pulmonary venules. Venules are the arterial equivalent on the venous side and they're smaller than veins created by the joining of two or more capillaries. From the pulmonary venules, blood travels into the pulmonary veins, which will eventually bring the oxygenated blood back to the heart to be pumped throughout the body. Usually, veins contain deoxygenated blood, but similar to the pulmonary arteries, the pulmonary veins are an exception to the oxygenated rule. The pulmonary arteries and veins have a similar branching pattern to the bronchioles.

So that's the system of getting deoxygenated blood to the alveoli and bringing oxygenated blood back to the heart, but how did the parts of the bronchial tree get oxygen and nutrients themselves? That's where the bronchial arteries come into play. These arteries usually arise from the thoracic aorta and bring oxygen and nutrient-rich blood to the bronchi and connective tissue of the lungs. As they travel through the lungs, they branch in a similar pattern to the bronchi and bronchioles giving off small branches along the way to supply the cells of these structures.

At the level of the terminal and respiratory bronchioles, the bronchial arteries may anastomose in one of three places. First, they can anastomose directly with the pulmonary artery causing oxygenated blood to mix with deoxygenated blood. They can also anastomose with the pulmonary capillaries surrounding the alveolus, and finally, it may form a capillary network of its own which it does when supplying structures such as the smooth muscle of the bronchial tree.

Collecting deoxygenated blood from the bronchial tree are the bronchial veins. We can see them highlighted in green in this image arising from the capillary network on the respiratory bronchiole. At this deep distal level of the bronchioles, some bronchial veins may drain into the pulmonary veins. Remember, the pulmonary veins are bringing oxygen-rich blood from the alveoli to the heart after gas exchange. This means that the bronchial veins adding a bit of deoxygenated blood into that oxygen-rich blood of the pulmonary veins. Some of the bronchial veins, however, will continue independently back along the bronchial tree to eventually drain into the azygous or the hemizygous veins.

Now what do you think would happen if one or more of these bronchioles was blocked? If perhaps due to an object that has been breathed in, air can't get to the alveoli. Well, this scenario could lead to segmental atelectasis. Atelectasis, in general, is a collapse of lung tissue, which can affect all or some of the lung. Segmental atelectasis is specifically the collapse of a segment of the lung.

So, do you remember which part of the bronchial tree divides into the segments of the lungs? These are the tertiary or segmental bronchi. And if a tertiary bronchus is blocked due to the aspiration of a foreign object, for example, air is prevented from reaching the bronchopulmonary segment. Air that is downstream from the blockage will be absorbed into the blood and the segments will collapse. And now, you're an expert on bronchioles and alveoli.

Before I let you go, let's have a quick review of what we looked at today.

So, we started by looking at the bronchial tree and following the path from largest to smallest components. First, we had our main or principal bronchi bifurcating from the trachea and entering the hilum of each lung. These then split into secondary or lobar bronchi which travel to the lobes of each lung. The next branches are the tertiary or segmental bronchi, which supply the bronchopulmonary segments of the lungs, and then we're getting quite small, and we get into the bronchioles themselves.

And first were the terminal bronchioles – the smallest type of conducting bronchioles which transport air but lack alveoli. Next, we had the respiratory bronchioles which do have alveoli, and therefore, facilitate some gas exchange, and at the end of the bronchial tree is where we found the alveoli – the basic structural unit of the lung and which also facilitates gas exchange.

The internal aspect of many alveoli forms the alveolar sac. Internally as well are the pores of Kohn – the openings between adjacent alveoli that allow for the movement of air. And three types of cells in the alveoli were identified – the type I pneumocytes which are the thin cells facilitating gas exchange, the type II pneumocytes which secrete surfactant, and the alveolar macrophages which are a line of defense for the alveoli phagocytosing invasive pathogens.

The only nervous structure we identified was the bronchial nerve. This nerve carries many different types of fibers including those from the parasympathetic and sympathetic nervous systems. And lastly, we looked at some vascular structures in this area – the first being branches of the pulmonary artery. The pulmonary artery is bringing deoxygenated blood to the lungs for oxygenation, and this branch into several smaller pulmonary arterioles. And, finally, we ended up with pulmonary capillaries which is where gas exchange occurs.

Moving into the venous system, we first came across these small pulmonary venules which joined together to form the larger pulmonary veins that are bringing oxygenated blood in the lungs back to the heart. The final structures are the arteries and veins that supply these structures of the lungs themselves and bringing oxygen and nutrients to the bronchioles are the bronchial arteries. And draining these deep structures of the lungs are the bronchial veins.

Finally, we looked at segmental atelectasis – a collapse of a bronchopulmonary segment of the lung affecting everything downstream from the blocked tertiary bronchus.

And that brings us to the end of the video. I hope you enjoyed it. Thanks for joining me and happy studying!