One tree, many branches of the bronchial system and the alveoli

When looking at the two-dimensional drawing from the outside, it seems that the expression “bronchial tree” fits it perfectly (except for the fact that the tree is upside down). There is a trunk (trachea), many branches (bronchi), which, becoming thinner, end in leaves (alveoli). But this image changes completely when you start to look at the bronchial system from the inside with a bronchoscope. On YouTube, you can find many videos on this topic. The trunk and branches suddenly turn into a three-dimensional system of tunnels, a real subway inside our body. And this metro has only one function-to deliver the inhaled air to the end of the tunnel, to the alveoli, where the gas exchange takes place. So it’s just a transport system for our life elixir.

A small amount of imagination can even decorate this subway. Imagine the billboards on the walls, the sign with the name of the station, the tracks, the sounds of a guitar and the vague hum of thousands of voices. When a doctor uses a bronchoscope to look into the respiratory tract, he sees a magnificent picture. After passing a rather nondescript entrance (nose), an escalator (pink, covered with some hair and mucus), a small ticket office (larynx) and an entrance door (glottis), he sees before him a real splendor, filled with symmetry, harmony, shape and color! The trachea is the vestibule where the airways converge. These are the starting and ending points of our metro. Do you think I’m exaggerating? Not at all. In Moscow, you can see the most beautiful metro stations in the world — “Mayakovskaya”, “Belorusskaya”, “Komsomolskaya”. Each of them is an architectural masterpiece and a landmark. For the majority of Muscovites, these are just public transport stops, purely functional structures. Behind the daily routine, all their splendor fades. So it’s just a matter of perception. But if you take your time, you will be able to see the true beauty.

From the inside, the trachea looks like the interior of a cathedral. The imposing longitudinal nave is 12-15 centimeters long with a vaulted ceiling. The ribs of the arch form 16-20 horseshoe-shaped cartilaginous strips, located strictly at a distance of 1.75 centimeters from each other. Between the cartilage is a flexible connective tissue. This design makes the trachea at the same time stable, elastic. It needs stability in order not to crumple with each breath, like a deflated bicycle camera, and elasticity-in order to be able to lengthen and shorten by a few centimeters during breathing. The posterior wall of the trachea is flat. It also consists of elastic muscle and connective tissue. The trachea differs in appearance from all other airways by its flat posterior wall, since the bronchi have a round cross-section. Thus, it seems to demonstrate its special position, since it is from it that all the airways begin.

Are you interested in this? Do you have some time? Then come with me, I’ll arrange a tour for you!

• Sat down?
• Yes.

“Are you comfortable?”

“Quite.”

“Worried?”

“A little.”

“Don’t worry. We’ll go slowly.

— Who else is with us?”

“Oh, these?” The components of the air — nitrogen, oxygen, a little carbon dioxide. Don’t pay any attention to them.

— Why did we stop?”

• The first attraction. Here, after 15 centimeters, the trachea divides into two main bronchi for the right and left lung.

— Are we going to the right or left bronchus?”

— It doesn’t matter which one, they’re practically the same.

— Is it possible to get lost here?

• In principle, no. If you get bored, just turn around and move back. There are no cross-connecting passages between the tunnels.

“Not exactly practical.

— There will be ramifications all along the way. After the 22nd, we will reach the goal.

— Here’s how!

— To be precise, the number of individual moves is a total of 223, which is almost 9 million. Their total length is about a kilometer.

“Here’s the next fork!” After it, the bronchi are called lobular, since they are responsible for supplying individual lobes of the lungs.

“Interesting!

— The right lung consists of three lobes — the upper, lower and middle, and the left — only two, the upper and lower, so it is slightly smaller. Further on, over there, you can see another branching into segmental bronchi.

“It’s getting a little cramped.”

“That’s right. Now we are at the 4th fork, beyond which the subsegmental bronchi begin. This place can still be reached with a bronchoscope. And then there is no more.

— I thought you could see everything with a bronchoscopy.”

• Unfortunately, no, we can only see a small part of the lungs. That is why bronchoscopy is always only an additional method of examination. We’ll go on without stopping. Starting from this point and up to the 17th branch, the airways are called bronchioles, that is, small bronchi. They are getting narrower, and their walls are getting thinner, like rolled dough.

— And I can’t see the cartilage anymore.”

“That’s right. The supporting cartilage structure completely disappears after the 10th branching. Then we see only the ring-shaped layer of muscles around the bronchioles.

— What do they do?”

“The same as all the muscles. They contract and relax. Due to this, you can control the diameter of the bronchioles and the amount of air passing through them.

“I see. But why?

— I haven’t the faintest idea!”

— What do you mean?”

“No one knows. In healthy people, the lumen of the bronchioles is constant and practically does not change. So it’s a mystery to everyone. In the upper parts of the bronchi, the cartilage is used to maintain a stable lumen, but here, at the bottom, nothing. The muscles are nothing but trouble.

“Which ones?”

— Sometimes they behave unpredictably. For no reason at all, they have a spasm, and then the person does not have enough air. You can ask any asthmatic person.

“That’s odd.

— That’s it.

— Where are we now?”

• At the 17th fork. The diameter here is less than half a millimeter.

— I don’t see much slime here.”

• A healthy person practically does not have it. The bronchial mucus glands produce less than 50 milliliters of mucus per day. This is very small compared to the nose.

“What about bronchitis?”

— Then it’s quite another matter. The production of mucus in this case reaches 300-400 milliliters per day.

“It’s dangerous, isn’t it?”

• The mucus is distributed along the entire length. But here, in the shallow airways, its accumulation can be a problem.

— It’s too narrow.”

“That’s right. Just a small swelling of the bronchial wall plus the mucosal plug is enough — and the lumen is blocked!

— And then it will be even narrower? I feel a little uneasy.

— No, there will be no more narrowing. We have the 18th fork ahead of us.

— What’s this?” There are some depressions in the walls.

As the alveoli. Here it is already more spacious, the area of the inner surface increases significantly. After the 23rd fork, there are almost 40 alveoli at each end of the bronchiole. And there are about 400 million of them in total.

• Impressive. And why do you need such a large area?
• For gas exchange. Oxygen enters the blood, and carbon dioxide is released from it.

— So this is where the breathing happens?”

• In principle, yes. Therefore, bronchioles after the 17th branching are called respiratory, in other words, “breathing”. However, this area accounts for only a relatively small part of the gas exchange. Real breathing takes place at the very end — in the alveolar sacs.

— Is it in that big cave over there?” How mixed up everything is!

“Don’t worry, it’s all right.

— Why is there a reddish light?”

— Because of the red blood cells in the smallest capillaries of the lungs. They wrap around the alveoli from behind. The walls of the capillaries and alveoli in this place are so thin that the blood shines through them. Only one thousandth of a millimeter.

“I see.

• Attention, we’re here! The doors open, you can go out!
• We got there quickly. How long are we here?”

“No more than a second. During this time, oxygen molecules pass into the blood, and carbon dioxide molecules are extracted from it.

• You won’t have time to blink an eye…

— It doesn’t really happen that fast. Only one in ten molecules is involved in the gas exchange process. The others must wait.

— Why not?”

— Because there must always be a certain amount of air in the alveoli. Even with a full exhalation. Otherwise, they may collapse. What do you think keeps the whole structure of the alveolar sac? Do you see any props here?

“No,” I said. Only a thin layer of mucus on the alveoli.

“That’s right. It’s a surfactant. It covers all the alveoli from the inside with a thin layer and thus ensures their stability. It’s like being in a soap bubble. It doesn’t look very spectacular, but it’s effective. In people with a lack of surfactant, everything sticks together, and this is very serious.

— What if you need to speed up the process?

— Do you mean increased physical activity? Then the accelerated transmission is activated. At the moment, the blood receives about 300 milliliters of oxygen per minute, that is, no more than a beer glass. But, if necessary, this volume can be quickly increased to 6 liters.

• Impressive…

— Don’t tell me, it’s a colossal energy!” The main thing is to get it to where it is needed, for this the heart pumps blood through the vessels of the lungs. If it works poorly, then the oxygen molecules have to wait in line for a long time. Even the most intense breathing will not help, the muscles will still experience oxygen starvation.

“I see.

— But sometimes it’s not the speed of transportation, but the volume of delivery. In this case, the lungs themselves are to blame.

— Why not?”

— Something is blocking the passage of air through the bronchi. This can happen when they narrow during an asthma attack. Sometimes its intake is blocked by the accumulation of mucus, for example, with pneumonia. If the air does not reach the alveoli, then the red blood cells cannot attach oxygen to themselves, they wait in vain. This also does not contribute to the efficiency of the body.

“Naturally.

— On the other hand, too much oxygen is not good either.

— I thought there wasn’t much fresh air.”

— It all depends on the concentration. It should not be too high, as in this case, the oxygen can even become toxic.

“Toxic oxygen?” How is that?

— The higher the concentration of oxygen in the air, the more radicals are formed.

— What the hell is that?”

— Some oxygen molecules lose control of themselves and start attacking anything that gets in their way. The damage can be huge.

— Is it possible to protect yourself from them?

• Of course! Of all the organs, the lungs have developed the most effective defense mechanisms. This is logical, because they constantly have to deal with oxygen. To fight the radicals in the lungs, there are devices that capture the radicals and neutralize them. They are located along the entire length of the respiratory tract, especially many such devices in the alveoli.

— And how effectively does it work?

It is very effective. But only as long as there are no other kind of radicals. That’s when it gets serious. Tobacco smoke and smog are a huge accumulation of radicals, they can cause enormous damage. And it’s time for us to go back, time is running out!

— One more question.

“Yes, please.”

— There’s a lot of life here, but on the way here…

— What’s bothering you?

— Does anyone even live there?” I haven’t met a single living soul, and that’s kind of unusual.

“Not a single living soul?” My dear fellow, this is a tour for tourists. All the people were previously dispersed to their homes.

— So there’s someone there?”

• Of course there is, there are all sorts of things. Many are completely peaceful and reasonable, but there are also absolutely violent, pugnacious, prone to crime. There are millions of vagabonds living in the airways, coming from all over the place — a continuous passageway. Lots of suspicious characters that you shouldn’t even see.

— Aren’t they disturbing public order here?”

“All right?” Oh, really! Some, on the contrary, are very concerned about the order. For example, immune cells. Without them, there would have been complete anarchy. People come across all sorts of things!