The Secondary Lobule (aka pulmonary lobule)

We will now move upstream toward the terminal bronchiole to gain an understanding of the secondary lobule. From a radiological point of view, it is structurally a more important unit than the acinus. It is a conglomerate of 3 to 5 acini enclosed in a membrane with a polyhedral shape.  From a structural and radiological point of view it is important to understand this entity, since it has implications in the understanding of the CXR, conventional CT, high resolution CT and in the structural changes that occur in some of the lung diseases.

We have already described the “buddy system” that exists in the lung, with the branches of arterioles and bronchi traveling together, and the venules and lymphatics going together.  Lymphatics sometimes also accompany the bronchovascular bundle.  A story called “Grapes of Exchange”  about two friends on a journey will describe the structures within the secondary lobule.

In the story there are two pairs of structures that travel together as they branch through the lungs.  The first pair is an artery and an airway and the second is a vein and a lymphatic.  The story is described by the airway – a female — who  travels with the artery – the male. They travel for many divisions together, dividing at the same time.  In the early phases they are both tubular in their form and equal in size.  As they enter close to the chambers of gas exchange the airway develops little sacs off her body and she becomes rounder and fatter, while the artery continues to be sleek thin and tubular.  While she becomes the chamber through which gas exchange takes place he becomes at one with the vein after traversing the capillaries.  Once revived by the oxygen, he becomes a vein and he picks up his new partner, the  skinny lymphatic, and continues on his travels accompanied by  the lymphatic.  The airway feels at this stage like a ditched lover but there is hope for her as she continues on her return journey.

Grapes of Exchange – A Romantic Story

The following audio is a story that describes the relationship between the pulmonary artery and the bronchiole as they branch through the parenchyma and eventually reach the secondary lobule.  It is told in the form of a story to promote understanding . If you do not have an audio system the story is in written form below.

From the minute I saw him I loved him and I knew that no matter what evolved, we would spend our lives together… little did I know what the working relationship would be.

Here follows a true story about me, the bronchiole, and my lifelong buddy the pulmonary arteriole.

He and I were on a mission.   I had my origins in the atmosphere and he from the heart.  We came from very different backgrounds and had come a long way on this trip on the highways and byways of the airways and circulation.  My mission was to take products from the air in the atmosphere and deliver them to the grapes of exchange and his was to deliver blue blood to the same grapes.  In this communication we will speak mostly of the journey to the grapes in the house of the pulmonary lobule.  The grapes of exchange in my imagination had something to do with bonding and marriage – perhaps the exchange of vows.  The story turns out quite differently.  We had great travels together and took a lot of pictures!!!

 

I think by this time you know what we had already been through.   He had started out as a large elastic vessel off the right heart called the main pulmonary artery (nickname MPA) and I had started out as the trachea. (nickname “trach”). We met at the doorway of the lung called the hilum, and took a fancy to each other right away and so we decided to travel together. We had traveled a long way by the time our story begins both experiencing many divisions and were right in the middle of an inspiration.  We both looked quite different at this point having given birth to many offspring.  In the new language – we had both “morphed” quite a bit but this was a necessary part of the mission.  Of course we were much smaller than we had been.  He had lost some of his elasticity and developed a bit of muscle.  I had lost my entire cartilagenous skeleton and had developed some muscle as well.  We were told that from now on we were both going to lose muscle and I in particular was going to change drastically.  I could not wait?!!!!   I secretly hoped that this change would make me more attractive and bring me closer to a happy union since I was promised a happy union in the end. My name at this stage was “terminal bronchiole”.  A foreboding and deathly chill rattled down my muscularis as I said the word “terminal” knowing that I was going to lose the small amount of muscle that I had.

We reached the doorway surrounding the secondary lobule and faced the polyhedral entrance.  It was quite beautiful I thought in my teal blue outfit.

We took a quick walk around the polyhedral structure.

We returned to the entrance and took a closer look at the goings on inside through the large front entrance windows of the pulmonary lobule. Below is a picture of what we saw.

As we were ushered into the lobule, we were faced by a refreshing atmosphere of comforting air and a hub of activity.  In addition to the swishing sounds of air movement we heard a hubbub of clinking and clanking. We were told that the sounds were coming from the grapes of exchange  (wedding bells?).  We also noticed that there were at least three other couples who looked like the people of our tribe – three other bronchovascular bundles.  We called our tribe the “bronchovascular bundle” with the one part of the bundle being the progeny of the bronchus and the other, the progeny of the pulmonary artery.  A group picture inside the lobule is shown below.  Fortunately the flash was working because it was a little dark inside.

I did not realize that my body had already started to morph at the time of the picture – little sac-like love handles spreading all along my formerly sleek and beautiful body  Yech!  I did not look good at all.   My buddy the arteriole on the other hand as well as members of the other tribe maintained their sleek tubular looks.  “Why me?”  I shouted.  “All for the good of the nation and good gas exchange!!” they shouted above the swishing of air and clanging of the factories.  “Easy for you to say,” I replied.  “Some of us have to suffer some bad morphs for the good of the whole” they said, “but those love handles kinda suit you,” they exclaimed with a wry smile.  I was so sad.

Well the divisions started coming rapidly and so we all became smaller very quickly. I was getting rounder and rounder while they were getting thinner and sleeker.  Things seemed to be rushing at an accelerated pace and I must say that there was some excitement in the air as we all got closer and closer to each other.  In the big picture we seemed to be coming together as a team, and the whole landscape seemed more colorful and more promising.  We reached out to the other tribe and they too us.

I remained on the plump side as my love handles grew more pronounced and rounder while the clanging noises grew louder and louder.  I was dividing into alveolar ducts and alveolar sacs and finally into alveolus which of course is a complete transformation from my former tube like shape to the spherical shape of the grape.  In the mean time my buddy the artery grew as small as an 8 micron red cell and remained true to his tubular form. He started to surround me completely but was more in concert with that beastly red vein as he ran headstrong in blind passion to join her in capillary union.  For a moment they each lost their identity becoming the tiniest of tubes neither arteriole nor venule.  I on the other hand became at one with the fruit of the vine and in stuporose state I think I became the grape itself.  Through my waist I felt the pleasant freshness of gusts of air going back and forth through my skin.  My delivery of fresh air moved very quickly into the capillary.  Little did I know that it was the oxygen that I carried which gave my competition the beautiful red glow of health.   The odious carbon dioxide moved the other way.  This toxic waste came directly from the man I had known and trusted for so long.  Thanks for nothing.

 

I do not like to show the next picture much since for me it is a sad one.  There I am a ditched lover, fat as a grape, in the middle of the capillary union between my blue “ex” and his new partner in crime, the red beast.  Yes of course it was for the better – they all say that – but what about me?

As you have learned already, the blood circulation needs two types of vessels; one to carry the blood to the lungs and a second to carry it from the lungs.  I on the other hand do it all by myself.  On inspiration I take air to the lungs and on expiration I take it away through the same vessel to the atmosphere.  This is the lot of all women.  We do the double the amount of work shlepping here and there and everywhere and get no respect for it.

So at this junction we are in the middle of an inspiration (for me – what kind of inspiration could I feel in my morphed format) and we were just about to start the expiration.   While I was traveling to the exit of the lobule during this phase of my life, my “ex” was going in the same direction – now transformed into a  beautiful vessel with that healthy glow.   Despite my odious load and my downtrodden feeling I moved with a sense of optimism.

 

What do you know? As I left the chamber of the lobule I shed my love handles one by one and my saccular form started to take on the sleek and tubular look again.  I looked and felt brand new.  “Hmm… I thought – perhaps he will fall in love with me again.  I was hoping for another inspiration, just to show him once again how beautiful I was both on the outside and the inside.  And that my friends was  my nightmare in the Grapes of Exchange.

The secondary lobule contains all the structural elements of the lung compacted into a small space and it is a recognizable structural entity that measures about 1-2.5cms.  Prior to entering the secondary lobule the bronchovascular bundle branches at approximately 1cm intervals.  However, once they enter the confines of the lobule the branching becomes fast furious and a new branch originates every 1-3mms.  A lot of structure subsequently packs into a small space.  The lobules are well formed in the apices and the peripheral aspects of the lungs and poorly formed posteriorly.   In the image below the lymphatic system has taken up carbon particles and the blackened lymphatics outline the interlobular septa, which in turn outline the lobule.

The interlobular septa and the secondary lobules are not normally visualized.  However any disease of the lung, particularly if the lymphatics are involved, will expose the lobules mostly by providing better visualization of the interlobular septa. If more than 3 contiguous secondary lobules are visualized then the lung is abnormal.  At the level of the terminal bronchiole the arteriole and bronchiole measure about 0.2 mms each and it is the challenge of our CT technology to resolve these structures.  At this level we are starting to push the limits of resolution.  Thin collimation allows us to optimally visualize the lobule.  We are thus better able to visualize the septa and the lobules with 1.5mm collimation than with 3mm collimation and our chances diminish as collimation gets thicker.  The evolution of high resolution CT scan has allowed us insight into the lobule and provides a better understanding of the diseases that affect the lobule.

Congestive Heart Failure

We are never able to visualize the normal lobule on a plain film but in diseases such as congestive cardiac failure when the lymphatics become overwhelmed they distend and become visible.  In congestive cardiac failure the heart fails as a pump.  In left ventricular failure the resulting congestion is reflected in the left ventricle (LV), left atrium (LA), pulmonary veins and capillaries as increased pressure.  It is reflected structurally as dilatation and of the chambers and vessels.  The increased fluid and pressure not only cause dilatation but also cause the fluid to leak out into the interstitium.  The lymphatics are able remove and mop up some of the excess fluid, but there is a point when the volume overwhelms the lymphatics and the excess fluid remains in the interstitium.   On CXR the findings include dilatation and cephalization of the arterioles, peribronchial cuffing, Kerley B lines (thickening of the interlobular septa), pleural effusion and subpleural edema.

Other Diseases Carcinoma Sarcoidosis

Sarcoidosis