I was blown away the first time I realised how Seriously Small the components of lung structure are. I have always seen pictures like the image here, of how the air travels through the lung to the alveoli, the air sacs where gas exchange happens, and it seemed so easy!
But here is a reality check.
The trachea brings air down your throat and divides in two behind your breastbone [sternum] taking the air to each lung.
That is the first of around 23 more divisions. They start to get smaller than the eye can see around 12-16th division where they are the thickness of a hair. The first 16 divisions are called the conducting portion of the lungs as they don’t do any gas exchange, but bring the air down to the final six divisions which are the respiratory portion.
The first 12 divisions are the bronchi, and they are distinct under a microscope as they have cartilage rings or plates in their walls to help them stay patent. By division 12 there is no more cartilage in the walls and the 12-16th divisions are the bronchioles. Their cell walls contain elastin fibres and smooth muscle tissue so the diameter of these airways can change significantly when the smooth muscle contracts ( as in asthma) or relaxes ( during exercise) When pollutants like dust or smoke enter the lungs, the bronchioles can constrict to protect the delicate tissue beyond it.
The first 16 divisions have a protective mechanism built into the cell walls. A few structures produce mucous that they secrete over the cell surface to trap any microorganisms and pollutants, and then amazing cilia move like a team of synchronised swimmers to waft the mucous up to your throat so you can swallow it.
(source) An individual cell can’t be seen without a powerful microscope, but there are about 200 cilia on the surface of each one. And inside each cilia there are 9 sets of microtubules that act like a muscle to enable the cilia to move. Seriously small structures that are vital to lung health!
Click on this link to see a short video of cilia movement. related animation
And here is a link to another article I wrote to give you more information about these extraordinary mechanisms.
The last 7 divisions are so small they only occupy a few mm. Around 2% of gas exchange occurs in the 17-22nd divisions. The final division is to the alveoli, the clusters of air sacs where the other 98% of gas exchange occurs. (source)
There are 170,000 alveoli in every cubic cm. Between the alveoli is a thin layer of cells called the interstitium, which contains blood vessels as shown in the picture, and elastic and structural material that help support them which have been removed in this image.
The air sacs are not a uniform size, but the smaller ones do not collapse because of the greater pressure in the bigger ones because and there are specialised cells in their lining that produce a surfactant that equalises the surface tension among them. Hence there is an even distribution of air throughout the lungs. (The surface tension of water is about 72mN/m, yet in alveoli we have remarkably low values of about 20mN/m.)
And inside the alveoli are macrophages which look like hairy blobs – and they engulf and destroy foreign matter that comes this far in the lungs. These only last about 3 days so every day at least 300.000 of them die and are wafted up in mucous and you swallow them, or they are taken away in the lymphatic system. Click on this link to see a short but lovely graphic of macrophages in the alveoli. (source)
Try the narrated and animated options.
Each of these pictures show a linear route of the divisions, getting smaller as they go, but the lungs are actually similar to a sponge which has air spaces throughout the whole structure. In this image of a cross section of the lung you can see alveoli like sponge holes with a blood vessel (bv ) and bronchiole inside the black box. They are all interspersed and joined to each other so the lung acts like a whole.
This is a link to an amazing video that shows the complexity of lung pathology using asthma as an example. (It does take 40 secs to start but it is worth the wait) It also uses a lot of specialist medical terms, but the graphics are extraordinary and worth a look to marvel at the tip of the iceberg of the lungs complexity. I hope it encourages any asthmatics who watch it to use their inhaled corticosteroids like flixotide or pulmicort regularly to control the inflammatory response in their cell linings.
(Animations in this article provided by Blausen.com staff. “Blausen gallery 2014“. Wikiversity Journal of Medicine. . – Own work)