Anatomy 101: Learning About Airway Walls
I’m going to make an attempt here to describe the asthmatic airway. In an earlier post on our sister site COPD.net, I took you on a journey down the respiratory tract. In this post, I want to describe to you the cells that line airways. In a future post, I will explain how all this might contribute to asthma. I promise to keep this pithy and easy to understand. And, I promise, there will be no test.
Ready! Here we go.
Picture an airway in your head. Picture it as the inside of a pipe. The opening of the pipe is called the lumen. Lining the walls on the inside of this pipe is the mucosa.
This is the first layer of cells.
- Epithelial Cells. They are cells that act as part of the innate immune system, as they work to prevent pathogens and particles from getting further into your lungs. If you really want to get technical, they are called pseudostratified ciliated columnar epithelium. You do not need to know what all those fancy terms mean. I will just refer to these cells in my posts as epithelial cells.
- Cilia. In our journey down the respiratory tract, you saw that cilia line airways from your nose all the way down to the smallest airways. They line the surfaces of epithelial cells. The move back and forth in a wave-like fashion and work to move pathogens to your upper airway to be swallowed.
- Goblet Cells. You also saw these on our journey. They are cells that are randomly scattered, and often squeezed nice and tight, between epithelial cells. They secrete mucus that covers the surfaces of epithelial cells. This is a sticky substance that is meant to keep airways moist but also to trap unwanted particles and pathogens. These particles and pathogens are balled up in sputum, which is moved to your upper airway by cilia.1
So, you can see, epithelial cells and goblet cells work together to keep particles and pathogens out of your lungs to keep them free from infections. This is part of your innate immune system.
Now, let’s go a little deeper into the airway wall. What we come to next is the…
Basement membrane (lamina propria)
Just under the single layer of epithelial cells is a thin membrane that separates epithelial cells from the next layer of cells, or the submucosa. This layer is made up of collagenous fibers that make the membrane strong and flexible (like the cartilage that makes up your ear). In a future post, I will explain how this membrane may become thicker in severe asthma.1-3
This is the layer of cells just under the Mucosa, or just under the basement membrane. It connects the basement membrane to the outer wall of the pipe. It consists of...
- Connective tissue. It’s a type of cell that adds support to tissues but also connects one group of cells to another. In our case, it adds stability to airway walls.2-3 Its tissue made up of fibers, ground substance (a gel-like substance) and cells such as mast cells and fibroblasts. It also contains blood vessels, nerve cells, and submucosal glands. It connects airway epithelial cells with airway smooth muscle cells. The basement membrane described above is also a type of connective tissue.
- Fibroblasts. They release proteins that are used to make connective tissue.4 They secrete type I Collagen used to make connective tissue. Some think they also play a role in severe asthma to cause airway scarring.
- Blood Vessels. Lots of blood vessels contain oxygenated blood that feeds the cells in this area. Their numbers may be increased in severe asthma, perhaps causing worse airway inflammation.
- Nerve Cells. There are a lot of these in this area too.
- Submucosal glands. Like goblet cells, they also produce mucus. They can be increased in number, particularly in severe asthmatics. These are absent in the bronchioles1
- Mast Cells. They are a special type of white blood cell called a granulocyte. This means that they contain granules, which are tiny proteins that cause airway inflammation and asthma symptoms. I describe them in detail during our journey.5
- Smooth muscle cells. This is a layer of cells that line the outer layer of the pipe or airway. They exist in bundles. They wrap or spiral around the pipe. They determine the diameter of the inside of the pipe or airway. When they constrict, airway narrows. This is what happens during asthma attacks. When they relax, the airway dilates. This is what happens when you take rescue medicine or when you are breathing easy.
This is the outer layer of the pipe. It is essentially a layer of connective tissue. It connects the airway with surrounding lung tissue.1
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