Left banner image adapted from amyloidosis gross node, by Ed Uthman MD, Creative Commons license

So, what exactly is mucus?

Mucin glycoprotein structure
You have to forgive me, I've been rattling on about mucus but never gave you a proper explanation of what it is. Oh sure, I mentioned water, salt (i.e. ions/electrolytes), and proteins, but that doesn't tell you much about why you have different mucus in different places or why it's slimy.

First of all, your genetic code includes a set of mucin genes. These particular sequences encode the mucin proteins. They are actually glycoproteins because after the protein portion is made sugars are added to create the final structure. Some of the mucins are secreted to live outside the cell in the tissue fluid, others bind to the membranes of the cells. More than 20 genes are known for humans, making for a complex family of proteins. In fact there are research groups dedicated to figuring these mucus proteins out.

Cervical mucus
Now, the composition of mucus is mostly water - 90% water by many of the estimates I've been finding. About half (or even less) of the remaining composition is mucins. How much glycoprotein determines how slimy it is, because mucins have special bonding characteristics with each other, forming a gel-like substance when they're able to bond in water. The more there are, the more gel-like the mucus is. The other components are either functional (like white blood cells) or simply being carried away (like blood from cracked nasal epithelium), and they're what affect the color.

So essentially, the cells of the different parts of the body produce mucus with varying concentrations of glycoproteins, causing differences in the sliminess of the mucus.

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