As a chemist, mucus is fascinating stuff.  It turns out that mucus is quite difficult to chemically characterize.  Mucus is a “non-Newtonian” fluid.  It does not act like a “predictable” fluid, water, for example.  When you pour a bucket of water, it seems to all pour as a single quantity.  Mucus, on the other hand, is a rubbery (viscoelastic) material: it can be compressed and relaxed, stirred, and sheared to thin it.  Basically it’s a mixture of water, salts, and mucin proteins that are sometimes cross-linked to form polymers of varying sizes.  The type of proteins in mucus, their concentrations and the amount of protein-crosslinking often determines mucus viscosity.  (Note: all of the technical information and the two pictures in this post (not the Zombie)  is from Michael M. Norton’s Master of Mechanical Engineering Thesis entitled Modeling Problems in Mucus Viscoelasticity and Mucociliary Clearance. Any errors in summarizing this excellent work are mine)

Observationally, mucus seems to have a life of its own.  It’s literally and figuratively creepy.   Zombies always seem to be expurgating copious amounts of it, mixed with the blood of their victims.  You know the story.

Imitation Zombie

While morbidly fascinating, mucus is critical for many bodily functions, and breathing is the function for the moment.  Breathing air, getting oxygen to our blood and expelling carbon dioxide is a dirty business.  Air is full of contaminants for humans: tiny particulates, viruses, bacteria, allergens, trash, plastics, you name it.  Anyone who says that the country has pure clean air is probably trying to sell you something, because they’re just blowing smoke.  Mucus traps the contaminants for us.  How we get the mucus and contaminants out of us is the important part.

Humans move the mucus by something called the mucociliary transport system.  This is a beautiful mechanism.  Tiny structures called cilia beat almost synchronously with neighboring cilia on a dense grid.  The cilia are bathed in a watery periciliary fluid.  The periciliary fluid layer is about as thick as the cilia are long (estimated about 5-7 microns in humans), and the fluid is held between cilia strands by capillary action. A layer of the much denser, viscous mucus floats on top of the cilia/periciliary fluid layer.  Here’s a cross-sectional view of this arrangement in a rabbit trachea.  The mucus layer boundary runs right through the middle of the picture.  It looks like the cilia strands are supporting it.

Cilia have a mechanically fascinating structure.  A single strand has two central columns, surrounded by 9 symmetrically spaced outer vertical structures, that move parallel to each other to whip the tip.  The tip has a cap structured to “grab” the upper mucus with each whipping stroke through the watery periciliary fluid.  (The top right inset photo is the “cap” and the top left inset shows the cross-sectional structure.)

Since the mucus is viscoelastic, each stroke of the cilia “nudges” the mucus, for lack of a better term, by compressing it and allowing it to relax in the direction of movement.  While one or two little strands won’t move much, millions of them can move a river of mucus in what looks like a stadium wave.  If you imagine that each strand is a person, you’ll get it.  Here’s a great video that illustrates this movement.  Start watching at the 1:30 mark and end at about 3:30.

The microscopy video of cilia moving crap out of the trachea shows how efficient it is.  Imagine if a section of the cilia just weren’t there.  And that’s what can happen when the bacteria Bortedela pertussis infects vulnerable humans.  Stay tuned for the next installment. . . .