(NaturalNews) Human lungs contain a tiny network of constantly moving "brushes" that flush contaminants out of the respiratory system, according to research conducted by scientists from the University of North Carolina and published in the journal Science.
Scientists have known for a long time that the respiratory system protects itself by means of a coating of mucus, which is sticky enough to trap pollutants and keep them from reaching the body's cells. When needed, the body can expel this mucus through a runny nose or a cough.
"The air we breathe isn't exactly clean, and we take in many dangerous elements with every breath," said lead researcher Michael Rubinstein.
"We need a mechanism to remove all the junk we breathe in, and the way it's done is with a very sticky gel, called mucus, that catches these particles and removes them with the help of tiny cilia. The cilia are constantly beating, even while we sleep.
"In a coordinated fashion, they push mucus, containing foreign objects, out of the lungs, and we either swallow it or spit it out. These cilia even beat for a few hours after we die. If they stopped, we'd be flooded with mucus that provides a fertile breeding ground for bacteria."
But until now, researchers have never understood why the mucus does not stick to or even infiltrate the respiratory cells themselves. The foremost theory, known as the "gel-on-liquid model," posited that an as-yet-undiscovered watery "periciliary" layer kept mucus and cilia separate. The problem with this theory was always that to the best of scientific knowledge, mucus should eventually dissolve into such a watery layer, not remain separate.
"We can't have a watery layer separating sticky mucus from our cells because there is an osmotic pressure in the mucus that causes it to expand in water," Rubinstein says. "So what is really keeping the mucus from sticking to our cells?"
To get to the bottom of the mystery, the researchers used modern imaging techniques to examine the interior of the lungs. They found a dense network of brush-like structures that sit atop the cilia. These brushes are composed of protective molecules that keep both mucus and contaminants from getting to the respiratory cells beneath. These molecules also function as a second line of defense against viruses or bacteria that manage to penetrate the mucus.
Stephen Spiro of the British Lung Foundation said the findings could help significantly improve scientific understanding of lung function.
"Mucus has a complex biological make-up and forms a vital part of the lungs' defense mechanism," he said.
"Research such as this helps our understanding [of] how this system works, and of the complex mechanisms deep within our lungs which protect us from the atmosphere we breathe in."
Rubinstein and his fellow researchers noted that their findings may also explain previously mysterious lung disorders from asthma to cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD). These diseases may stem from a collapse of the protective brushes.
"We found that there is a specific condition, below which the brush is healthy and cells are happy," Rubinstein said. "But above this ideal condition, in diseases like CF or COPD, the brush becomes compressed and actually prevents the normal cilia beating and healthy flow of mucus."
In such conditions, the mucus would then stick directly to the lung's cells.
"The collapse of this brush is what can lead to immobile mucus and result in infection, inflammation and eventually the destruction of lung tissue and the loss of lung function," Rubinstein said. "But our new model should guide researchers to develop novel therapies to treat lung diseases and provide them with biomarkers to track the effectiveness of those therapies."