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Phlegm

What is it? Phlegm is the mucus produced in the respiratory system, the thick stuff you cough up when you have a respiratory illness. 

Why? Like other mucus in the body, phlegm is a water-based mucin mixture secreted by the mucous membranes lining the interior structures to protect them. When you breathe in, snot stops foreign particles in the nose, phlegm stops them in the lungs. Small hairs (aka cilia) in the respiratory system push the mucus up to the throat where it is swallowed along with the other mucus from the nose and mouth. You don't even know it's there until you have too much of it!

Fancy term for this: 

Cilia on bronchial epithelium in the lungs

What about different colors? Just like snot, the mucus thickens when the airway is irritated. Smokers, people with chronic respiratory illnesses (like COPD and cystic fibrosis), people with bronchitis or pneumonia, and someone who swallowed "down the wrong pipe" experience thicker phlegm and may even cough some up (expectoration). Yellow and green tend to be associated with bacterial infections, red or brown with blood in the lungs or airway, and grey with smokers' hack, but what's coughed up could also include nasal mucus that hasn't yet been swallowed and mean absolutely nothing.
 
Gross fact: Mucus that's coughed up is called sputum, but mucus that just runs out of an orifice is called discharge. 

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