The substance you cough up, known as phlegm or sputum, is often confused with regular mucus, but the two are not the same. Mucus is a thin, slippery gel produced constantly to lubricate and protect internal linings, including the respiratory system. Phlegm is a specific type of mucus originating in the lower airways—the lungs and throat—mixed with inflammatory cells, debris, and pathogens when the body fights an illness. Coughing forcefully expels this thick, protein-rich mixture from deep within the chest.
The Anatomical Source of Mucus
The respiratory tract, from the nasal passages down to the bronchi, is lined with a specialized protective layer of mucus. This sticky blanket is continuously produced by two primary structures embedded within the airway lining.
The first are the goblet cells, specialized epithelial cells that secrete mucins, the large glycoproteins that give mucus its sticky consistency. The second source is the submucosal glands, found deeper within the airway walls. These glands produce a more watery fluid that helps hydrate the mucus layer, ensuring it remains fluid enough to be moved. This continuous process generates an estimated one to two quarts of secretions daily, most of which is silently swallowed.
The Essential Role of the Mucociliary Escalator
The respiratory tract has a built-in self-cleaning mechanism known as the mucociliary escalator. The surface of the airway cells features thousands of microscopic, hair-like projections called cilia. These cilia beat in a synchronized, wave-like pattern, moving the mucus layer upward in a coordinated motion.
This continuous movement sweeps trapped dust, inhaled particles, and microorganisms away from the lungs and toward the throat. Once the mucus reaches the pharynx, it is usually swallowed unconsciously and destroyed by stomach acid, ensuring the lower airways remain clean. This constant clearance is the body’s first line of defense, maintaining healthy lung function without requiring a cough.
The Hypersecretion Response to Irritation and Infection
When the airways encounter irritants like smoke, pollution, viruses, or bacteria, the body initiates an inflammatory response. This signals the respiratory lining to rapidly increase mucus output, a process termed hypersecretion. This aims to flush out invading pathogens or foreign material that has overwhelmed the normal cleaning capacity of the mucociliary escalator.
During an infection, immune cells like neutrophils rush to the site to fight invaders and become incorporated into the thickened mucus. The rapid, excessive production of this inflamed mucus (phlegm) can become too voluminous and viscous for the cilia to move effectively. When the mucociliary escalator becomes overloaded, the cough reflex is triggered as a mechanism to physically expel the accumulated phlegm from the chest and restore clear airflow.
What Phlegm Color and Consistency Reveal
The appearance of phlegm offers clues about the underlying cause of irritation or infection. Clear or white phlegm often suggests normal production, allergic reactions, or viral infections, as it is composed primarily of mucins and water. A change to a yellow or green hue signals that the immune system is actively fighting an infection. This color change is caused by the presence of large numbers of neutrophils, a type of white blood cell containing a greenish enzyme.
Thick and dense phlegm usually indicates significant inflammation or dehydration because the mucins become more concentrated. Pink or red streaks suggest the presence of blood, often due to irritation or damage to the airway lining from forceful coughing. Brown or black phlegm can result from inhaling environmental pollutants, such as smoke or dust, or signify old blood or a fungal infection.