Mucus and phlegm are two terms often used interchangeably, leading to confusion about what they represent in the body. While related, they are distinct substances defined by their origin and purpose within the respiratory system. Mucus is a general, protective substance produced continuously as part of normal bodily function, whereas phlegm is a specific, often thicker form of mucus arising from the lower airways, typically signaling inflammation or infection.
The Role and Composition of Mucus
Mucus is a biological hydrogel that acts as a primary defense mechanism, lining the moist surfaces of several bodily systems. It is primarily composed of water, electrolytes, and large, gel-forming proteins called mucins. Specialized cells, particularly goblet cells, produce this viscoelastic substance throughout the respiratory, digestive, and reproductive tracts.
The main function of mucus is to lubricate and hydrate the underlying tissue, preventing it from drying out. A continuous blanket of mucus in the airways traps inhaled foreign particles, such as dust, pollen, and bacteria. This contaminated mucus is then moved out of the lungs and toward the throat by tiny, hair-like structures called cilia, a process known as mucociliary clearance.
Mucus contains various immune components, including antibodies and enzymes, that actively neutralize or degrade pathogens. Under normal, non-diseased conditions, the body produces approximately one to one and a half liters of this clear, thin substance daily. Most of which is unconsciously swallowed and processed by the digestive system.
Phlegm: A Symptom from the Lower Airways
Phlegm is a specific type of mucus produced deep within the lower respiratory tract, originating from the bronchi and lungs. The appearance of phlegm is often considered a pathological sign, meaning it is a byproduct of disease or significant inflammation. It is generally expelled from the body by a productive cough.
Phlegm is thicker and more viscous than normal mucus because it is heavily laden with components of the immune response. Its composition includes water and mucins, but also high concentrations of inflammatory cells, such as white blood cells, along with dead cellular debris and the pathogens the body is fighting. When phlegm is coughed up and expelled, it is clinically referred to as sputum.
Phlegm production increases significantly in response to conditions like bronchitis, pneumonia, or chronic lung disease. This occurs as the body attempts to clear the deeper air passages of irritants or infection.
Interpreting Changes in Consistency and Color
The visual characteristics of expelled respiratory secretions, whether mucus or phlegm, can offer clues about the underlying cause. Normally, mucus is clear and thin, or sometimes white and slightly thicker if there is minor congestion or dehydration. An increase in clear, watery secretions can often be a sign of environmental irritants or allergies as the body tries to flush them out.
If the color changes to yellow or green, it typically indicates a significant presence of neutrophils, a type of white blood cell, fighting an infection. These immune cells release an enzyme that contains a green pigment, leading to the color change. However, the color alone cannot definitively distinguish between the two, as both types of infection can cause yellow or green secretions.
Other colors warrant closer attention; brown or black phlegm can result from inhaling heavy pollutants, smoking, or may contain old, dried blood. Red or pink-tinged secretions indicate the presence of fresh blood, which can be due to minor irritation from forceful coughing, but also suggests the need for immediate medical evaluation. Any significant change in color or consistency, particularly if accompanied by fever, chest pain, or shortness of breath, requires monitoring by a healthcare professional.