Most of us rarely think about snot or mucus production. Yet, it is always present. What is the point of having this viscous substance?
There are two types of mucous membranes in the nasal cavity. The olfactory epithelium mucosa is in the superior region of the nasal cavity and is where we find the smell receptors. The rest of the nasal mucosa is called respiratory mucosa and contains goblet cells containing both mucous and serous glands. As one might expect, mucous glands produce mucus while the serous glands secrete an enzyme-rich, watery fluid. On a daily basis these glands secrete approximately a quart of mucus that contains lysozyme. Lysozyme is an enzyme that is helpful in the function of material introduced into the respiratory passage. Additionally, mucus is quite sticky allowing it to trap dust, bacteria, and debris that are inspired. The respiratory mucosa also contains epithelial cells which produce defensins. Once the mucus does its job trapping various inhaled particles and acting upon bacteria, ciliated cells in the respiratory mucosa produce a gentle movement of the contaminated mucus towards the throat where it is swallowed and then digested by gastric secretions. The mucus film is also rich in water which is helpful as it humidifies inhaled air.1
Typically, snot/nasal mucus is clear because it has a high-water content that also contains proteins, dissolved salts, and other products of the immune system. When the movement of mucus slows within the system, the mucus can become white as it loses moisture and becomes thicker and cloudier. The mucus may turn yellow as white blood cells congregate to particular sites. When the white blood cells are spent, they become part of the mucus giving it a yellow color. The more white blood cells you have congregating, the more likely the mucus may become greener in color. Occasionally, mucus becomes pink or red indicating that it is mixing with capillary blood. This often occurs when the respiratory mucosa becomes dry, irritated, or damaged. Sometimes the mucus may become brown from dried blood or, more commonly, from the inhalation of dirt.
While typically we blame snot and mucus for nasal congestion, this is actually usually caused by vascular changes. Certainly, plasma leakage, the movement of immunocompetent cells, and long-term remodeling of the nasal mucosa may also be relevant. The nasal mucosa contains resistance and capacity blood vessels (capacitance vessels and venous sinusoids) used in mucosal defense and various physiologic nasal functions. The nervous system controls their volume and is regulated by hormonal changes and mediators from immunocompetent cells in the nasal mucosa. Together these give rise to the nasal cycle, a way of controlling nasal airflow resistance. For example, cold, dry air will slow the movement of mucus, whereas exercise may cause a decrease in nasal airflow resistance. Longstanding experiences of slow-moving mucus is likely due to the release of cell mediators. This is particularly the case in the late-phase response and is marked by the presence of eosinophils in the nasal mucosa.2
In some circumstances, there are changes to the tight junction and adherens junction components of nasal epithelial cells affecting the nasal epithelial junction. Further, this is associated with increased mucus production, increased vascular permeability and additional immune effects as a result of decreased mRNA levels of zonulin, occludin, claudin-1 and E-cadherin.3
REFERENCES
1. Marieb EN et al. Human anatomy and physiology. 8th ed. San Francisco: Benjamin-Cummings Publishing 2010.
2. Spronsen EV et al. Allergy. 2008;63(7):820-33.
3. Yeo N-K et al. Laryngoscope. 2010 Feb;120(2):346-52.
