<P> The term was coined in 1866 and is a combination of the Greek terms rhino - ("of the nose") and - rhoia ("discharge" or "flow"). </P> <P> Rhinorrhea is characterized by an excess amount of mucus produced by the mucous membranes that line the nasal cavities . The membranes create mucus faster than it can be processed, causing a backup of mucus in the nasal cavities . As the cavity fills up, it blocks off the air passageway, causing difficulty breathing through the nose . Air caught in nasal cavities, namely the sinus cavities, cannot be released and the resulting pressure may cause a headache or facial pain . If the sinus passage remains blocked, there is a chance that sinusitis may result . If the mucus backs up through the Eustachian tube, it may result in ear pain or an ear infection . Excess mucus accumulating in the throat or back of the nose may cause a post-nasal drip, resulting in a sore throat or coughing . Additional symptoms include sneezing, nosebleeds, and nasal discharge . </P> <P> Rhinorrhea is especially common during winter months and certain low temperature seasons . Cold - induced rhinorrhea occurs due to a combination of thermodynamics and the body's natural reactions to cold weather stimuli . One of the purposes of nasal mucus is to warm inhaled air to body temperature as it enters the body . In order for this to happen, the nasal cavities must be constantly coated with liquid mucus . During cold, dry seasons, the mucus lining nasal passages tends to dry out, meaning that mucous membranes must work harder, producing more mucus to keep the cavity lined . As a result, the nasal cavity can fill up with mucus . At the same time, when air is exhaled, water vapor in breath condenses as the warm air meets the colder outside temperature near the nostrils . This causes an excess amount of water to build up inside nasal cavities . In these cases, the excess fluid usually spills out externally through the nostrils . </P> <P> Rhinorrhea can be a symptom of other diseases, such as the common cold or influenza . During these infections, the nasal mucous membranes produce excess mucus, filling the nasal cavities . This is to prevent infection from spreading to the lungs and respiratory tract, where it could cause far worse damage . It has also been suggested that rhinorrhea is a result of viral evolution, and may be a response that is not useful to the host, but which has evolved by the virus to maximise its own infectivity . Rhinorrhea caused by these infections usually occur on circadian rhythms . Over the course of a viral infection, sinusitis (the inflammation of the nasal tissue) may occur, causing the mucous membranes to release more mucus . Acute sinusitis consists of the nasal passages swelling during a viral infection . Chronic sinusitis occurs when one or more nasal polyps appear . This can be caused by a deviated septum as well as a viral infection . </P>

Where does the water from a runny nose come from
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