Each day, a large amount of mucous is produced by the tissues of the nose. The major causes of a runny nose are excess mucous production and blockage of the normal flow of mucous. There are several causes of nasal obstruction. Anatomic causes such as a deviated septum or an abnormally narrow nasal passage can result mucous drainage. One sided nasal drainage which is chronic is suspicious for a foreign body, such as a piece of plastic or food, blocking the nose.
Tissue swelling from allergy and infection can also plug the nose.
In many cases in children, chronic nasal obstruction and drainage is caused by excessively large adenoids. The adenoids are a lump of tissue high in the throat behind the nose which can be likened to a tonsil. Often the child grows out of the problem. If symptoms are severe and conservative treatment fails, surgery is required to correct the problem. If one likens white blood cells to soldiers fighting the human body's war against infection, then lymphoid tissue is analogous to the fort in which they reside.