The nasal septum is the wall between the left and right sides of the nose. It is firm, but bendable, and it is covered by skin that has a rich supply of blood vessels. Ideally, the nasal septum should lie exactly in the center, so that the left and right sides of the nose are of equal size. In about 80% of us, however, the nasal septum is a little off-center, although most of us never notice. Less often, the septum is more dramatically off-center. This is called a deviated septum.
In people who have a deviated septum, one side of the nose is wider than normal and one is narrower. This alters the pattern of airflow in the nose and sometimes blocks the narrowed side. In some cases, sinus openings can be blocked, triggering a sinus infection (sinusitis) that lasts a long time or keeps returning. The altered airflow pattern within the nose can cause the skin of the nasal septum to become dry and cracked, which can cause frequent nosebleeds.
Some people are born with a deviated septum because the nose developed that way before birth. A deviated septum can also be caused by injury to the nose during birth. Later in life, a deviated septum can be caused by trauma, although adults and teenagers often cannot remember the injury that caused the problem. In most cases, however, it was probably a blow to one side of the nose — often during contact sports, playground games, or a traffic accident — that knocked the nasal septum out of position.