Just inside the nostril is a collection of capillaries (called Hasselbach's plexus) on the middle wall of the nose. This is the most common site of spontaneous nosebleeds in children. Simple inspection of this area after a nosebleed can often identify the offending vessels - a pleasant job most often left to the pediatrician. These blood vessels, which are simply tubes constructed of cells that are adherent to one another, become dilated when inflamed. This dilatation creates gaps between the cells, which can no longer withstand increasing pressure of the blood that fills the vessels.
Among the common reasons for the lining of the nose to be inflamed are colds, allergies (even in the absence of other allergy symptoms) and overuse of decongestant nose drops. Other topical or oral medications may also trigger nosebleeds.
at 12:21 AM on Nov. 15, 2009