Two common examples are disorders of the nose and sinuses and of the digestive tract. What Are the Sinuses ?The sinuses are a series of bony cavities located in the skull, lined by a surface layer called the epithelium. This thin membrane closely resembles the bronchial tube lining. Each sinus opens into the nasal passages, which have a similar surface lining. Although this thin surface membrane has a pale, innocent appearance, it may become severely swollen and produce a copious discharge. The nasal and sinus passages perform several functions vital to healthy breathing, including filtering air as it is inhaled and trapping foreign particles and germs, as well as warming and adding moisture to inspired air. Symptoms of Nasal and Sinus DiseaseWhen the lining of the nasal and sinus passages becomes irritated and inflamed, the body's response is similar to what occurs in the bronchial tubes during an asthma attack. This reaction may occur in patients with and without allergies. Swelling of this lining creates congestion and stuffiness. In the nose this inflammation is called rhinitis, and when inflammation occurs too much mucus may be produced, adding to the obstruction. A watery discharge or drip is common with many irritations of the nose and sinuses. Because some of this discharge may be directed back toward the throat, it is often termed "postnasal drip." In the bony sinuses inhaled germs may take advantage of these wet, warm conditions to produce an infection called sinusitis. When that happens, the back drip may carry disease organisms into the throat and lower respiratory passages. Sinus infections may be slow to resolve and difficult to treat because of lowered blood supply to bony areas. A recent study of sinuses during common colds highlighted the extensive sinus infection and congestion that may exist then. If not adequately treated, sinus disease can become chronic, forming the basis for repeated infections.