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The nose is the main organ that allows air to flow into the lungs. For most adults, about 18,000 to 20,000 liters of air pass through the nose every day. The nose offers an entrance to the respiratory system (the body system that exchanges gases between air and blood), through two passageways (the nostrils). The structure supporting the upper part of the external nose is made of bone, while the lower part is made of cartilage.

Each nostril opens up into its own nasal cavity, which is separated by the nasal septum. The septum is made of bone and cartilage and runs from the nostrils to the back of the throat. Bones that project into the nasal cavity (nasal conchae) form a series of folds (turbinates) that increase the surface area of each nasal cavity. The back of the nasal cavity opens up into the airways that lead to the lungs. The nose also acts as the main passageway for air to leave the body when a person exhales. Air can also enter and leave the body through the mouth, or oral cavity.

The most important function of the nose and nasal cavity is to process each breath before it enters the lungs. This filtering process is important for several reasons. Considering the amount of air that passes through the respiratory system each day, it is important that the nose:

• Filter out dangerous particles (e.g., bacteria, viruses, dust, pollen) that could otherwise enter into the lungs, causing damage. Tiny hairs (cilia) and a mucous membrane line the inside of the nose to trap particles before they enter the body. The cilia are capable of small movements and can direct the flow of mucus (a substance secereted by mucous membranes), removing it from the nasal cavity. Sneezing forces air through the nasal cavity and is also effective at removing particles and mucus.
  
  
• Warm each breath to prevent cold air from damaging sensitive lung tissue. The large amount of surface area and many blood vessels in the nasal cavity help warm each breath quickly as heat transfers from the blood to the passing air.
  
  
• Add moisture to each breath to prevent the airways and lungs from becoming dry and damaged. As air passes through the nasal cavity, moisture is transferred from the mucus secreted by the lining of the nasal cavity into the air.

Because the nose and nasal cavity are so effective at filtering, warming and moisturizing each breath, it is generally better to breathe through the nose than the mouth. Though the mouth can allow a person to inhale more quickly, over time, mouth-breathing can dry out the delicate tissue in the airways and lungs. Furthermore, oxygen may be easier for the lungs to extract from air that has been filtered, warmed and humidified through the nose and nasal cavities.

The nose and nasal cavity are also important for several other reasons:

• The nasal cavity contains smell receptor cells that signal the brain when they encounter different chemicals. The brain can interpret these signals as various smells. The sense of smell also affects a person’s perception of the taste of foods. When people sniff, air flow is increased over the smell receptor cells. This increases a person’s exposure to odors.
  
  
• Tears drain into the nasal cavity from the eyes when crying. This drainage occurs through the tear duct known as the nasolacrimal duct. This drainage helps prevent the entrance of foreign particles through the nose and explains why a person’s nose “runs” when crying.
  
  
• The eustachian tube runs from the back of the nasal cavity to the ears, equalizing the pressure between them. This connection also allows for ear drainage and ventilation, which helps prevent infection.
  
  
• The nasal cavity is connected to several of the sinuses, allowing for drainage from these areas. This allows for the easy removal of foreign bacteria and viruses from the sinuses, which helps prevent infection.

At the back of the nasal cavity is the nasopharynx, which forms the upper part of the throat. The nasopharynx contains adenoid tissue. The adenoid is made up of lymph tissue, which plays a role in fighting infection during the first year of life. After childhood, adenoid tissue shrinks back, reducing the threat of poor sinus drainage and sinus disease. A large adenoid can cause a blockage of the eustachian tubes and may need to be removed by a procedure known as an adenoidectomy.

Common colds and other infections can cause inflammation of the nasopharynx, This can block the eustachian tubes, which results in a feeling of clogged ears.

Patients experiencing prolonged or recurrent nasal problems, such as congestion, sneezing, runny nose or thick nasal discharge, are encouraged to contact their physician. These symptoms may require prompt medical attention because they can aggravate lung problems and lead to other conditions. For example, untreated nasal allergies can increase a person's risk of developing asthma (a chronic inflammation of the airway tissues), or make existing asthma worse.