Nose

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Nose, organ of smell, and also part of the apparatus of respiration and voice. Considered anatomically, it may be divided into an external portion—the visible projection portion, to which the term nose is popularly restricted—and an internal portion, consisting of two principal cavities, or nasal fossae, separated from each other by a vertical septum, and subdivided by spongy or turbinated bones that project from the outer wall into three passages, or meatuses, with which various sinuses in the ethmoid, sphenoid, frontal, and superior maxillary bones communicate by narrow apertures.

The margins of the nostrils are usually lined with a number of stiff hairs (vibrissae) that project across the openings and serve to arrest the passage of foreign substances, such as dust and small insects, which might otherwise be drawn up with the current of air intended for respiration. The skeleton, or framework, of the nose is partly composed of the bones forming the top and sides of the bridge, and partly of cartilages. On either side are an upper lateral and a lower lateral cartilage, to the latter of which are attached three or four small cartilaginous plates, termed sesamoid cartilages. The cartilage of the septum separates the nostrils and, in association posteriorly with the perpendicular plate of the ethmoid and with the vomer, forms a complete partition between the right and left nasal fossae.

The nasal fossae, which constitute the internal part of the nose, are lofty and of considerable depth. They open in front through the nostrils and behind end in a vertical slit on either side of the upper pharynx, above the soft palate, and near the orifices of the Eustachian tubes, leading to the tympanic cavity of the ear.

In the olfactory region of the nose the mucous membrane is very thick and colored by a brown pigment. The olfactory nerve, or nerve of smell, terminates in the nasal cavity in several small branches; these ramify in the soft mucous membrane and end in tiny varicose fibers that in turn terminate in elongated epithelial cells projecting into the free surface of the nose.

For diseases of the nose, see Cold, Common; Rhinitis.

Regulation of Respiratory System

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The flow of air in and out of the lungs is controlled by the nervous system, which ensures that humans breathe in a regular pattern and at a regular rate. Breathing is carried out day and night by an unconscious process. It begins with a cluster of nerve cells in the brain stem called the respiratory center. These cells send simultaneous signals to the diaphragm and rib muscles, the muscles involved in inhalation. The diaphragm is a large, dome-shaped muscle that lies just under the lungs. When the diaphragm is stimulated by a nervous impulse, it flattens. The downward movement of the diaphragm expands the volume of the cavity that contains the lungs, the thoracic cavity. When the rib muscles are stimulated, they also contract, pulling the rib cage up and out like the handle of a pail. This movement also expands the thoracic cavity. The increased volume of the thoracic cavity causes air to rush into the lungs. The nervous stimulation is brief, and when it ceases, the diaphragm and rib muscles relax and exhalation occurs. Under normal conditions, the respiratory center emits signals 12 to 20 times a minute, causing a person to take 12 to 20 breaths a minute. Newborns breathe at a faster rate, about 30 to 50 breaths a minute.

The rhythm set by the respiratory center can be altered by conscious control. The breathing pattern changes when a person sings or whistles, for example. A person also can alter the breathing pattern by holding the breath. The cerebral cortex, the part of the brain involved in thinking, can send signals to the diaphragm and rib muscles that temporarily override the signals from the respiratory center. The ability to hold one’s breath has survival value. If a person encounters noxious fumes, for example, it is possible to avoid inhaling the fumes.

A person cannot hold the breath indefinitely, however. If exhalation does not occur, carbon dioxide accumulates in the blood, which, in turn, causes the blood to become more acidic. Increased acidity interferes with the action of enzymes, the specialized proteins that participate in virtually all biochemical reaction in the body. To prevent the blood from becoming too acidic, the blood is monitored by special receptors called chemoreceptors, located in the brainstem and in the blood vessels of the neck. If acid builds up in the blood, the chemoreceptors send nervous signals to the respiratory center, which overrides the signals from the cerebral cortex and causes a person to exhale and then resume breathing. These exhalations expel the carbon dioxide and bring the blood acid level back to normal.

A person can exert some degree of control over the amount of air inhaled, with some limitations. To prevent the lungs from bursting from overinflation, specialized cells in the lungs called stretch receptors measure the volume of air in the lungs. When the volume reaches an unsafe threshold, the stretch receptors send signals to the respiratory center, which shuts down the muscles of inhalation and halts the intake of air.

Alveoli

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The bronchioles divide many more times in the lungs to create an impressive tree with smaller and smaller branches, some no larger than 0.5 mm (0.02 in) in diameter. These branches dead-end into tiny air sacs called alveoli. The alveoli deliver oxygen to the circulatory system and remove carbon dioxide. Interspersed among the alveoli are numerous macrophages, large white blood cells that patrol the alveoli and remove foreign substances that have not been filtered out earlier. The macrophages are the last line of defense of the respiratory system; their presence helps ensure that the alveoli are protected from infection so that they can carry out their vital role.

The alveoli number about 150 million per lung and comprise most of the lung tissue. Alveoli resemble tiny, collapsed balloons with thin elastic walls that expand as air flows into them and collapse when the air is exhaled. Alveoli are arranged in grapelike clusters, and each cluster is surrounded by a dense hairnet of tiny, thin-walled capillaries. The alveoli and capillaries are arranged in such a way that air in the wall of the alveoli is only about 0.1 to 0.2 microns from the blood in the capillary. Since the concentration of oxygen is much higher in the alveoli than in the capillaries, the oxygen diffuses from the alveoli to the capillaries. The oxygen flows through the capillaries to larger vessels, which carry the oxygenated blood to the heart, where it is pumped to the rest of the body.

Carbon dioxide that has been dumped into the bloodstream as a waste product from cells throughout the body flows through the bloodstream to the heart, and then to the alveolar capillaries. The concentration of carbon dioxide in the capillaries is much higher than in the alveoli, causing carbon dioxide to diffuse into the alveoli. Exhalation forces the carbon dioxide back through the respiratory passages and then to the outside of the body.

Trachea, Bronchi, and Bronchioles

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Air passes from the larynx into the trachea, a tube about 12 to 15 cm (about 5 to 6 in) long located just below the larynx. The trachea is formed of 15 to 20 C-shaped rings of cartilage. The sturdy cartilage rings hold the trachea open, enabling air to pass freely at all times. The open part of the C-shaped cartilage lies at the back of the trachea, and the ends of the “C” are connected by muscle tissue.

The base of the trachea is located a little below where the neck meets the trunk of the body. Here the trachea branches into two tubes, the left and right bronchi, which deliver air to the left and right lungs, respectively. Within the lungs, the bronchi branch into smaller tubes called bronchioles. The trachea, bronchi, and the first few bronchioles contribute to the cleansing function of the respiratory system, for they, too, are lined with mucous membranes and ciliated cells that move mucus upward to the pharynx.

Larynx

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Air moves from the pharynx to the larynx, a structure about 5 cm (2 in) long located approximately in the middle of the neck. Several layers of cartilage, a tough and flexible tissue, comprise most of the larynx. A protrusion in the cartilage called the Adam’s apple sometimes enlarges in males during puberty, creating a prominent bulge visible on the neck.

While the primary role of the larynx is to transport air to the trachea, it also serves other functions. It plays a primary role in producing sound; it prevents food and fluid from entering the air passage to cause choking; and its mucous membranes and cilia-bearing cells help filter air. The cilia in the larynx waft airborne particles up toward the pharynx to be swallowed.

Food and fluids from the pharynx usually are prevented from entering the larynx by the epiglottis, a thin, leaflike tissue. The “stem” of the leaf attaches to the front and top of the larynx. When a person is breathing, the epiglottis is held in a vertical position, like an open trap door. When a person swallows, however, a reflex causes the larynx and the epiglottis to move toward each other, forming a protective seal, and food and fluids are routed to the esophagus. If a person is eating or drinking too rapidly, or laughs while swallowing, the swallowing reflex may not work, and food or fluid can enter the larynx. Food, fluid, or other substances in the larynx initiate a cough reflex as the body attempts to clear the larynx of the obstruction. If the cough reflex does not work, a person can choke, a life-threatening situation. The Heimlich maneuver is a technique used to clear a blocked larynx (see First Aid). A surgical procedure called a tracheotomy is used to bypass the larynx and get air to the trachea in extreme cases of choking.

Pharynx

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Air leaves the nasal passages and flows to the pharynx, a short, funnel-shaped tube about 13 cm (5 in) long that transports air to the larynx. Like the nasal passages, the pharynx is lined with a protective mucous membrane and ciliated cells that remove impurities from the air. In addition to serving as an air passage, the pharynx houses the tonsils, lymphatic tissues that contain white blood cells. The white blood cells attack any disease-causing organisms that escape the hairs, cilia, and mucus of the nasal passages and pharynx. The tonsils are strategically located to prevent these organisms from moving further into the body. One tonsil, called the adenoids, is found high in the rear wall of the pharynx. A pair of tonsils, the palatine tonsils, is located at the back of the pharynx on either side of the tongue. Another pair, the lingual tonsils, is found deep in the pharynx at the base of the tongue. In their battles with disease-causing organisms, the tonsils sometimes become swollen with infection. When the adenoids are swollen, they block the flow of air from the nasal passages to the pharynx, and a person must breathe through the mouth.

Nasal Passages

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The flow of air from outside of the body to the lungs begins with the nose, which is divided into the left and right nasal passages. The nasal passages are lined with a membrane composed primarily of one layer of flat, closely packed cells called epithelial cells. Each epithelial cell is densely fringed with thousands of microscopic cilia, fingerlike extensions of the cells. Interspersed among the epithelial cells are goblet cells, specialized cells that produce mucus, a sticky, thick, moist fluid that coats the epithelial cells and the cilia. Numerous tiny blood vessels called capillaries lie just under the mucous membrane, near the surface of the nasal passages. While transporting air to the pharynx, the nasal passages play two critical roles: they filter the air to remove potentially disease-causing particles; and they moisten and warm the air to protect the structures in the respiratory system.

Filtering prevents airborne bacteria, viruses, other potentially disease-causing substances from entering the lungs, where they may cause infection. Filtering also eliminates smog and dust particles, which may clog the narrow air passages in the smallest bronchioles. Coarse hairs found just inside the nostrils of the nose trap airborne particles as they are inhaled. The particles drop down onto the mucous membrane lining the nasal passages. The cilia embedded in the mucous membrane wave constantly, creating a current of mucus that propels the particles out of the nose or downward to the pharynx. In the pharynx, the mucus is swallowed and passed to the stomach, where the particles are destroyed by stomach acid. If more particles are in the nasal passages than the cilia can handle, the particles build up on the mucus and irritate the membrane beneath it. This irritation triggers a reflex that produces a sneeze to get rid of the polluted air.

The nasal passages also moisten and warm air to prevent it from damaging the delicate membranes of the lung. The mucous membranes of the nasal passages release water vapor, which moistens the air as it passes over the membranes. As air moves over the extensive capillaries in the nasal passages, it is warmed by the blood in the capillaries. If the nose is blocked or “stuffy” due to a cold or allergies, a person is forced to breath through the mouth. This can be potentially harmful to the respiratory system membranes, since the mouth does not filter, warm, or moisten air.

In addition to their role in the respiratory system, the nasal passages house cells called olfactory receptors, which are involved in the sense of smell. When chemicals enter the nasal passages, they contact the olfactory receptors. This triggers the receptors to send a signal to the brain, which creates the perception of smell.