Microorganisms that colonize the respiratory system enter an environment quite different than the one faced by skin organisms. The dark caverns and tortuous passages of the respiratory system are relatively hot and humid, and the atmosphere contains less oxygen and more carbon dioxide than occurs in air. The eyes and the nose are the two gateways, or "portals of entry," to the respiratory system. They are more than entrances; respiratory infections often first establish themselves there and then spread to other parts of the system.
Structures commonly involved in infections of the upper respiratory system (figure 23.1) are:
■ the conjunctiva, composing the moist surfaces of the eyes and eyelids. Infection of the conjunctiva is called conjunctivitis;
■ the nasolacrimal, or tear ducts, from the eyes to the nasal chamber. Infection of the tear ducts is called dacryocystitis;
■ the middle ear. Infection of the middle ear is called otitis media. The external ear canal is not part of the respiratory system. It is a blind tunnel lined with skin and is part of the skin ecosystem; infection there is called external otitis;
■ the air-filled chambers of the skull, the sinuses and mastoid air cells. Infections of these chambers are called sinusitis and mastoiditis, respectively;
■ the nose. Infection of the nose is called rhinitis;
■ the throat, or pharynx. Infections of the throat are called pharyngitis;
■ the epiglottis is a little muscular flap that covers the opening to the lower respiratory system during swallowing, thereby preventing material from entering. Infection of the epiglottis is called epiglottitis.
The tonsils are composed of lymphoid tissue, strategically located to come into contact with incoming microorganisms. The tonsils are important producers of antibodies to infectious agents, but paradoxically they can also be the sites of infection, resulting in tonsillitis. Enlargement of the pharyngeal tonsil, also called "adenoids," can contribute to ear infections by interfering with normal drainage from the eustachian tubes. These tubes, which extend from the middle ear to the nasopharynx, are 3 to 4 cm long. They equalize the pressure in the middle ear and drain normal mucous secretions. ■ lymphoid system, p. 396
A person normally breathes about 16 times per minute, inhaling about 0.5 liters of air with each breath, or more than 11,500 liters of air per day, with any accompanying microorganisms or other pollutants. The air enters the respiratory system at the nostrils, flows into the nasal cavity, and is deflected downward through the throat.
One of the main functions of the upper respiratory tract is to regulate the temperature and water content of inspired air.
When cold air enters the upper respiratory tract, nervous reflexes immediately increase the blood flow to the spongy tissues in the nose, thereby transferring heat to the air. This mechanism usually adjusts the temperature to within 2 to 3 degrees of body temperature by the time it reaches the lungs. Inspired air also becomes saturated with water vapor, the nasal tissues giving up as much as a quart of water per day to keep the air humidified. The warmth and moisture provide optimum conditions for the body's defenses against infection.
The air then enters the lower respiratory tract below the epiglottis. Structures commonly involved in lower respiratory system infections include:
■ the voice box, or larynx (see figure 23.1). Inflammation of the larynx is called laryngitis, and is manifest as hoarseness.
■ The larynx is continuous with the windpipe, or trachea, which branches into two bronchi. Inflammation of the bronchi is called bronchitis, commonly the result of infection or smoking tobacco.
■ The bronchi branch repeatedly, becoming bronchioles, site of an important viral infection called bronchiolitis.
■ The smallest branches of the bronchioles end in the tiny, thin-walled air sacs called alveoli that comprise the bulk of lung tissue. Inflammation of the lungs is called pneumonitis, often the result of viral infections. Pneumonitis that results in the filling of alveoli with pus and fluid is called pneumonia. Macrophages are numerous in the lung tissues and readily move into the alveoli and airways to engulf infectious agents, thus helping to prevent pneumonia from developing.
■ The lungs are surrounded by two membranes: One adheres to the lung and the other to the chest wall and diaphragm. These membranes, termed pleura, normally slide against each other as the lung expands and contracts. Inflammation of the pleura is called pleurisy, characterized by severe chest pain aggravated by breathing or coughing.
The moist membranes of the respiratory system are coated with a slimy glycoprotein material called mucus, which is produced by specialized unicellular glands called goblet cells. Goblet cells, so called because of their shape, narrow at the base and wide at surface, are sprinkled among the other cells composing the membrane. Most of the respiratory system mucous membranes are composed of ciliated epithelium. Ciliated cells have tiny hairlike projections called cilia along their exposed free border. The cilia beat synchronously at a rate of about 1,000 times a minute, continually propelling the mucous film out of the mastoids, middle ear, nasolacrimal duct, sinuses, and the lungs. The mucus is then swallowed, and any entrapped microorganisms and viruses are exposed to the killing action of stomach acid and enzymes. This mechanism, called the mucociliary escalator, normally keeps the middle ears, sinuses, mastoids, and lungs completely
Figure 23.1 Anatomy and infections of the Respiratory System (a) Lateral view, upper respiratory system, showing the inside of the nasal chamber and its connections with the eyes, middle ears, and sinuses, and details of the respiratory epithelium. (b) Frontal view of the upper and lower respiratory system, including details of the alveoli.
Upper Respiratory — System
Lower Respiratory — System
Nasolacrimal duct opening
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