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The oral cavity is a secondary opening for the respiratory tract. The oral and nasal cavities connect through the fauces to the pharynx, or throat. The pharynx can be divided into three regions: the nasopharynx, the oropharynx , and the laryngopharynx . Air inhaled through the mouth does not pass through the nasopharynx; it proceeds first through the oropharynx and then through the laryngopharynx. The palatine tonsils , which consist of lymphoid tissue, are located within the oropharynx. The laryngopharynx, the last portion of the pharynx, connects to the larynx , which contains the vocal fold ( [link] ).

a) diagram of ear; a closeup shows the bones and membranes of the middle ear. The eardrum is a flat disk labeled tympanic membrane. Behind this is the tympanic cavity (middle ear) which contains the bones. A tube boing downward from the middle ear is labeled Eustachian tube (auditory tube). b) A diagram of a cross section of the head. Above the nose is a space in the bone labeled frontal sinus.  The space in the nose is the nasal cavity and a duct in the nose is the nasolacrimal duct. A space in the bone behind the nose is the sphenoid sinus. At the back of the nose is the opening of the Eustachian tube (auditory tube). Behind that is the pharyngeal tonsil. Below that is a tube labeled nasopharynx which becomes the pharynx which because the oropharynx (behind the mouth) which becomes the laryngopharynx, which becomes the esophagus. Vocal folds are found just beyond the laryngopharynx in the larynx a tube which becomes the trachea. The epiglottis is a flap the determines if material in the pharynx travels to the esophagus or the trachea because the mouth also leads to the pharynx. The mouth contains the tongue. Underneath the tongue is the lingual tonsil and at the back of the mouth is the palatine tonsil. At the very back of the mouth is the fauces. In front of the trachea is the thyroid gland.
(a) The ear is connected to the upper respiratory tract by the eustachian tube, which opens to the nasopharynx. (b) The structures of the upper respiratory tract.
  • Identify the sequence of anatomical structures through which microbes would pass on their way from the nares to the larynx.
  • What two anatomical points do the eustachian tubes connect?

Anatomy of the lower respiratory system

The lower respiratory system begins below the epiglottis in the larynx or voice box ( [link] ). The trachea , or windpipe, is a cartilaginous tube extending from the larynx that provides an unobstructed path for air to reach the lungs. The trachea bifurcates into the left and right bronchi as it reaches the lungs. These paths branch repeatedly to form smaller and more extensive networks of tubes, the bronchiole s. The terminal bronchioles formed in this tree-like network end in cul-de-sacs called the alveoli . These structures are surrounded by capillary networks and are the site of gas exchange in the respiratory system. Human lungs contain on the order of 400,000,000 alveoli. The outer surface of the lungs is protected with a double-layered pleural membrane. This structure protects the lungs and provides lubrication to permit the lungs to move easily during respiration.

A drawA drawing of the lower respiratory system. The epiglottis is a flap that can allow material into the larynx. The larynx is a tube that leads to the trachea. The trachea branches to become the primary bronchi. These branch to become the secondary bronchi, these branch to become the tertiary bronchi. These branch to become the bronchioles. Terminal bronchioles end in clusters of balloon shapes called alveolar sacs. Each balloon shape is an alveolus. Thin, webbed capillaries cover the outside of the alveolus and are connected to pulmonary veins and pulmonary arteries. Oxygen from the alveolus travels into the capillary and carbon dioxide from the capillary travels into the alveolus. ing of the lower respiratory system. The epiglottis is a flap that can allow material into the larynx. The larynx is a tube that leads to the trachea. The trachea branches to become the primary bronchi. These branch to become the secondary bronchi, these branch to become the tertiary bronchi. These branch to become the bronchioles. Terminal bronchioles end in clusters of balloon shapes called alveolar sacs. Each balloon shape is an alveolus. Thin, webbed capillaries cover the outside of the alveolus and are connected to pulmonary veins and pulmonary arteries. Oxygen from the alveolus travels into the capillary and carbon dioxide from the capillary travels into the alveolus.
The structures of the lower respiratory tract are identified in this illustration. (credit: modification of work by National Cancer Institute)

Defenses of the respiratory system

The inner lining of the respiratory system consists of mucous membrane s ( [link] ) and is protected by multiple immune defenses. The goblet cell s within the respiratory epithelium secrete a layer of sticky mucus. The viscosity and acidity of this secretion inhibits microbial attachment to the underlying cells. In addition, the respiratory tract contains ciliated epithelial cells. The beating cilia dislodge and propel the mucus, and any trapped microbes, upward to the epiglottis, where they will be swallowed. Elimination of microbes in this manner is referred to as the mucociliary escalator effect and is an important mechanism that prevents inhaled microorganisms from migrating further into the lower respiratory tract.

A micrograph showing a space at the top labeled lumen of trachea. Underneath this are long cells with a brush border at the top. These cells are called pseudostratified columnar epithelia. The brush border is many cilia. Vase shaped cells in this layer are called goblet cells. Below this layer is tissue with small spheres labeled seromucous gland in submucosa.
This micrograph shows the structure of the mucous membrane of the respiratory tract. (credit: modification of micrograph provided by the Regents of University of Michigan Medical School © 2012)

The upper respiratory system is under constant surveillance by mucosa-associated lymphoid tissue (MALT) , including the adenoids and tonsils. Other mucosal defenses include secreted antibodies (IgA), lysozyme, surfactant, and antimicrobial peptides called defensins . Meanwhile, the lower respiratory tract is protected by alveolar macrophage s. These phagocytes efficiently kill any microbes that manage to evade the other defenses. The combined action of these factors renders the lower respiratory tract nearly devoid of colonized microbes.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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