Chapter 24 - Respiratory System
 
Overview of Respiratory System
     The respiratory system consists of all the tissue and organs designed to bring air to the gas exchange surface where oxygen is absorbed and carbon dioxide is released.
     The respiratory system can be divided into:
  upper respiratory system that includes the nose, nasal cavity, paranasal sinuses and pharynx; and
  lower respiratory system that includes the larynx, trachea, bronchi and lungs.
     The respiratory system is essentially designed to support the respiratory tract that consists of the passageways through which the air travels to reach the gas exchange surface. 
     The respiratory tract can be divided into:
  conducting portion from the nasal cavity to the terminal bronchioles through which no gas exchange occurs; and
  respiratory portion that includes the respiratory bronchioles and alveoli where gas exchange occurs.
 
  Functions 
       The primary functions of the respiratory system are to:
  1. Provide a gas exchange surface;
  2. Move air to and from exchange surface.
       These functions create problems for the body that are solved by features in its design. So the respiratory system also functions to:
  3. Protect respiratory surfaces from environment;
  4. Defend against invasion by pathogens.
       Auxiliary functions of the respiratory system include the:
  5. Production of sound;
   6. Involvement in regulation of blood volume and pressure, and control of body pH.
 
  Respiratory Epithelium
       Most of the mucous membrane lining the conducting portion of the respiratory tract is lined by pseudostratified, ciliated, columnar epithelium (PCCE), which is also know as respiratory epithelium.
       PCCE serves the protective function of the respiratory system. Goblet cells, and mucous glands found in the lamina propria, produce mucus that traps particles on the surface. The cilia move sheets of mucus with trapped debris and pathogens out of the tract and toward the esophagus.
 
  Upper Respiratory System  
  Nose and Nasal Cavity
     The nose protects the entrance to the upper respiratory tract. The openings of the nose are called external nares (nostrils). The nose surrounds a portion of the nasal cavity called the nasal vestibule where coarse hairs filter out large particles and insects. The nasal cavity is separated into halves by a nasal septum. Cartilage and bone forms the septum and also supports the dorsum of the nose (bridge) and the apex of the nose (tip).
     The nasal cavity is the chamber between external and internal nares. Coiled, shelf-like extensions of bones called superior, middle and inferior nasal conchae (a.k.a. turbinates) extend from the lateral walls of the nasal cavity toward the nasal septum. Narrow passageways between the conchae are called the superior, middle and inferior meatuses. The air that passes through the meatuses is filtered, warmed and humidified. The hard palate is a bony partition that separates the nasal cavity from the oral cavity. The fleshy extension of the hard palate is called the soft palate.
     Paranasal sinuses are present in bones that surround the nasal cavities. These sinuses are confluent with the nasal cavity and they produce secretions that flow into the nasal cavity to keep it moist.
     The internal nares is the passageway between the nasal cavity and the pharynx.
  Pharynx
     The pharynx is the passageway connecting the nasal cavity, oral cavity, esophagus and larynx. It is shared by the digestive and respiratory tracts.
     The pharynx is divided into three regions:
  1. Nasopharynx 
     The nasopharynx is the superior portion of pharynx. It begins at the internal nares and is separated from oral cavity by the soft palate. It has typical respiratory epithelium. The pharyngeal tonsil is located on posterior wall and the openings of auditory tubes are found in the lateral walls.
  2. Oropharynx
     The oropharynx is between the soft palate and base of tongue at the level of the hyoid. It is lined by stratified squamous epithelium.
     A fleshy protuberance on posterior margin of soft palate is called the uvula. Curving folds of mucous membrane called pharyngeal arches extend from the soft palate to the base of the tongue. The palatine tonsils lie between pharyngeal arches.
     The fauces is the passageway between the oral cavity and oropharynx It is bounded by the anterior pharyngeal arches and the uvula.
  3. Laryngopharynx
     The laryngopharynx is the most inferior part and lies between hyoid bone and opening of esophagus. It is also lined by stratified squamous epithelium.
 
  Lower Respiratory System
  Larynx
     Air enters the lower respiratory system through an opening called the glottis. The larynx is a cylindrical structure that surrounds, protects and controls the glottis.
     The larynx is a complex structure made up of pieces of cartilage held together by muscles and ligaments. There are three unpaired cartilages:
  1. Thyroid cartilage (shield-shaped).
     The thyroid cartilage is the largest cartilage and forms anterior and lateral walls of the larynx. Like a shield, it is open in the back and on the anterior surface has a prominent ridge called the laryngeal prominence (Adam’s apple).
  2. Cricoid cartilage (ring-shaped).
     This cartilage is inferior to thyroid and forms a complete ring of which the posterior portion is greatly expanded (like a signet ring). Ligaments attach the inferior surface to the first cartilage of the trachea.
  3. Epiglottis
     The epiglottis is a shoehorn-shaped structure that is supported by the epiglottic cartilage. The cartilage is attached to the anterior and superior borders of the thyroid and the hyoid bone. It projects superior to glottis (Hence, “epi” glottis). During swallowing the epiglottis folds over the glottis and prevents the entry of liquids and solids.
     There are three paired cartilages:
  1. Arytenoid cartilages (ladle-shaped).
     This cartilage articulates with the superior border of the cricoid cartilage.
  2. Corniculate cartilages (horn-shaped).
     The corniculates articulate with the arytenoids and together these two pairs of cartilage control the opening and closing of the glottis.
  3. Cuneiform cartilages (wedge-shaped).
     These are cartilages that lie in the aryepiglottic fold that extends from the arytenoids to the epiglottis.
  Laryngeal Ligaments
     The intrinsic ligaments bind the cartilage of the larynx together and the extrinsic ligaments attach thyroid to hyoid and cricoid to the trachea. The vestibular and vocal ligaments extend between the thyroid and arytenoids. Both of these intrinsic ligaments are covered by epithelium and form the:
  vestibular folds that are superior to the vocal folds and protect the glottis;
  vocal folds that form the boundaries of the glottis.
     The vocal folds contain elastic tissue that vibrate to produce sound and are also called the true vocal folds. The vestibular ligaments are stiffer and the vestibular folds are also called the false vocal folds.
  Laryngeal Musculature
     The larynx has both intrinsic laryngeal muscles that regulate tension on vocal cords opens and closes glottis and extrinsic laryngeal muscles positions and stabilizes larynx.
 
  Trachea
     The trachea is a tough, flexible tube that is supported by 15 to 20 C-shaped tracheal cartilages which are attached by annular ligaments. The tracheal cartilages are open in the back where the esophagus is located. This allows the trachea accommodate the expansion of the esophagus as food passes through it. The ends of the C-shaped cartilages are spanned by the trachealis m. Contraction and relaxation of this smooth muscle is under autonomic control and results in a change in the diameter of the tracheal lumen.
     The trachea is lined by typical respiratory epithelium, PCCE. The thick layer of connective tissue under the mucosa called the submucosa often contains mucous glands that are connected by ducts to the surface.
 
  Primary Bronchus
     The trachea divides into right and left primary bronchi which are similar to the trachea in design. At the point of separation there is an internal ridge called the carina (“keel”). The primary bronchi are also called extrapulmonary bronchi because they are still outside the lungs. The primary bronchi enter each lung at an indentation called the hilus and they enter the lung with blood vessels nerves and lymphatics. This complex of structures that arises from the mediastinum with supportive connective tissue is called the root of the lungs.
 
  Superficial Anatomy of Lungs
     The right and left lungs are situated in the pleural cavities and are shaped like blunt cones with a blunt superior apex within the base of the neck and a concave base on the surface of the diaphragm.
  Lobes of lungs:
     Each lung is divided into distinct lobes. The right lung has three lobes, a superior, middle and inferior lobe. The left lung has only two lobes, a superior and inferior lobe.
  Lung surfaces:
     The curving anterior portion of lungs underneath the rib cage is the costal surface. The medial or mediastinal surface presses against the structures in the mediastinum and as a consequence has an irregular surface.
     The heart and the great vessels are mostly on the left side and press into the mediastinal surface of the left lung. As a result, the medial border of the left lung has a deep indentation called cardiac notch.
     The surface of the lung is divided in lobules by connective tissue septa (“walls”) that are extensions of the connective tissue of the visceral pleura.
 
  Internal Structure of the Lungs
  Bronchi
     After the primary bronchi enter the lungs they immediately branch into smaller and smaller passage ways giving rise to what is called the bronchial tree. The bronchi within the lungs are now referred to as intrapulmonary bronchi.
     The primary bronchi first divide into the secondary or lobar bronchi that supply the lobes of each lung. The right lung has three lobes ands as a consequence has a superior lobar bronchus, middle lobar bronchus and inferior lobar bronchus. The left lung has two lobes and a superior lobar bronchus and a inferior lobar bronchus.
      Inside the lobes of the lungs, the lobar bronchi divide into tertiary or segmental bronchi that provide the bronchopulmonary segments within each lung. Each bronchopulmonary segment has its own blood supply and drainage in addition to its own segmental bronchi. Each lung has 10 bronchopulmonary segments.
  Bronchioles
     As the tertiary bronchi branch within a segment and become smaller, the cartilaginous plates that supported the wall disappear and the outer wall is dominated by smooth muscle. These passageways are now called bronchioles (“little bronchi”).
     The bronchioles continue to divide and give rise to smaller bronchioles until the bronchiole is supplying air to a pulmonary lobule. At this point the bronchiole is called a terminal bronchiole and this marks the end of the conducting system of the respiratory tract. The terminal bronchioles still have smooth muscle in their walls that control the flow of air into the lobule. Contraction of the smooth muscle is called bronchoconstriction which causes the opening to narrow. Relaxation is called bronchodilation which causes the opening to widen.
     Within the lobule the terminal bronchiole divides into bronchioles whose walls become thinner and develop thin pouches called alveoli. These bronchioles are now called respiratory bronchioles and the alveoli are line by simple squamous epithelium across which gas exchange occurs. The respiratory bronchioles are the beginning of the respiratory portion of the respiratory tract.
  Alveolar Ducts and Alveoli
     The respiratory bronchioles divide into multiple linear passageways lined by alveoli that end in sacs surrounded by alveoli. The linear passageways are alveolar ducts and the sacs are alveolar sacs. The alveolar walls are associated with capillaries and elastic fibers that enable the alveolar ducts and sacs to maintain their relative positions after expansion.
  Alveolus and Respiratory Membrane
     The alveolar epithelium is primarily simple squamous epithelium. The alveolar epithelium consists of:
  1. Squamous epithelial cells also called Type I cells or respiratory epitheliocytes;
  2. Large cells called septal cells, surfactant cells, Type II cells or large alveolar cells. This cell (of many names!) produces an oily secretion called surfactant. Surfactant consists of a mixture of phospholipids that reduce the surface tension of the fluid in alveoli;
  3. Alveolar macrophages that phagocytize any particulate matter or pathogens that managed to get through the defenses of the respiratory tract.
     The respiratory membrane consists of:
  1. alveolar epithelium;
  2. capillary endothelium;
  3. fused basal laminae of the alveolar and endothelial cells.
  The respiratory membrane is thin and permits rapid exchange of the lipid soluble respiratory gases.