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ANATOMY OF THE RESPIRATORY SYSTEM

External Nares - Openings in nose to the nasal cavities.

Vestibule - The space just behind the external nares and inside the nose proper. It is lined with skin containing sweat and sebaceous glands, as well as, hairs. The lining epithelium is stratified squamous.

Nasal Cavities - Space behind the vestibule covered with a respiratory mucosa lined with a ciliated columnar epithelium containing goblet cells and serous glands. The serous glands produce a fluid containing lysozymes which are bacteriocidal.

Nasal Conchae - The lateral walls of each nasal cavity contain three bony projections called the conchae. The superior and medial conchae are part of the ethmoid bone. The inferior concha is a separate bone.

Nasal Septum - The region from the external nares to the nasal cavity is separated by a septum. The anterior portion is composed of soft tissues and hyaline cartilage. Posteriorly, it is formed by the Vomer (lower) and perpendicular plate of the ethmoid bone (upper).

The roof of nasal cavity is formed by the ethmoid (cribriform plate) and sphenoid bones. The floor is formed by the hard palate (palatine processes and palatine bones) and the soft palate. A rich blood supply to the nasal cavity helps to warm and moisten incoming air. The ciliated lining moves trapped debris back to the beginning of the pharynx (throat) where it is swallowed.

Paranasal Sinuses - Air spaces in the frontal, ethmoid, maxillary and sphenoid bones connect with the nasal cavities. These spaces function to lighten the skull, manufacture mucous and produce a resonance to amplify our vocalizations.

Internal Nares - Constrictions at the end of the nasal cavities which open into the superior portion of the pharynx.

The Pharynx - a tubular structure about 5" long. Commonly called the throat. It consists of:

A. Nasopharynx - lies above the point of food entry into the pharynx. It is a passageway for air only. Its ciliated lining continues to move mucous and debris down for swallowing. The pharyngeal tonsils are located in its posterior wall. The tonsils produce an immune response to pathogens in the upper respiratory tract. Eustacian tubes open into the side walls of the nasopharynx. They are used to equalize air pressure within the middle ear. In young children these tubes are a source of frequent middle ear infections (otitis media).

B. Oropharynx - is located directly inferior to the nasopharynx and is continuous with the oral cavity by way of the fauces. Within the walls of the fauces are the palatine tonsils. A pair of lingual tonsils are found at the base of the tongue. This common passageway for food and air is lined with stratified squamous epithelium.

C. Laryngopharynx - like the oropharynx, a common passage for food and air. It is lined with stratified squamous and extends from the Hyoid bone to the larynx where the food and air passages divide. The laryngopharynx is then continuous with the esophagus.

The Larynx - is about 2" long. It is attached to the hyoid bone superiorly and the trachea inferiorly. It functions to provide an opening to the airway, route food and air into the appropriate tract and to produce most of the sounds we call "voice".

Anatomy of the Hyoid bone - The Hyoid helps to support the larynx from above. The hyoid doesn't articulate with any other bone. The hyoid is anchored by ligaments to the styloid processes of the temporal bones. The hyoid is used as a point of origin by muscles which insert on the larynx raising it during swallowing and speech.

Anatomy of the Larynx - composed of 9 pieces of cartilage:

 
Text Box:
 
Text Box: a. thyroid cartilage - also called the "Adam's apple" or laryngeal prominence, it is composed of hyaline cartilage

b. cricoid cartilage - a ring-shaped piece of hyaline cartilage located just below the thyroid cartilage

c. arytenoid cartilage - a pair of pyramidally-shaped pieces of hyaline cartilage which anchor and move the true vocal cords.

d. cuneiform cartilage - a pair (hyaline)

e. corniculate cartilage - a pair (hyaline)

f. epiglottis - a spoon-shaped piece of elastic cartilage attached to the anterior rim of the opening to the larynx (glottis). The surface of the epiglottis is covered with mucous membrane and taste buds.

   In the larynx above the vocal cords the epithelium is stratified squamous. Below the vocal cords it becomes pseudostratified ciliated columnar. The cilia in this region beat up moving mucous and debris away from the lower respiratory tract.

The Voice - Tension on the vocal cords is altered by contractions of intrinsic muscles attached to the arytenoid cartilages and the cords. As air is forced past the cords, they vibrate. Pitch of sound is directly proportional to the tension on the cords. Loudness depends on the force of the air. Resonance produced by the spaces of the pharynx, oral and nasal cavities and the sinuses. Shaping of sounds is produced through the action of muscles of the throat, tongue, soft palate and lips. The "valsalva maneuver" involves using the vocal cords in a sphincter-like manner. After inhalation air is held in the lungs by closing the glottis, i.e., bringing the vocal cords together. This action, together with contractions of abdominal muscles, is used to assist us during defecation and micturation.

The Trachea - begins just below the larynx in the cervical region. At the thoracic inlet it enters the mediastinum. It is 4 to 5" long and 1" wide. At the level of the 5th thoracic vertebra it divides into right and left bronchi. The first branches of the trachea are called the primary bronchi. The right primary bronchus is wider, shorter and more vertical than the left. This results in inhaled objects being found there more often. Both bronchi enter the lungs at medial depressions - the hilus. In the lungs, the primaries branch into secondary or lobar bronchi.

                    Bronchopulmonary Segments

The branching of the bronchial tree leads to the functional subdivision of each lung into ten broncho-pulmonary segments. Each of these segments is somewhat autonomous. If a particular segment is damaged by trauma or infection, the blood flow and air to that particular segment can be reduced by constriction of the smooth muscle in the walls of blood vessels and bronchioles.

Bronchioles - The smallest branches of the bronchi in the lungs. There are several changes in the basic structure of the air tube, at this level.


1. The epithelium is simple columnar rather than pseudostratified.

2. The predominent cell types of the epithelium include:

  1. Ciliated cells appear a bit shorter than cells in the bronchi. They have long microvilli.
  2. Bronchiolar cells are tall, dome-shaped cells that protrude into the lumen of the bronchiole as far as the tips of the cilia of the adjacent ciliated cells. This gives the lining a scalloped appearance. These cells appear to be very active metabolically. They contain the apparatus associated with active protein synthesis, fat metabolism, carbohydrate and cholesterol synthesis. These cells are very abundant in bronchioles of 1mm or less in diameter. The serous secretions of the bronchiolar cells contain mucolytic and proteolytic enzymes. These secretions may be used to digest the viscous mucous produced more proximately in the bronchial tree. Supporting this is the fact that serous cells predominate over mucous cells in the more distal regions of most branched exocrine glands.
  3. The neuroepithelial body contains a number of neurosecretory cells that are positive for dopamine. Possible function may be the local release of catecholamines to affect the smooth muscle of the bronchiolar wall. For example, during anoxic or hypoxic episodes this would lead to a dilation of bronchioles.

The table below summarizes the histological changes seen in the bronchial   tree from the bronchi to the terminal bronchioles.

                  Primary Bronchi Secondary Bronchi Terminal Bronchioles
Cartilage Cartilage rings Irregular plates of cartilage No cartilage
Epithelium Pseudostratified ciliated columnar Ciliated columnar Simple cuboidal
Smooth Muscle There is a relative increase in smooth muscle as the diameter of the tube decreases. Contractions of the smooth muscle in the terminal bronchioles produces a large resistance to air flow.

 

Alveolus - a sac-like outpocket from the wall of the respiratory bronchiole or alveolar duct. The cells making up the alveolar wall are:

  1.    1.  Simple squamous, type I pneumocytes line the alveoli. They are characteristically very thin (about 0.2um).

     2.  Great alveolar or type II pneumocytes are found singly or in small groups
          between the type I cells. These cells are globular with a granular cytoplasm.
          Under EM, these cells can be seen to contain lamellar bodies. These bodies
          appear to exude from the pneumocytes and form a thin coat on the inside
          surface of the alveolus. This coat contains the substance surfactant.

     3.  Endothelial cells of capillaries can be seen in the interalveolar septa between
          two adjacent alveoli.

     4.  Blood cells of all kinds may be found in the septal capillaries.

     5.  Migratory macrophages may be seen outside the capillary in the septum or
          within the alveolus itself. These cells may show evidence of phagocytic
          activity in the form of cytoplasmic lipid granules, carbon particles scoured
          from the inner alveolar surface or hemosiderin, an iron containing pigment.
         This last inclusion results from the destruction of erythrocytes by the
          macrophages. During an interruption in blood flow through the lungs, the
          stasis in pulmonary blood flow leads to an increase of these siderophages or
          "heart failure cells".

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