Chapter 36:  Anatomy of the Respiratory System 



            Primary function of respiratory system is transport of O2 and CO2. This requires the four processes collectively known as respiration:

1.      Pulmonary ventilation is the movement of air into and out of the lungs (breathing).  This involves gas pressures and muscle contractions.

2.      External respiration is the exchange of O2 (loading) and CO2 (unloading) between blood and alveoli (air sacs).

3.      Transport of respiratory gases between lungs and tissues.

4.      Internal respiration is gas exchange between blood and tissue cells.


Cellular respiration - the includes the metabolic pathways which utilize O2 and produce CO2, which will not be included in this unit.



Upper Respiratory System Structures

The upper respiratory tract consists of the nose and the pharynx, or throat.


I.                  Nose

a.      Divided into the external nares and the internal nasal cavity.

b.      Nasal cavity is divided in half by the nasal septum.

c.       Floor of the nasal cavity is formed by the palate.

                                                                          i.      Anterior is the hard palate. 

                                                                        ii.      Posterior is the soft palate.  It is primarily composed of skeletal muscle.

                                                                      iii.      Cleft palategenetic defect where failure of the palatine bones &/or the palatine processes of the maxillary bones fuse medially; it causes difficulty in breathing & oral cavity functions such as sucking & mastication & speech.  A cleft lip is a separation of the two sides of the lip. Sometimes this includes the maxilla bone of the upper jaw.  

d.      On the lateral walls of the nasal cavity we find the 3 pairs of nasal conchae (turbinals).

                                                                          i.      The superior and middle and inferior nasal conchae.

                                                                        ii.      The nasal conchae make airflow within the nasal cavity turbulent.  The turbulence slows the flow of air down which increases the available time for inspired air to be filtered, warmed, and humidified. Also, provide a sensory surface for olfaction.

e.      The majority of the nasal cavity is lined by respiratory epithelium.

                                                                          i.      Respiratory epithelium is pseudostratified columnar epithelium with goblet cells.

                                                                        ii.      The mucus secreted by goblet cells, as well as mucous glands, helps filter and trap inspired particulate matter.  The mucus also contributes to the humidification of inspired air.

                                                                      iii.      The cilia help sweep mucus to the pharynx where it can be swallowed.

                                                                      iv.      The respiratory epithelium is underlain by a dense vasculature.  The blood helps warm inspired air.  Cold, dry air can damage the delicate alveolar tissue.

                                                                        v.      Mucus also contains enzymes and IgA’s, which help prevent infection.

f.        The nasal cavity is continuous with the nasopharynx via the internal nares.


 II.             Pharynx

Divided into 3 sections: nasopharynx, oropharynx, & laryngopharynx. 


a.      Nasopharynx

                                                                                     i.      Continuous with the nasal cavity via the internal nares.

                                                                                    ii.      Lined by respiratory epithelium.

                                                                                 iii.      Contains the opening to the auditory tube (a.k.a. the Eustachian tube).

1.      The auditory tube connects the pharynx to the middle ear cavity.

2.      It functions to ensure that the air pressure within the middle ear cavity is equal to atmospheric pressure.

                                                                                  iv.      Contains the pharyngeal tonsil.


b.      Oropharynx

                                                                                      i.      Inferior to the uvula and superior to the epiglottis.

                                                                                    ii.      Lined by nonkeratinized stratified squamous epithelium b/c it is a common pathway for food and air.

                                                                                  iii.      The palatine tonsils are located near the opening of the oral cavity into the pharynx.


c.       Laryngopharynx

                                                                                      i.      Inferior to the epiglottis and superior to the split between the larynx and the esophagus.

                                                                                   ii.      Lined by nonkeratinized stratified squamous epithelium b/c it is a common pathway for food and air.

                                                                                  iii.      Continuous with the larynx inferiorly.

                                                                                  iv.      Lingual tonsils are located on the posterior surface of the tongue, which also places them near the opening of the oral cavity into the pharynx.



III.            Larynx

a.      Routes food and air down their correct passages.

b.      Contains the vocal cords, which function in voice production.

c.       Structure

                                                              i.      Arrangement of 9 cartilages connected by membranes and ligaments and lined by respiratory epithelium.

                                                            ii.      Cartilages include thyroid, cricoid, epiglottis and 3 small paired cartilages.

                                                          iii.      All cartilages are hyaline with the exception of the epiglottis, which is elastic cartilage.

                                                          iv.      Thyroid cartilage is the largest and its midline laryngeal prominence is the male “Adam’s apple.”

                                                            v.      Inferior to the thyroid is the signet-ring shaped cricoid cartilage.

                                                          vi.      The 3 pairs of small cartilages form much of the posterior and lateral larynx.

                                                        vii.      The epiglottis extends from the base of the tongue to its hinge on the superior thyroid cartilage.  During swallowing, the epiglottis tips and covers the entrance to the larynx and ensures that food enters the esophagus.



d.      Deep to the laryngeal mucosa in the lateral walls of the larynx are the vocal ligaments.  These are the core of the lower vocal folds or true vocal cords.

                                                              i.      Vibration of the vocal ligaments creates sounds that are then modified by the tongue, lips, etc., to produce the sounds with which we are familiar.

                                                            ii.      Superior to the vocal folds are the vestibular folds or false vocal cords.  These play no role in voice production.

                                                          iii.      The space between the vocal folds on the left and those on the right is known as the glottis.





Lower Respiratory System Structures

Consists of the parts found in the thoracic cavity: the lower trachea and the lower trachea and the lungs.


IV.            Trachea

a.      Extends from the larynx to the mediastinum, where it splits into 2 primary bronchi.

b.      Lined by respiratory epithelium and associated with abundant mucus secretion.

Note: smoking destroys cilia.  Without cilia, it becomes difficult to sweep the mucus up and out of the trachea.  The individual coughs violently. 

The persistent mucus may lead to chronic bronchitis (inflammation of the bronchi).

c.       The trachea is reinforced by about 18 C-shaped rings of cartilage.

                                                                          i.      These rings prevent the trachea from collapsing during inspiration.

                                                                        ii.      The open portion of the cartilage rings is posterior and there you find the trachealis muscle.  The lack of posterior cartilage is important b/c it provides the esophagus with room to expand when a large bolus of food is swallowed.

d.      Carina - the point where the trachea divides into the left and right primary bronchus.


V.               The Bronchial Tree

a.      The trachea divides into 2 primary bronchi.

                                                                          i.      The right primary bronchus is wider, shorter, and more vertical than the left.  Thus, it’s more likely to be obstructed by a foreign object.

b.      Air reaching the bronchi has been significantly filtered, warmed, and humidified.

c.       Within the lungs, each primary bronchus divides into secondary bronchi.  There are 3 secondary bronchi on the right (one for each of the 3 lobes of the right lung) and 2 secondary bronchi on the left (one for each of the 2 lobes of the left lung).

d.      Secondary bronchi divide to yield tertiary bronchi which then divide to yield quaternary bronchi and so forth until about 23 branchings have occurred.

e.      Once the passageways have a diameter <1mm they are known as bronchioles.

f.        The terminal bronchioles are the last bronchioles without alveoli. 

g.      Bronchioles with alveoli are known as respiratory bronchioles and lead into alveolar ducts, which lead to alveolar sacs and to alveoli where the exchange of oxygen and carbon dioxide takes place.

h.      As the bronchial tree progresses (divides), its histology changes noticeably.

                                                                          i.      Cartilage rings are replaced by cartilage plates and within the bronchioles, cartilage is absent entirely.

                                                                        ii.      The epithelium changes from pseudostratified columnar to simple columnar to simple cuboidal.

                                                                      iii.      The number of cilia declines.

                                                                      iv.      The number of goblet cells declines.

                                                                        v.      The amount of smooth muscle increases. 

VI.            Lungs

a.      Occupy the entire thoracic cavity except for the mediastinum.

b.      Each lung is cone-shaped and suspended within its own pleural cavity and connected to the mediastinum by bronchial and vascular attachments.

c.       Anterior, lateral, and posterior surfaces hug the ribs and form the costal surface.

d.      Deep to the clavicle is the apex, the narrow superior lung tip.

e.      Base the concave diaphragmatic surface of the lung.

f.        On the medial side of each lung is an indentation known as the hilus.  Pulmonary vessels, nerves, and lymphatics enter/exit at this point.

g.      Left lung is slightly smaller than the right and contains a concavity known as the cardiac notch – which normally accommodates the bulk of the heart.

h.      Left lung has only 2 lobes – superior and inferior separated by an oblique fissure. In cat left lung has 3 lobes – anterior , middle , and posterior .

i.        Right lung has 3 lobes – superior, middle, and inferior separated by oblique and horizontal fissures respectively. In cat right lung has 4 lobes – anterior , middle , posterior, and mediastinal .

j.        Lungs largely consist of air space.  The remainder is mostly elastic connective tissue. Allows recoiling. 



VII.         Pleurae

a.      Thin, double-layered serosa that covers each lung.

b.      Parietal pleura covers the thoracic wall and superior surface of the diaphragm.  Continues around the heart and between the lungs as well.

c.       Visceral pleura covers the external surface of the actual lungs themselves.

d.      Pleurae produce pleural fluid which fills the slit-like pleural cavity between them.  Pleural fluid allows for friction-free movement and helps the parietal and visceral pleurae adhere to one another.