Respiratory System Organs

• Respiratory organs include the nose, nasal cavity, paranasal sinuses, pharynx, larynx, trachea, bronchi, bronchioles, and lungs containing alveoli.
• Alveoli, singular alveolus, are terminal air pouches at the end of bronchioles.
• The respiratory organs facilitate air passage and gas exchange.

Organs of the Respiratory System

• Divided into the conducting zone for air passage and the respiratory zone for gas exchange between blood and atmosphere within the lung.
• The upper respiratory tract contains two types of mucosa.
• Olfactory mucosa houses olfactory receptors for smell and is located on the superior concha and upper nasal septum.
• Respiratory mucosa lines the nasal cavity, nasopharynx, trachea, and bronchi.

Respiratory Mucosa

• Consists of ciliated, pseudostratified columnar epithelium.
• Goblet cells are present within the epithelium.
• Mucus traps fine particles, like dust.
• The underlying layer of lamina propria contains mucosal glands.
• Mucosal glands secrete lysozyme to destroy bacteria, also secrete mucus to trap debris.
• There is no muscularis mucosa in the respiratory mucosa.
• Outer layers vary with bone, cartilage, and smooth muscle.
• Cilia move contaminated mucus to the oropharynx, to be swallowed or spat out.

The Nasal Cavity

• External nares are nostrils.
• The vestibule is the interior and lateral space within the flared region of the nose, containing hairs to trap particles.
• Stratified squamous epithelium is the site of transition within the nasal cavity.
• The nasal cavity is divided by the nasal septum.
• The anterior nasal septum is cartilage, and the posterior is bone of the ethmoid and vomer.
• The nasal septum is covered by respiratory mucosa to trap particles.
• The nasal cavity is continuous with the nasopharynx.
• Choanae are posterior nasal apertures.
• This allows an opening between nasal cavity and nasopharynx.

Nasal Conchae (Turbinates)

• Superior and middle nasal conchae are part of the ethmoid bone.
• Inferior nasal conchae are the maxilla.
• Nasal conchae project medially from the lateral wall of the nasal cavity.
• These are scroll-like projections forming large ridges covered by respiratory mucosa.
• They cause turbulent airflow.
• Particulate matter is deflected to mucus-coated surfaces and gets trapped.

Conchae/Nasal Meatuses

• A meatus is a groove that is inferior to each concha.
• This includes the superior, middle, and inferior meatuses.
• Conchae with their meatuses filter air of particulate matter, warm inhaled air, and moisten inhaled air with water from the mucus coating.
• Heat and moisture are reclaimed from air before exhalation.

Paranasal Sinuses

• These are air-filled spaces within skull bones surrounding the nasal cavity, opening into meatuses.
• There are four paranasal sinuses: frontal, sphenoidal, ethmoidal, and maxillary.
• Sinuses are continuous with the nasal cavity and lined by respiratory mucosa, thus having the same functions as the nasal cavity mucosa.
• These functions include warming and moistening air while trapping dust, and also lightening the skull and supporting sound resonance.

The Nasopharynx

• It is superior to the oropharynx, serving only as an air passageway.
• The nasopharynx is closed off during swallowing.
• The mucosa traps dust.
• The pharyngeal tonsil (adenoids) is located on the posterior wall and destroys entering pathogens.
• The nasopharynx contains the opening to the pharyngotympanic, auditory, tube.

The Larynx

• It lies at the entrance to the trachea.
• Its functions include voice production with vocal cords, providing an open airway and routing air and food into the proper channels -via the epiglottis.
• The structure is mainly cartilage, with five major and two minor cartilages, aided by the hyoid bone for tongue movement.
• Most of the larynx's cartilage components are hyaline, except for the elastic cartilage epiglottis.
• The epiglottis attaches to the posterior tongue and closes off the trachea during swallowing.
• The thyroid cartilage, or "shield," is the largest and most anterior cartilage, featuring a superior notch and laryngeal prominence known as the "Adam's Apple," connected via the thyrohyoid membrane (ligament).
• The cricoid cartilage is ring-shaped and inferior to the thyroid cartilage, featuring the cricothyroid membrane.
• The arytenoid cartilages are paired and pyramid-shaped, sitting on the posterior cricoid cartilage.
• They anchor "true" vocal cords which include vocal ligaments made of elastic fibers covered by mucosa.
• The vocal cords extend between the arytenoid and thyroid cartilages, leaving a space between them named Rima Glottidis
• Vocal cords move via movements of arytenoids.
• There are two types of movement: lateral-medial to increase or decrease the rima glottidis, and rotation to change the length and tension of cords to control pitch.

The Larynx, Continued

• Voice production includes length and tension changes of the vocal folds to alter the pitch.
• During puberty in males, the thyroid cartilage narrows and lengthens, therefore the vocal cords elongate resulting in a lower pitch.
• Loudness depends on the force of air across the vocal folds.
• Innervation of the larynx is by the recurrent laryngeal nerves, which are a branch of the vagus nerve.

The Trachea

• Descends into the mediastinum and divides into two main bronchi.
• C-shaped cartilage rings, numbering 16-20, keep the airway open, allowing the elastin in the wall to recoil during exhalation.
• Contains respiratory mucosa which includes ciliated, pseudostratified columnar epithelium with goblet cells.
• Cilia move trapped particles in mucous toward the pharynx to be swallowed.

Bronchi in the Conducting Zone

• The bronchial tree features extensively branching respiratory passageways.
• There are approximately 23 orders of branching.
• The primary bronchi are the largest bronchi.
• Includes right and left primary bronchi.
• The right main bronchus is wider and shorter than the left.
• Secondary bronchi are the lobar bronchi.
• There are three on the right, which has three lobes, and two on the left for its two lobes.
• Tertiary bronchi are the segmental bronchi.
• They branch into different lung segments in each lobe.
• Bronchioles are "little bronchi," less than 1 mm in diameter and have no cartilage in their walls.
• Terminal bronchioles are less than 0.5 mm in diameter and represent the end of the conducting zone.

Changes Along Conducting Pathways

• Supportive connective tissues change from C-shaped rings to cartilage plates beginning in the 2° bronchi to the end of bronchi, but there is no cartilage in bronchioles.
• Respiratory mucosa transitions from ciliated, pseudostratified columnar epithelium to the end of bronchi.
• The tissue is then replaced by ciliated, simple columnar in larger bronchioles, and finally ciliated, simple cuboidal epithelium in the smallest bronchioles.
• Smooth muscle becomes more important; contraction and relaxation change the diameter.
• The trachea has a small amount of smooth muscle to reduce its lumen during coughing.
• Bronchioles have more smooth muscle which regulate airflow to the lung.

Histology of Bronchioles

• Epithelium transitions from simple columnar epilithelium (larger bronchioles) to simple cuboidal epilithelium(smaller bronchioles).
• There is no cartilage in their wall, but there is smooth muscle.
• Asthma affects smooth muscle of bronchioles.
• Bronchioles have a connective tissue outer layer.

Lungs

• Bronchopulmonary segments are functional and anatomical subdivisions of each lung.
• Segments are separated by thick sheets of connective tissue.
• Connective tissue minimizes infection spread, which facilitates surgical removal of segments.
• Each segment is supplied by a tertiary bronchus.
• The right lung has 10 segments, and the left lung has 9-10 segments, due to some fusing.
• The respiratory zone consists of air-exchanging structures, notably respiratory bronchioles, alveolar ducts, and alveoli.
• Respiratory bronchioles branch from terminal bronchioles.
• A few alveoli open directly into respiratory bronchioles, which then lead to alveolar ducts that contain a few alveoli.
• Alveolar ducts open into alveolar sacs, or clusters of several alveoli.
• In the bronchioles summary, Primary Bronchi lead to Secondary Bronchi which lead into Teriary Bronchi then to Multiple Branches Bronchioles.
• Terminal Bronchioles transition to Respiratory Bronchioles then to Alveolar Ducts and finally Alveoli.

Alveoli

• These are sites for gas exchange between blood and air.
• Has a round, sac-like shape with about 150 million per lung, totaling a surface area of 140 square meters.
• Type I cells and their basal laminae compose alveoli = Simple squamous epithelial cells.
• Cuboidal Type II epithelial cells secrete pulmonary surfactant to reduce surface tension on the inner alveolar wall, preventing alveoli from collapsing.
• Macrophages also exist, they are often called "dust cells," and phagocytize pathogens and debris.
• An extensive network of blood capillaries surrounds alveoli, facilitating gas exchange between blood in capillaries and air inside alveoli, in addition to being surrounded by elastic fibers.
• These fibers provide some recoil of stretched alveoli during exhalation.
• Alveoli interconnect by way of alveolar pores to equilibrate pressures.

Respiratory Membrane

• This membrane acts as a barrier for diffusion of gases across the alveolar wall, its thinness of 0.5 mm supporting rapid diffusion; consists of the following components.
• Plasma membrane of type I alveolar cells consisting of simple squamous epithelium.
• Basal laminae; with contributions from both type I alveolar cells and from endothelial cells of capillaries.
• Plasma membrane of capillary endothelial cells also composed of simple squamous epithelium.