Respiratory system

Nasal cavity

The upper respiratory tract begins with the nasal cavity. The nasal cavity opens anteriorly on the face through the two nares, and posteriorly into the nasopharynx through the two choanae. The floor of the nasal cavity is formed by the hard palate, while the roof consists of the cribriform plate of the ethmoid bone posteriorly, and the frontal and nasal bones anteriorly. The nares and anterior portion of the nasal cavity contain sebaceous glands and hair follicles that serve to prevent any larger harmful particles from passing into the nasal cavity. 

The lateral walls of the nasal cavity contain three bony projections called nasal conchae (superior, middle and inferior), which increase the surface area of the nasal cavity. The nasal conchae also disrupt the laminar flow of air, making it slow and turbulent, thereby helping to humidify and warm up the air to body temperature. 

The roof of the nasal cavity contains the olfactory epithelium which consists of specialized sensory receptors. These receptors pick up airborne odorant molecules and transform them into action potentials that travel via the olfactory nerve to the cerebral cortex, allowing the brain to register them and provide a sense of smell.

Another pathway for the entry of air is the oral cavity. Although it is not classified as a part of the upper respiratory tract, the oral cavity provides an alternative route in the case of obstruction of the nasal cavity. The oral cavity opens anteriorly on the face through the oral fissure, while posteriorly, it opens into the oropharynx through a passage called the oropharyngeal isthmus. 

Paranasal sinuses

Several bones that form the walls of the nasal cavity contain air-filled spaces called the paranasal sinuses, which are named after their associated bones; maxillary, frontal, sphenoidal and ethmoidal sinuses.

The paranasal sinuses communicate with the nasal cavity via several openings, and thereby also receive the inhaled air and contribute to its humidifying and warming. In addition, the mucous membrane and respiratory epithelium that lines both the nasal cavity and the paranasal sinuses traps any harmful particles, dust or bacteria.

Pharynx

After passing through the nasal cavity and paranasal sinuses, the inhaled air exits through the choanae into the pharynx. The pharynx is a funnel-shaped muscular tube that contains three parts; the nasopharynx, oropharynx and laryngopharynx. 

• The nasopharynx is the first and superiormost part of the pharynx, found posterior to the nasal cavity. This part of the pharynx serves only as an airway, and is thus lined with respiratory epithelium. Inferiorly, the uvula and soft palate swing upwards during swallowing to close off the nasopharynx and prevent food from entering the nasal cavity.
• The oropharynx is found posterior to the oral cavity and communicates with it through the oropharyngeal isthmus. The oropharynx is a pathway for both the air incoming from the nasopharynx and the food incoming from the oral cavity. Thus, the oropharynx is lined with the more protective non-keratinizing stratified squamous epithelium.
• The laryngopharynx (hypopharynx) is the most inferior part of the pharynx. It is the point at which the digestive and respiratory systems diverge. Anteriorly, the laryngopharynx continues into the larynx, whereas posteriorly it continues as the esophagus. 

Larynx

Following the laryngopharynx, the next and last portion of the upper respiratory tract is the superior part of the larynx. The larynx is a complex hollow structure found anterior to the esophagus. It is supported by a cartilaginous skeleton connected by membranes, ligaments and associated muscles. Above the vocal cords, the larynx is lined with stratified squamous epithelium like the laryngopharynx. Below the vocal cords, this epithelium transitions into pseudostratified ciliated columnar epithelium with goblet cells (respiratory epithelium). 

Besides its main function to conduct the air, the larynx also houses the vocal cords that participate in voice production. The laryngeal inlet is closed by the epiglottis during swallowing to prevent food or liquid from entering the lower respiratory tract.

If you want to learn more about the anatomy and function of the larynx, take a look at the study unit below!

Lungs

The lungs are a pair of spongy organs located within the thoracic cavity. The right lung is larger than the left lung and consists of three lobes (superior, middle and inferior), which are divided by two fissures; oblique and horizontal fissure. The left lung has only two lobes (superior and inferior), divided by one oblique fissure. 

Each lung has three surfaces, an apex and a base. The surfaces of the lung are the costal, mediastinal and diaphragmatic surface, which are named after the adjacent anatomical structure which that surface faces. The mediastinal surface connects the lung to the mediastinum via its hilum. The apex of the lung is where the mediastinal and costal surfaces meet. It is the most superior portion of the lung, that extends into the root of the neck. The base is the lowest concave part of the lung that rests upon the diaphragm.

Each hilum of the lung contains the following:

• Principal bronchus
• Pulmonary artery
• Two pulmonary veins
• Bronchial vessels
• Pulmonary autonomic plexus
• Lymph nodes and vessels

Microanatomy

On the microscopic level, the lower respiratory tract is characterized by several changes of epithelial lining, serving different purposes. Beginning from the inferior part of the larynx to the tertiary segmental bronchi, the lower respiratory tract is lined with pseudostratified ciliated columnar epithelium with goblet cells. The goblet cells produce mucus that lubricates and protects the airway by trapping any inhaled harmful particles. These trapped particles are then propelled towards the upper respiratory tract by the cilia of the epithelial cells and eventually expelled by coughing. 

As the larger tertiary segmental bronchi divide into smaller bronchi, the epithelium begins to change from respiratory epithelium to a simple columnar ciliated epithelium. This epithelium is continued in the larger terminal bronchioles, and transitions into a simple cuboidal epithelium in smaller terminal bronchioles. The epithelium of the terminal bronchioles contains exocrine bronchiolar cells called club cells, formerly known as Clara cells. These are non-ciliated cuboidal cells that contribute to the production of surfactant. In addition, the terminal bronchioles contain smooth muscle in their walls, that allows for bronchoconstriction and bronchodilation to occur. 

Terminal bronchioles then branch into respiratory bronchioles, which are also lined by simple cuboidal epithelium. The walls of the respiratory bronchioles extend into alveoli, and the epithelium changes into a simple squamous epithelium composed of type I and type II pneumocytes. Type I pneumocytes are thin, squamous cells that carry out the gas exchange, while type II pneumocytes are larger cuboidal cells that produce surfactant.