The conducting division of the respiratory system cannot exchange gases with blood because the walls are too thick. The lumen of these passages is therefore called the anatomical dead space (Saladin, 2016).   

(Young et al., 2013)     

The upper respiratory tract starts with the nose. Its functions are to warm, to cleanse, to humidify, to inhale air, olfaction, to amplifier the voice. The nasal cavity NC begins with a small dilated chamber called vestibule. It is lined with stratified squamous epithelium with cilia. It is divided by the nasal septum NS into two nasal fossae. In H&E/Alcian blue, the cartilage stains blue. The inferior nasal septum is composed of the bony vomer, the superior part is composed of ethmoid bone and the anterior part of aa hyaline septal nasal cartilage. 

Moreover, the nasal cavity is lined by a mucous membrane called the conchae or turbinate T. Its main function is to cleanse the nose, warm and humidify the air (Saladin, 2016).

 

While the rest of the nasal cavity is covered by a ciliated pseudostratified columnar epithelium RE called the respiratory epithelium. Those cilia are mobile and serve to move nasal mucous. It is composed of wineglass-shaped mucous-producing goblet cells and ciliated cells which propel the mucous posteriorly towards the pharynx. The mucosa contains mucous cells M and the lamina propria is rich in lymphocytes, blood vessels V (which help to warm the air) and serous glands S (Saladin, 2016).

The larynx is a cartilages chamber which prevents the passage of food and/or drinks in the airways, and aids phonation. The passage of food/or drinks is prevented by the flap of tissue called epiglottis located in the superior opening of the larynx. 

It compromises two folds: the false fold which covered by columnar ciliated respiratory-type epithelium RE with seromucous glands, and lower fold called the true vocal cords TC -surgically removed in this photomicrograph- which are lined by stratified squamous epithelium (SE).

The true vocal cord contains vocalis muscle and vocalis ligament Li. The larynx is defined as cartilage chambers as it consists of nine cartilage such as the spoon-shaped supportive epiglottic cartilage, the shieldlike-shaped thyroid cartilage, the ringlike cricoid cartilage, the arytenoid cartilages and the corniculate cartilages. 

The thyroid cartilage and the cricoid cartilage constitute the voice box, while the arytenoid cartilages and the corniculate cartilages function in phonation. 

Several extrinsic and intrinsic fibrous ligaments suspend and operate each cartilage: the extrinsic ligaments link the larynx to the organs such as the thyrohyoid ligament, while the intrinsic ligaments are contained within the larynx and link its nine cartilages such as the cricotracheal ligament. Moreover, the vestibular ligaments support the vestibular folds and close the larynx during swallowing. While the vocal ligaments Vi support the vocal cords. Each vocal cord is covered with stratified squamous epithelium 

The larynx consists of intrinsic and extrinsic muscles. The intrinsic muscles control the voice cords by pulling on the corniculate and arytenoid cartilages while the extrinsic muscles connect the larynx to the hyoid bone. Its function is to elevate the larynx during swallowing (Saladin, 2016).

The trachea is located anteriorly to the oesophagus. It is supported by 16 to 20 C-shaped rings of hyalanine cartilage C. From a histological standpoint, it is composed of pseudostratified columnar epithelium -respiratory epithelium RE- with goblets cells, ciliated cells and short stem cells. The first two cells are responsible for the mucociliaryescalator mechanisms ad the goblet cells trap inhaled particles, while the ciliated cells drive the debris-laden mucous towards the pharynx. Under the tracheal epithelium, it is possible to find the connective tissue which is rich in lymphatic nodules, mucous and serous glands and tracheal cartilages. The adventitia of the trachea is composed of fibrous F connective tissue that blends into the adventitia of other organs of the mediastinum. The carina is the most inferior tracheal cartilage (Orchard et al. 2013).

(Young et al., 2013)

 

The main muscle found in the trachea is the trachealis (T), its contraction or relaxation allows the trachea to be widened or narrows to adjust airflow. Moreover, it is possible to find also some strands of longitudinal muscle (LM). (Orchard et al. 2013).

 

(Young et al., 2013)

 

It starts from the main bronchus and terminates with the terminal bronchioles. Just like the trachea, the main bronchi are supported by C-shaped hyaline cartilage, while the lombar and segmental bronchi are supported by overlapping crescent-shaped cartilaginous plates. Each bronchus is lined by ciliated 

CC pseudostratified columnar epithelium -respiratory epithelium RE- with goblet cells GC.  While towards the ends, it is thinner. In submucosa, it is possible to find mucous glands G and many mucosa-associated lymphatic tissue. Moreover, it is possible to find elastic fibres El -which helps the recoiling after the respiratory cycle- and mast cells Ma. In the mucosa, it is possible to find a smooth layer of muscles called the muscolaris mucosae. It regulates airway diameter and airflow (Saladin, 2016) (Orchard et al. 2013).

The bronchi are all lined by ciliated pseudostratified columnar epithelium that gets shorter and thinner throughout the bronchial tree, becoming simple columnar -as it is possible to see in the tertiary bronchus T- and then simple cuboidal epithelium. While goblets cells are replaced by Clara cells and there is no cartilage or submucosal glands as it is possible to see in the final branch of the conducting division. Therefore, the wall of the respiratory bronchioles R compromise of scanty smooth muscle and lack ciliation.  

The word respiratory indicates that alveoli are present and that internal respiration (gas exchange) occurs at the airways. Thus are not conducting division but respiratory division (Saladin, 2016) (Orchard et al. 2013).

Consequently, the alveolar ducts have non-ciliated simple squamous epithelia and it is the site of respiratory.  Each alveolar ducts AD end in alveolar sacs AS with a central space called the atrium. The lumen of the conducting division is called anatomical dead space as there is no exchange of gasses. While gas exchange between the inspired air and the blood occurs at the alveoli of the lungs, therefore it is called

An alveolus A is composed of 95% of squamous (type 1) alveolar cells P₁ which allows the rapid gas diffusion between it and bloodstream, and 5% of cuboidal great (type II) alveolar cells P₂. Their function is to repair the alveolar epithelium and to secret pulmonary surfactant (a mixture of phospholipids and proteins) which prevent the alveoli from collapsing. 

The alveolar macrophages are found in the lumen of the alveoli and in the connective tissue. Their function is to prevent to phagocytise dust particles. Each alveolus is surrounded by blood capillaries supplied by the pulmonary artery. The barrier between the air in the alveoli and the blood, consisting of two layers of squamous epithelial cells and their fused membranes is called the respiratory membrane 

The respiratory membrane consists of squamous alveolar cells, squamous endothelial cells of the capillary (Saladin, 2016).

 

Asthma is a chronic respiratory illness which causes breathing difficulties. It is classified as an allergic reaction to airborne antigens. The airways in asthma are occluded by tenacious plugs of exudate and mucus, there is fragility of airway surface epithelium, thickening of the reticular layer beneath the epithelial basal lamina and bronchial vessel congestion and oedema. Some of the common symptoms are wheezing, breathlessness, a tight chest and coughing. 

From a histological standpoint, it is possible to see 

1.     Smooth muscle thickness 

2.     Significant amount of chronic inflammation lymphocytes 

3.     Mucous material accumulating within the lumen 

4.     Narrowed lumen 

Emphysema is characterized by damaged alveolar walls and replacement of spongy tissue with air-filled cavities. The patient will have reduced alveolar surface and thus reduced ability to oxygenate the blood. Histologically, emphysema histological slides present an enlargement of alveoli because the wall between them have ruptured (Saladin, 2016).

(Klatt, 2014)

While chronic bronchitis characterized by air-way congestion, chronic infections and bronchial inflammation. From a histological point of view, it is possible to see squamous metaplasia and mucous gland hyperplasia.  Some of the common symptoms are increased breathlessness, persistent chesty cough with phlegm, frequent chest infections and persistent wheezing (Saladin, 2016).

 

Klatt, E., C. (2014). Robbins and Contran Atlas of Pathology. 3^rd edition. Saunders

 

Orchard, G., Nation, B. (2013). Cell Structure & Function. OUP Oxford.

 

Saladin Kenneth. (2016). Human Anatomy. 5^th edition. McGraw-Hill Education.

 

Young, B., O’Dowd,  G. and Woodford, P. (2013). Wheater’s Functional Histology: A text and Colour Atlas. 6th edition. Churchill Livingstone.