The Respiratory System

The integumentary system | Main Anatomy Index | Gastrointestinal system I

Last updated 30 March 2006

This page was contributed by Ross Keen.

The Respiratory System

• This consists of the lungs and the air passages that lead to and form the lungs.
• The air passages branch as they enter the lungs to finally form alveoli.
• This system has 3 functions:

1. Air conduction;
2. Air filtration;
3. Gas exchange (respiration).

• Also air passing through the larynx gives rise to speech and air passing over the olfactory mucosa leads to our sense of smell.

• Air passages consist of a conducting portion and a respiratory portion.
• The conducting portion is the air passages that lead to the sites of respiration so gas exchange can occur.
• The passages external to the lungs are:

1. Nasal cavities;
2. Nasopharynx and oropharynx;
3. Larynx;
4. Trachea;
5. Paired primary bronchi.

• Bronchi within the lungs branch extensively to form bronchioles, which are the terminal part of the conducting system.
• The respiratory portion is the part of the tract where gaseous exchange takes place and includes:

1. Respiratory bronchioles;
2. Alveolar ducts;
3. Alveolar sacs;
4. Alveoli.

• Capillaries within the lungs come into intimate contact with the alveoli and are the structural basis of gas exchange in the lung parenchyma.

• Conditioning of the air before it reaches the respiratory portion occurs and consists of warming, moistening, and removal of particulate material.
• Mucous and serous secretions are very important in the conditioning process and also stops dehydration of the underlying epithelium.

• Mucous is produced from goblet cells and mucus-secreting glands.
• Cilia push mucous into the pharynx.
• Vibrissae are small hairs that trap particulate material.

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• The nasal cavities have 3 regions:

1. Vestibule;
2. Respiratory segment;
3. Olfactory segment.

• The paranasal sinuses are filled with respiratory epithelium.

• The vestibule communicates anteriorly with the environment and is lined by stratified squamous epithelium and contains vibrissae.
• Sebaceous glands are present and also assist in the entrapment of particulate matter.

• Posteriorly, the epithelium changes to ciliated pseudostratified columnar epithelium of the respiratory portion.

• The respiratory portion has its lamina propria attached to the underlying bone.
• The medial wall is the nasal septum.
• The lateral wall contains conchae.
• The conchae increase the surface area and cause turbulence in the airflow to allow more efficient conditioning of air and also the removal of particulate matter via turbulent precipitation.

• The epithelium consists of 5 types:

1. Ciliated cells;
2. Goblet cells;
3. Brush cells (have short blunt microvilli);
4. Small granule cells (contain secretory granules);
5. Basal cells (stem cells).

• The lamina propria of the respiratory segment has a rich vascular network.
• This allows inhaled air to be warmed, and also contains mucus glands, many with serous demilunes.
• It is directly contiguous with the periosteum of the underlying bone.
• The laminae propria also contains blood and lymphatic vessels, unmyelinated olfactory nerves, myelinated nerves and the olfactory glands (Bowman's glands).

• The olfactory segment is lined by the olfactory mucosa.
• It is also pseudo-stratified but contains very different cells:

1. Olfactory cells, bipolar neurons;
2. Supporting or sustentacular cells (columnar cells providing mechanical and metabolic support of olfactory cells, and microvilli);
3. Basal cells;
4. Brush cells (have short blunt microvilli and are columnar, they appear to be involved in the transduction of general sensory stimulation of the mucosa to CNV).

• Olfactory cells are bipolar neurons that possess an apical projection called the olfactory vesicle bearing cilia.
• The olfactory glands are tubuloalveolar serous glands, which deliver secretions via ducts to the surface.
• The serous secretions remove odours so that new scents can be smelt.

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• This part connects the nasal and oral cavities to the larynx and the oesophagus.
• It provides a resonating chamber for speech.
• This is divided into nasopharynx, oropharynx and laryngopharynx.

• This is the passageway of air between the oropharynx and the trachea.
• It serves as the organ of speech.

• The vocal folds are 2 folds of mucosa that project into the lumen of the larynx in an anteroposterior direction.
• In each are the vocal ligament and the vocalis muscle.

• Intrinsic skeletal muscles join cartilage plates and generate tension in the vocal folds and open and close the glottis.
• This is important for pitch.

• Extrinsic laryngeal muscles move larynx during swallowing.

• Above the vocal folds is the laryngeal ventricle, an elongated recess.
• Above this are the false vocal cords, or the vestibular folds.
• These are important for resonance.

• The larynx lined by ciliated, pseudostratified columnar epithelium, but the luminal surface of the vocal fold is lined by stratified squamous epithelia.

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• This extends from larynx to the middle of the thorax and divides into two primary bronchi.
• The wall of the trachea has 4 layers:

1. Mucosa (ciliated, pseudostratified epithelium and elastic fibre-rich lamina propria);
2. Submucosa (denser connective tissue);
3. Cartilaginous layer (C-shaped cartilages);
4. Adventitia (binds trachea to other structures).

• The trachealis muscle bridges the gap in the cartilage.

• The epithelium is similar to respiratory epithelium in other parts of the conducting system.

1. Ciliated cells (remove small inhaled particles from the lungs);
2. Mucous cells (lack cilia and look like goblet cells);
3. Brush cells (columnar cells that bear microvilli, and the basal surface is in synaptic contact with an afferent nerve ending so is a receptor cell);
4. Small granule cell;
5. Basal cell (serve as a reserve population by maintaining individual cell replacement in the epithelium).

• A thick basement membrane is characteristic of tracheal epithelium.
• The lamina propria appears as typical loose connective tissue.
• It is very cellular and contains many lymphocytes, which infiltrate the epithelium.
• Furthermore, there are also plasma cells, mast cells, eosinophils and fibroblasts.

• The lymphatic tissue in diffuse and nodular forms.

• There is an elastic membrane that marks the boundary between the lamina propria and the submucosa.
• The submucosa is loose connective tissue.
• Diffuse lymphatic tissue and nodular lymphatics are found.

• The submucosa also has mucous-secreting acini with serous demilunes.
• These have ducts that extend to the epithelium (simple cuboidal).
• This layer ends when the connective tissue blends with the perichondrium of the cartilage.

• The adventitia lies peripheral to the tracheal rings and the trachealis muscle.
• It contains large blood vessels and nerves supplying the tracheal wall and also the lymphatics draining it.

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• The trachea divides into the primary bronchi (extrapulmonary, left and right).
• The right is wider and shorter than the left.
• The primary bronchi enter the lung and become intrapulmonary and branch to give lobar bronchi.

• The left lung has 2 lobes and the right lung has 3 lobes.
• Each lobe receives a lobar bronchus.

• The left lung is further divided into 8 bronchopulmonary segments and the right into 10.
• Each segment gets a segmental bronchus.

• Bronchi have the same structure as the trachea but where they becomes intrapulmonary, the cartilage rings become cartilage plates of irregular shape.
• These cartilage plates are arranged to give the circular shape of the bronchi.

• As branching occurs plates become smaller and less.
• Smooth muscle appears upon entering the lung and increases as cartilage decreases.
• As there is smooth muscle in the wall of the bronchus, it can be considered to have 5 layers:

1. Mucosa;
2. Muscularis;
3. Submucosa;
4. Cartilage layer;
5. Adventitia.

• At 1 mm diameter, the cartilage plates disappear and it becomes a bronchiole.

• The bronchopulmonary segments are further subdivided into pulmonary lobules; each supplied by a bronchiole.

• Lobules are composed of pulmonary acini.
• Each pulmonary acinus is made up of a terminal bronchiole, and the respiratory bronchioles and alveoli it aerates.
• The respiratory bronchiolar unit consists of a single respiratory bronchiole and alveoli.

• The epithelium changes from ciliated, pseudostratified columnar to simple cuboidal as the size decreases.
• The number of glands decreases.
• Clara cells increase as the ciliated cells decrease in number.
• These cells produce a lipoprotein (a surfactant) that prevents luminal adhesion should airway fold on itself.
• There are small amounts of connective tissue.

• Outpocketings of the respiratory bronchiole are the alveoli.
• It is here that the gas exchange occurs.

• Each alveolus is confluent with a respiratory bronchiole by means of an alveolar duct (elongated airways with almost no walls only alveoli) and an alveolar sac (spaces surrounded the alveolus).
• Alveoli are separated from one another by a thin connective tissue layer (alveolar septum) with many capillaries.

• Alveolar epithelium is composed of:

1. Type I pneumocytes (squamous, 95 % of surface area);
2. Type II pneumocytes (secretory, cuboidal cells, 5% of surface area, they bulge into the lumen and are filled with granules called lamellar bodies which contain a surfactant that is secreted onto the surface of the alveoli to reduce surface tension);
3. The occasional brush cell.

• Components of the alveolar septum:

1. Alveolar epithelial cells;
2. Basal laminae of alveolar epithelium;
3. Basal laminae of the capillary endothelium;
4. Endothelial cells;
5. Fibroblasts, macrophages, collagen fibres and elastic fibres.

• The air-blood barrier is the cells and cell products across which gases may diffuse between the alveolar compartment and the capillary compartment.
• The thin portion is for most of the gas exchange and the thick portion is a site in which tissue fluid can accumulate, which is drained by lymphatics of the terminal bronchiole.
• Alveolar macrophages remove inhaled particulate matter from the air spaces and red blood cells from the septum.
• Collateral air circulation through alveolar pores allows air to pass between alveoli.

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• The pulmonary circulation supplies the capillaries of the alveolar septum and is derived from the pulmonary artery.
• The bronchial circulation, via bronchial arteries, branches from the aorta and supplies the whole lung.

• The 2 circulations anastomose at the level of the junction between the conducting and respiratory passages.

• One set of lymphatic vessels drains the parenchyma of the lung and follows the air passages.
• The second set drains the surface of the lung and travels in the connective tissue of the visceral pleura.

• There are parasympathetic and sympathetic divisions of the ANS.
• These mediate reflexes that moderate the dimensions of the air passages and blood vessels by contraction of smooth muscle.

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