Next Lesson - Development of the Respiratory System
Core
The respiratory system allows the body to take in oxygen and expel carbon dioxide. This process oxygenates the blood and removes carbon dioxide produced by respiring cells within the body. This article will cover the anatomical structures that form the respiratory system as well as the neurovascular supply to it.
The nasal cavity is divided into the left and right nasal cavities by the medial nasal septum. Air enters the nasal cavity through the nostrils and passes into the nasal vestibule, which is lined by keratinised, stratified squamous epithelium (a tough, layered, skin-like lining). The nasal vestibule contains nasal hairs, which filter dust and any other foreign materials. Beyond the nasal vestibule, the lining changes to ciliated pseudostratified columnar epithelium (respiratory epithelium), in which the cilia (tiny hair-like projections) move mucus along.
On the lateral wall of each side of the nasal cavity, there are three outgrowths called conchae or turbinates. They increase the surface area for inspired air to be warmed and moistened on the way in, and for expired air to be cooled on the way out.
As well as warming and moistening inspired air, the nasal cavity is also used as a resonance chamber in the production of speech and in olfaction (smell).
Diagram - The structures in the nasal cavity
Creative commons source by Stella Gänger and Katharina Schindowski [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
There are four paranasal sinuses located around the nasal cavity and they are connected to the nasal cavity via small orifices called ostia. The ostia can become blocked by inflammation, for example in allergic reactions and infections. Blocked ostia can stop mucus draining from the sinuses, causing sinusitis (inflammation of the sinuses) to develop. The four paranasal sinuses and where they are found can be seen in the diagram below.
Diagram - The paranasal sinuses
Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
In the pharynx, air and food have a common passage, which connects the nasal and oral cavities to the larynx and oesophagus. The pharynx is split into three sections: nasopharynx, oropharynx and laryngopharynx. Both the pharynx and larynx work together to ensure food enters the oesophagus and air enters the trachea.
The vocal cords are found within the larynx. During the process of swallowing, the larynx closes at several levels to protect the airway: the vocal cords adduct (come together), the laryngeal inlet (the opening into the larynx) becomes narrowed and the epiglottis (the flap of cartilage above the larynx) folds downwards. Together these act as a sphincter to close off the entrance to the trachea, thus preventing food or any other foreign objects from entering.
If the larynx and vocal cords become dysfunctional, there is an increased risk of inhalation of food or liquid due to the inlet to the trachea not properly closing.
The coughing mechanism allows particles and objects to be expelled from the trachea. In this mechanism the vocal cords adduct (close), allowing intra-thoracic pressure to build up. This is followed by quick abduction (opening) of the vocal cords, which expels air and particles out of the trachea in a cough.
Diagram - The sections of the pharynx, with the nasopharynx in green, the oropharynx in blue and the laryngopharynx in purple
Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The airways are split into two regions: the conducting portion and the respiratory portion. The conducting portion is described as being from the nasal cavity to the terminal bronchioles and is responsible for the transport of air to the gaseous exchange surfaces. The respiratory portion runs from the respiratory bronchioles to the alveoli and is the site of gaseous exchange.
The trachea is supported by cartilage in a C shape, and the presence of this cartilage ensures the trachea remains open under the negative pressure that occurs during breathing. The cartilage stays C-shaped in the primary bronchi (the right and left main bronchi) and changes to crescent-shaped plates in the secondary and tertiary bronchi (the lobar and segmental bronchi, described below).
Alveoli are found at the end of respiratory bronchioles, attached to alveolar sacs (collection of alveoli). Alveoli are the sites of gaseous exchange of carbon dioxide and oxygen.
Diagram - The structure of alveolar sacs
Public Domain Source by LadyofHats [Public domain]
Most of the epithelium found between the nasal cavity and the secondary bronchi is pseudostratified ciliated epithelium with goblet cells. The exceptions are the nasal vestibule, the oropharynx, the laryngopharynx and the upper larynx, which are lined with stratified squamous epithelium instead. The goblet cells produce and release mucus, which traps microorganisms and inhaled particles. The cilia then waft the mucus up to the pharynx to be swallowed and digested in the gastrointestinal tract. From the bronchioles onwards, the goblet cells are replaced by the club cells (the secretory cells that line the bronchioles), which produce and secrete surfactant-like material that helps reduce airway collapse during expiration.
Diagram - Histological section of the trachea
Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Chronic Obstructive Pulmonary Disease
In Chronic Obstructive Pulmonary Disease (COPD), long-term inflammation of the airways, usually caused by tobacco smoke, leads to hyperplasia (an increase in number) of the goblet cells and hypertrophy (enlargement) of the submucosal glands (the mucus-producing glands beneath the airway lining). These changes are seen particularly in people with chronic bronchitis (a cough with sputum for at least 3 months in each of 2 consecutive years). COPD is now the preferred term for both chronic bronchitis and emphysema (loss of lung tissue). The ratio of mucus cells to ciliated cells rises and there is increased mucus production. Patients with this disease therefore have an increased risk of developing infections, as microorganisms cannot be transported from the airways to the pharynx as efficiently anymore. For more information see our article on COPD.
Diagram - The lungs of someone with COPD versus the lungs of someone without COPD
Public Domain Source by National Heart Lung and Blood Institute [Public domain]
Patients with cystic fibrosis (CF) have more viscous mucus (thick and sticky). This is due to a defect in the cystic fibrosis transmembrane conductance regulator (CFTR), which is a chloride channel present on the luminal side of the cells lining the airways (the side facing the airway). The defect leads to fewer chloride ions being transported from inside the cell into the mucus, so less water moves out of the cells and the mucus becomes more viscous. As a result, patients with cystic fibrosis are more likely to have repeated pulmonary infections, because the mucus in the airways struggles to move up to the pharynx.
The bony thorax is composed of the sternum, twelve pairs of ribs and the twelve thoracic vertebrae of the spine. The lungs are ventilated by the bony thorax, the intercostal muscles and the diaphragm functioning together.
The first seven ribs are attached to the sternum through the costal cartilage (the cartilage at the front end of each rib), whilst the eighth, ninth and tenth ribs are each attached to the costal cartilage of the rib above. The eleventh and twelfth ribs terminate in the abdominal musculature.
Diagram - The thoracic cavity, showing the sternum, ribs and costal cartilages
Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
There are three muscles found in each intercostal space and they are all innervated (supplied with nerves) by the intercostal nerves. These muscles are (from superficial to deep):
External Intercostal Muscles - the most superficial intercostal muscles. The fibres of these muscles project inferiorly and anteriorly. The external intercostal muscles elevate the ribs, which expands the chest during inspiration.
Image - The external intercostal muscles
Creative commons source by Anatomography [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Internal Intercostal Muscles - the fibres of these muscles project inferiorly and posteriorly. They pull the ribs inferiorly during forced expiration.
Image - The internal intercostal muscles
Creative commons source by Anatomography [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Innermost Intercostal Muscles - the deepest intercostal muscles. These muscles are used alongside internal intercostal muscles during forced expiration.
Image - The innermost intercostal muscles
Creative commons source by Anatomography [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The Intercostal Arteries, Veins and Nerves
The neurovascular bundle that supplies the intercostal muscles runs on the inferior border of each rib, and therefore superiorly in the intercostal space.
The first nine intercostal spaces are supplied by three vessels each: two anterior intercostal arteries and one posterior intercostal artery. The final two intercostal spaces (tenth and eleventh) are supplied solely by the posterior intercostal arteries.
The first two posterior intercostal arteries are branches of the superior intercostal artery whereas the final nine posterior intercostal arteries derive from the thoracic aorta.
The first six anterior intercostal arteries arise from the internal thoracic artery and the final three anterior intercostal arteries arise from the musculophrenic arteries (terminal branches of the internal thoracic artery that supply the lower intercostal spaces and the diaphragm).
Regarding the venous drainage of the intercostal spaces, there are eleven pairs of posterior intercostal veins and nine pairs of anterior intercostal veins (supplying spaces 1-9). With the anterior intercostal veins, the first six pairs drain directly into the internal thoracic vein and the final three pairs drain firstly into the musculophrenic vein before draining into the internal thoracic vein. The majority of the posterior intercostal veins drain into the azygos or hemiazygos veins (the veins that run alongside the vertebral column).
Diagram - The azygos and hemiazygos veins alongside the other veins in the thoracic region
Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The intercostal nerves arise from the anterior rami (the front branches of the spinal nerves) of the first eleven thoracic spinal nerves. The anterior ramus of the twelfth thoracic spinal nerve lies in the abdomen as the subcostal nerve.
When performing a chest drain, the tube is classically inserted into the intercostal space at the superior border of the lower rib (the bottom part of the space, away from the bundle under the rib above) to avoid damaging the neurovascular structures found in the intercostal space. In practice, chest drain insertion should always follow local clinical guidelines and be performed by appropriately trained clinicians.
The diaphragm is the main muscle responsible for chest expansion during inspiration. The muscle is dome-shaped at rest but during contraction it flattens with the muscle moving downwards, increasing the vertical diameter of the chest cavity.
The diaphragm is split into a right and a left dome or hemidiaphragm. The right dome of the diaphragm is found at the level of the fifth rib and the left dome is found at the level of the fifth intercostal space. The right dome is higher in comparison to the left dome due to the liver pushing the right dome superiorly.
Diagram - The lungs and the diaphragm
Public Domain Source by National Cancer Institute [Public domain]
The diaphragm is innervated by the left and right phrenic nerves (made up of cervical nerves 3, 4 and 5) which supply the left and right hemidiaphragms respectively. This means if either nerve is damaged, there is paralysis of the affected side of the diaphragm. This can be seen on a chest X-ray, as the affected side appears raised (elevated) even when the patient is breathing in. On inspiration the paralysed side can move upwards instead of downwards (paradoxical movement).
The diaphragm has three openings (called hiatuses) for three important structures. These are the inferior vena cava at the level of thoracic vertebrae 8 (T8), the oesophagus at the level of T10 and the aortic hiatus at the level of T12. The aortic hiatus allows the passage of the aorta through the diaphragm. This can be remembered by the vena cava having 8 letters (T8), the oesophagus having 10 letters (T10), and the aortic hiatus having 12 letters (T12).
The Movements of the Thoracic Wall and Diaphragm During Respiration
During the process of inspiration, the ribs swing superiorly and laterally, which increases the lateral diameter of the thoracic space. This is known as the 'bucket handle' movement, because the ribs swing up like the handle of a bucket.
The sternum moves anteriorly during inspiration to increase the anterior-posterior diameter as the diaphragm descends to increase the thoracic capacity. The forward movement of the sternum is called the 'pump handle' movement, because it is similar to the movement of the handle of an old-fashioned water pump.
The Pleura and the Pleural Cavity
The pleura is the serous membrane (a thin membrane that secretes a lubricating fluid) that lines the lungs and the thoracic cavity. The pleura is composed of two membranes: the parietal pleura and the visceral pleura. The parietal pleura lines the inside of each hemi-thorax (one half of the chest), meaning it is stuck to the inside of the chest wall. The visceral pleura lines the outside of the lungs and extends into the interlobar fissures, meaning that it is stuck to the outside of the lungs. The function of the pleura is to ensure the lungs expand and inflate as the thorax expands during inspiration.
Between the parietal and visceral pleura is a small cavity, known as the pleural cavity, that contains a small volume of fluid, the pleural fluid. This fluid allows the pleura to slide effortlessly over each other during the movements of respiration. The pleural fluid also produces surface tension, which pulls the parietal and visceral pleura together and so helps to keep the lung inflated against the chest wall. The pressure within the pleural cavity is also negative (below atmospheric pressure).
The lungs do not completely fill the thoracic space. This leaves potential spaces (spaces that are normally closed) between the pleura covering the diaphragm and the pleura lining the inside of the chest wall. These spaces are called the costo-diaphragmatic recesses. The recesses are clinically important because pleural effusions (collections of fluid in the pleural cavity) can develop here through gravity, and can be seen on a chest X-ray.
Diagram - The pleura and the pleural cavity
Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The Trachea and the Bronchial Tree
The trachea commences at the lower border of the cricoid cartilage and then terminates by dividing into the right and left main bronchi around the level of T4/5 opposite the sternal angle. At the tracheal bifurcation (where the trachea divides in two), the carina is a ridge of cartilage between the openings of the right and left main bronchi.
The right main bronchus is shorter, wider and more vertical in comparison to the left main bronchus, meaning inhaled objects are more likely to lodge in the right main bronchus.
Diagram - The anatomy of the trachea
Creative commons source by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436
[CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The right and left bronchi can be known as the primary bronchi and these divide into the lobar or secondary bronchi. There are three right lobar bronchi and two left lobar bronchi, relating to the number of lobes in each lung. Each lobar bronchus divides into segmental or tertiary bronchi, which each supply a bronchopulmonary segment.
A bronchopulmonary segment is an area of lung with its own segmental bronchus and its own segmental branch of the pulmonary artery. It is drained by tributaries of the pulmonary vein, which classically run in the planes between adjacent segments. It is possible if needed to remove one bronchopulmonary segment with very little bleeding and air leakage, because each segment is relatively separated from the next.
The left lung has two lobes (upper and lower) whereas the right lung has three lobes (upper, middle and lower).
Diagram - The lobes and bronchi of the lungs
Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The apex of each lung ascends to the root of the neck, above the first rib. This area is close to many vascular and neurological structures (like the brachial plexus and subclavian arteries), meaning that a patient with a tumour in this area can present with vascular or neurological symptoms.
For example, a patient with a Pancoast tumour (a lung cancer at the very top of the lung) can present with symptoms of Horner’s syndrome. This happens because the tumour presses on the sympathetic nerves present in this area. The signs are ptosis (a drooping eyelid), miosis (a small pupil) and anhidrosis (reduced or absent sweating) on the same side of the face, including the eye.
The base of each lung rests on a hemidiaphragm.
The hilum of each lung is found on the mediastinal surface (closest to the heart) of each lung. It is where structures enter and leave the lung, and it contains the following structures: main bronchi, pulmonary artery and vein, lymphatics (lymph vessels), bronchial arteries and veins.
The structures next to the mediastinal surface of the left lung include the heart, aortic arch, descending aorta, oesophagus and nerves including the phrenic, vagus and recurrent laryngeal nerves.
Structures next to the mediastinal surface of the right lung include the superior vena cava, azygos vein, right atrium, oesophagus and nerves such as the phrenic nerve, vagus nerve and the sympathetic trunk.
The Neurovascular Supply of the Lungs
The lungs have a dual blood supply via the bronchial and pulmonary arteries.
The bronchial arteries supply the bronchial tree (airways) and the visceral pleura with oxygenated blood. They do not supply the alveoli. Bronchial venous drainage has two routes. The superficial bronchial veins drain into the azygos and hemiazygos veins, and so return blood to the right side of the heart. The deep bronchial veins drain into the pulmonary veins, and so return blood to the left side of the heart. Bronchial artery blood comes to the lungs oxygenated, and leaves deoxygenated after supplying the lung tissue.
The pulmonary arteries carry the entire cardiac output of the right ventricle to the lungs for the process of gaseous exchange and supply the alveoli at the same time. This is the blood that comes to the lungs deoxygenated, and leaves oxygenated.
The pulmonary veins return the oxygenated blood to the left side of the heart. The heart then pumps this blood around the body to supply oxygen to the tissues.
Diagram - The pulmonary circulation, showing the pulmonary arteries and veins
Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The lymphatics of the visceral pleura and the lungs drain into the hilar nodes (bronchopulmonary nodes) and from these nodes, lymph drains into the tracheobronchial nodes (the lymph nodes beside the trachea and main bronchi).
The nerve supply to the lungs includes the right and left vagus nerves, and the sympathetic trunk.
- The vagus nerve contains some parasympathetic efferent fibres which supply motor innervation to bronchial smooth muscle. Activation of these fibres leads to bronchoconstriction (narrowing of the airways) and stimulates secretion from the mucous glands (a secretomotor effect).
- The afferent fibres of the vagus nerve are sensory and are responsible for the coughing reflex.
- The sympathetic efferent fibres cause bronchodilation (widening of the airways) and vasoconstriction when they are activated.
Visualising the Lobes of the Lungs and The Pleural Cavity
The upper and lower lobes of the left lung are separated by the oblique fissure. The right upper and middle lobes are separated by the horizontal fissure, whilst the oblique fissure separates the middle and lower lobes of the right lung.
The oblique fissure on both sides extends from the spinous process (the bony point you can feel down the middle of the back) of T3 posteriorly to the sixth costal cartilage anteriorly.
The horizontal fissure is only present on the right lung. It extends from the mid-axillary line (a vertical line down the side of the chest from the middle of the armpit) anteriorly along the fourth rib to the anterior edge of the lung.
Edited by: Dr. Maddie Swannack and Dr. Thomas Burnell
Reviewed by: Dr. Marcus Judge
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