Explain the broad functions of the respiratory system in health + Define the Upper and Lower Respiratory Tract
- In order to ensure that all tissues receive the oxygen they need and can dispose of the CO2 they produce, transport and exchange occurs.
-Blood carries gases to and from tissues
-Lungs exchange with atmosphere
- Upper respiratory tract: nose, nasal cavity, mouth, pharynx and larynx
- Lower respiratory tract: trachea, bronchi, bronchioles and alveoli
What is the intrinsic capacity of blood?
•Has the intrinsic capacity to pick up oxygen and lose CO2 if exposed to the right gaseous environment
•This is what the lungs do: they expose blood to a gas mixture which has a carefully controlled composition
What is the Kinetic Theory of Gases?
•Gases are a collection of molecules moving around a space, independently of one another
•Pressure is generated by collisions of molecules with the walls
•The more frequent and harder the collisions, the higher the pressure
What is Boyle's Law?
•If a given amount of gas is compressed into a smaller volume, molecules will hit the wall more often (more frequent collisions) and pressure will rise.
•Therefore pressure is inversely proportional to volume
•P = 1/V
What is Charles' Law?
•Increasing temperature increases the kinetic energy of molecules so they hit the walls more often so pressure increases.
•Therefore pressure is proportional to absolute temperature starting at Absolute Zero.
•This is -273oC aka OoK (Kelvin)
•Molecules have zero kinetic energy – no movement occurs
What is the Universal Gas Law?
•P.V = nR.T
•R is a universal gas constant
•n is the number of molecules of gas in moles
•This equation allows calculation of how volume will change as pressure and temperature changes
•Volume is usually corrected to STP (Standard Temperature and Pressure, 0oC and 1 atmosphere pressure)
•STP 273oK (Or 0oC), 101.1kPa
Explain about Partial Pressure
•Pressure depends on number of molecules in the space not what the molecules are therefore pressure is the same in two equal-sized spaces containing the same number of a different gas (e.g. pressure of A = pressure of B)
•In a mixture of gases, molecules of each type behave independently so each gas exerts a partial pressure.
•This partial pressure is calculated as the same fraction of the total pressure as the volume fraction of the gas in the mixture
•E.g. if A and B were mixed together in double the space and there were equal numbers of gas molecules, pressure would still be the same but ½ the pressure would be attributable to gas A and ½ the pressure would be attributable to gas B
What is pO2 in Air?
•pO2 = total pressure x 0.209
•= 101.1 x 0.209
•NB: we measure pressure in kPa (in CVS, we measured pressure in mmHg which is a clinical convention)
What is meant by Water Vapour and Saturated Vapour Pressure?
•Water is the primary component in the human body therefore it is inevitable that gas mixtures will come into contact with water
•Water molecules evaporate
-Water molecules entering the gas (evaporation) exert vapour pressure which causes water molecules to enter the water again.
-When water molecules leave gas and then enter the water (again) at same rate (no net movement), this gives the constant Saturated Vapour Pressure
-This process is only affected by temperature – independent of total pressure
•When gases enter our body they are completely saturated with water vapour, so they don’t dry out our lungs
-SVP is 6.28kPa at 37oC
What is meant by Gas Tension?
•Gas molecules dissolve in water
-Gas molecules enter water and exert ‘tension’-like pressure if water is not there
-They collide with the upper surface of water and the gas molecules leave
-At equilibrium tension is the same as partial pressure of gas in gas mixture
Explain about Gas Tension in Liquids
•Indicates how readily gas will leave the liquid
•DOES NOT (at least directly) indicate how much gas is in the liquid
•The higher the tension, the more the gas molecules want to move out the liquid
What is meant by Solubility and Content of a Gas in Liquid?
•The amount of gas which enters the liquid to establish a particular tension is determined by solubility (the amount of gas molecules that interact with components of the liquid --> no longer free to move)
•Content = solubility x tension – (how easily the gas will dissolve x how readily it will leave the liquid)
Give an example of how the content of a gas in a liquid can be calculated
•If a gas reacts with a component of the liquid, this reaction must complete before tension is established.
•Total content is therefore reacted gas + dissolved gas
•E.g. Plasma just dissolves oxygen. At pO2 of 13.3kPa
-Blood contains Haemoglobin which reacts chemically with oxygen at 13.3kPa and binds 8.8mmol.l-1 and 0.13mmol.l-s is dissolved in the water
-Total content = O2 bound to haemoglobin + O2 dissolved in plasma
-Therefore content is 8.93mmol.l-1
Describe briefly general gas exchange in the lungs
•At rest 5L of blood must pick up 12mmol of oxygen per minute so a very large surface area is needed to allow this exchange to occur at such a rate – tennis court size
•You need a very large number of small compartments; each lung has approximately ~300 million alveoli, each surrounded by a capillary
•Air reaches the alveoli via a complex tree of airways – over 20 divisions
Trachea --> main bronchi --> lobar bronchi (3R, 2L) --> segmental bronchi --> sub-segmental bronchi etc till bronchioles
Bronchi have cartilage in walls, bronchioles do not but do have more smooth muscle
•Bronchioles divide and divide to form 200,000 terminal bronchioles.
•A complex ‘tree’ of conduction airways leads to the alveoli and an equally complex network of blood vessels directs virtually the entire cardiac output through plexuses of capillaries surrounding them
•Each terminal bronchiole is connected to a set of respiratory bronchioles, alveolar ducts and alveoli
Explain about the two circulations the lungs have
•The lungs have two circulations – pulmonary and bronchial.
•The bronchial circulation is part of the systemic circulation, and meets the metabolic requirements of the lungs. The pulmonary circulation is the blood supply to the alveoli, required for gas exchange.
•The pulmonary circulation must accept the entire cardiac output, and works with low resistance due to short, wide vessels, lots of capillaries connected in parallel (lower resistance) and arterioles with relatively little smooth muscle. This low resistance leads to the circulation operating under low pressure.
Why is low pressure and ventilation perfusion important in the pulmonary circulation? And how are they achieved?
•Low pressure (due to low resistance of capillaries): lower than colloid osmotic pressure so tissue fluid is not normally formed except occasionally in the base of the lungs where the effects of gravity increase transmural hydrostatic pressure
•Ventilation perfusion matching is vital and often disturbed by disease
•For efficient oxygenation, ventilation of the alveoli needs to be matched with perfusion. The optimal Ventilation/Perfusion ratio is 0.8. Maintaining this means diverting blood from alveoli that are not well ventilated.
•This is achieved by hypoxic pulmonary vasoconstriction. Alveolar hypoxia results in vasoconstriction of pulmonary vessels, and the increased resistance means less flow to the poorly ventilated areas and greater flow to well ventilated areas.
•Chronic hypoxic vasoconstriction can lead to right ventricular failure. The chronic increase in vascular resistance puts a high afterload on the right ventricle, leading to its failure.
Explain about Ventilation, defining Tidal Volume ,Respiratory Rate and Pulmonary Ventilation Rate
•Air drawn into lungs by increasing volume of terminal and respiratory bronchioles as lungs expand in inspiration
•Each breath draws a tidal volume into and out of the lungs
•Tidal volume: normal volume of air displaced between normal inhalation and exhalation – the volume of gas inhaled and exhaled during one respiratory cycle – when extra effort is not applied.
•Ventilation can be changed to match needs of the body by varying
•Respiratory rate: rate of breathing (number of breaths in a set time, normally 60 seconds)
•Pulmonary Ventilation Rate (Minute ventilation) : the amount of air a person breathes in a minute – tidal volume x respiratory rate
Discuss the role of Hydrostatic Pressure and Oncotic Pressure in the Pulmonary Circulation
•The hydrostatic pressure gradient = capillary hydrostatic pressure – interstitial space hydrostatic pressure.
•This gradient drives fluid from the pulmonary microcirculation into the interstitium
•The hydrostatic pressure gradient is opposed by the oncotic pressure gradient
•The rate of movement of fluid across the microvascular endothelium in the interstitium is determined by the net hydrostatic and oncotic pressures acting across the microvascular walls.
•In the normal lung, oncotic pressure tending to draw fluid back into the circulation is always greater than hydrostatic pressure tending to force it out.
What is meant by Surface Marking?
•Allows us to identify the position, shape and extent (the surface projection) of the lungs and the pleural cavity on the external chest wall, using bony landmarks as reference points
•Important to know during clinical examination of the lungs and in interpreting chest x-rays
What is meant by the Pleural Cavity?
•Thoracic cavity has 2 pulmonary cavities separated by an area known as the mediastinum which contains all the tissues and organs of the chest other than the lungs and pleura.
•Each lung, surrounded by 2 layers of pleura, lies within each pulmonary cavity (hemi thorax)
•The parietal pleura is the outside layer and lines the bony thoracic cage, diaphragm and mediastinal surface which make up the walls of each hemi thorax
•The parietal pleura is continuous at the root (hilum) of the lung with the visceral pleura which is adherent to the surface of the lung (cannot be dissected away) and also extends between the lobes of the lung
•The potential space between the 2 layers of pleura is the pleural cavity.
•The lungs fill the pleural cavity except inferiorly. During inspiration the lungs expand to occupy most (but not all) of this available space in the pleural cavity
What is meant by Pleural Reflection?
•The sharp lines along which the parietal pleura changes direction from its costal surface to the diaphragmatic and mediastinal surfaces are known as the lines of pleural reflection and are used to mark the extent of the pleural cavity
Describe the Surface Marking of the Pleural Cavity above the 4th Costal Cartilage?
•The apex of the lung and pleural cavity extends ~3cm above the medial 1/3rd of the clavicle, into the root of the neck
•From the apex, the line of pleural reflection of each side slopes down behind the sternoclavicular joints to meet each other near the midline, behind the sternal angle. Here the two pleural cavities are virtually touching
•Both pleural reflections pass vertically down behind the sternum, up to the level of the 4th costal cartilage
What happens at the 4th Costal Cartilage?
•The right pleura continues vertically near the midline down to the 6th costal cartilage
•The left pleura deviates laterally to the edge of the sternum then descends lateral to the border of the sternum down to the 6th costal cartilage.
•At the 6th costal cartilage, each side turns laterally and passes around the chest wall to cross
-The 8th rib at the mid-clavicular line
-The 10th rib at the mid-axillary line
-The 12th rib in the scapular line (line of medial border of the scapula) and lateral edge of the paravertebral muscles (yet to be covered)
•Then the medial border of the pleural cavity ascends vertically along the lateral border of the paravertebral muscles, up to the apex
What is the Sternal Angle?
•The 2nd costal cartilage articulates with the sternum at the level of the sternal angle (angle formed by the junction of the manubrium and body of the sternum).
•Hence the sternal angle is useful to identify the 2nd rib and to count the other ribs
Describe the surface marking of the lung
•The lungs fill the pleural cavity except at their lower margins and in front of the heart
•Therefore the surface marking of the lung is the same as that of pleural reflection at the apex and anteriorly at the 2nd, 4th and 6th ribs (except for the cardiac notch)
•However inferiorly, the lower margin of the lung is about 2 ribs higher than the pleura:
-Crosses the 6th rib at the mid-clavicular line
-8th rib in the mid axillary line
-10th rib in the scapular line and lateral edge of paravertebral muscles
•Thereafter the medial border of the lung ascends, along with the pleural vertically at the edge of the paravertebral muscles, up to the apex
What is the Costodiaphragmatic Recess?
•The inferior part of the pleural cavity not occupied by lung
Why is knowing the extent of the lung and pleura clinically important?
•Their lower parts overlap abdominal organs such as the liver, kidney and spleen
•On the apices of the pleura lie the subclavian vessels and the brachial plexus
•Stab wounds of the lower neck may injure the pleura and lungs
•Procedures such as exposure of the kidney, liver biopsy and insertion of a cannula into the subclavian vein may easily (and silently) produce a pneumothorax (air in the pleural cavity) that reveals itself later when the patient deteriorates as the lung collapses
Surface marking of the lobes of the lung (and fissures of the lung)
•Left lung has 2 lobes (upper and lower), which are separated by the oblique fissure. NO Horizontal fissure
•Right lung has 3 lobes (upper, middle and lower).
-The oblique fissure separates the upper and middle lobes from the lower lobe
-The horizontal fissure separates the upper and middle lobes
• The oblique fissure extends from spinous process of T2 vertebra posteriorly to the 6th costal cartilage anteriorly
•In the right lung, the horizontal fissure extends from the mid-axillary line, anteriorly along the 4th rib, to the anterior edge of the lung.
NB: to auscultate the lower lobe, do it at the back!
Describe the respiratory and conducting parts of the respiratory tract
•Mucous membrane lines the conducting portion of the respiratory tract – bearing mucus-secreting cells to varying degrees
•Serous membranes line the pleural sacs
•Conducting portion includes everything from nasal cavity to terminal bronchioles
•Respiratory portion includes respiratory bronchioles, alveolar ducts and alveoli
•The walls of the passageways become thinner as their lumens decrease in diameter (from exterior to interior)
Describe the changes in the respiratory epithelium down the respiratory tract
•Nasal cavity, pharynx, larynx, trachea, primary bronchi and secondary bronchi have pseudostratified epithelium with cilia and goblet cells, lines the airways from the nasal cavity to the largest bronchioles
•Bronchioles and terminal bronchioles have simple columnar epithelium with cilia and Clara (surfactant-producing) cells but no goblet cells
•Respiratory bronchioles and alveolar ducts have simple cuboidal epithelium with Clara cells and a few sparsely scattered cilia
•Alveoli have simple squamous / type 1 (+septal/type 2) cells. Type 2 cells produce surfactant.
What are Clara Cells?
•As bronchioles get smaller, goblet cells give way to Clara cells, interspersed between ciliated cuboidal cells.
•Clara cells secrete a surfactant lipoprotein, which prevents the walls sticking during expiration – holds surfaces apart
•Clara cells also secrete abundant Clara cell protein (CC16)
-A measurable marker in bronchoalveolar lavage fluid (if lowered then lung damage)
-A measurable marker in serum (if raised then leakage across air-blood barrier)
•Terminal bronchioles are the smallest airways of the conducting portion (<0.5mm)
•Absence of goblet cells in these very narrow airways is important to prevent individuals from ‘drowning’ in their own mucus
What is the epithelium in Non-Olfactory regions?
- Non-olfactory regions in the nasal cavities have pseudostratified ciliated epithelium. Mucous glands and venous sinses in lamina propria.
- Venous plexuses swell every 20-30 minutes, alternating air flow from side to side, preventing overdrying
- Arterial blood flows warm inspired air.
- Patency (shape of the space) is maintained by surrounding cartilage or bone
What is the epithelium in the Olfactory region?
Olfactory regions have particularly tall, thick pseudostratified columnar epithelium, without goblet cells, located in posterior, superior region of each nasal fossa.
The olfactory cells are bipolar neurons – one dendrite extends to the surface to form a swelling from which non-motile cilia extend parallel with the surface. These cilia increase surface area and respond to odours.
Axons of olfactory cells join each other form an olfactory nerve
Lamina propria blends with submucosa – serous glands (Bowman’s glands) flush odorants from epithelial surface, with a clear non-mucous substance
Describe the epithelium of the larynx
The ventricular folds of the larynx are lined by pseudostratified epithelium and contain mucous glands and numerous lymph nodules (arrows – dark purple)
The ventricles, together with the ventricular folds, contribute to the resonance of the voice.
The vocal cords beat 100xs a minute
Each vocal cord, of the larynx, is lined by stratified squamous epithelium and contains:
(1) A vocal ligament (large bundle of elastic fibres, running front to back).
(2) A vocalis muscle (bundle of skeletal muscle).
The vocal cords help stop foreign objects from reaching the lungs; they close to build up pressure when coughing is required.
Discuss the epithelium of the trachea (part 1)
•10cm long; 2.5cm wide
•Pseudostratified ciliated epithelium
•Divides into two primary bronchi in the mid-thorax
•Primary bronchi also have hyaline cartilage incomplete rings and spiral muscle that completely encircle the lumen
•The cricoid cartilage is the only complete ring of cartilage in the trachea
•In the trachea, fibroelastic membrane (mucous) contains the trachealis muscle
•C-shaped cartilaginous tracheal ring can transform in part to bone during ageing
•Layers: epithelium, respiratory mucosa, submucosa, fibroelastic membrane, adventitia
Discuss the epithelium of the trachea (part 2)
Secretions from the epithelium and submucosal glands of the trachea and bronchi contain mucins, water, serum proteins, lysozyme (destroys bacteria), antiproteases (inactivate bacterial enzymes).
Lymphocytes contribute immunoglonulins (esp. IgA)
Tracheal mucosa: approx. 250 cilia/cell. Cilia beat synchronously at 12 Hz beneath a viscoelastic mucus blanket (5μm deep).
There is an unusually thick basement membrane.
Lamina propria is rich in immune cells and with layer of elastic fibres
Explain briefly about the effect of CF on the respiratory tract
•A defective CFTR (cystic fibrosis transmembrane regulatory) or is absent in the apical membranes of the epithelial cells means chloride ion transport is substantially compromised
•Consequently, in the respirator tract, water does not leave the epithelium in sufficient quantities to adequately hydrate the secreted mucus. The mucus becomes viscous and can less readily be moved to the oropharynx for swallowing. Serious pulmonary infection often results.
Describe the epithelium in primary, secondary and tertiary bronchi
•The histology of secondary and tertiary bronchi are similar to primary bronchi except cartilages are arranged as irregular crescent plates or islands rather than rings (cartilage rings completely encircle the lumen of primary bronchi). They still have glands in submucosa.
Histology of primary bronchi is similar to trachea
Describe the epithelium in bronchioles and the blood supply
•Pulmonary arteries (PA) carry deoxygenated blood
•Bronchial arteries carry oxygenated blood to the lungs
•Bronchioles have no cartilage or glands (simply epithelium with more smooth muscle than bronchi)
•Bronchioles draw air into the lungs by increasing their volume, using the smooth muscle in their walls.
•Bronchioles are 1mm or less in diameter. Surrounding alveoli keep the lumen open.
~200,000 terminal bronchioles
•Entire pulmonary vasculature contains up to 500ml blood (40% of lung weight)
What happens to the bronchioles in asthmatics?
•Absence of cartilage in walls of bronchioles can be problematic because it allows these air passages to constrict and almost close down when smooth muscle contraction becomes excessive
•Such bronchoconstriction can become excessive in asthma and cause more difficulty with expiration than inspiration (during expiration, the bronchial walls no longer held open by the alveoli)
Describe passage openings into the alveoli
•Terminal bronchioles have no alveolar openings
•Respiratory bronchiole walls open onto some alveoli
•Duct wall has ubiquitous openings into alveoli
•Alveolar sac – composite air space onto which many alveoli open (hotel corridor / lobby analogy)
•Alveoli can open into
-A respiratory bronchiole
-Another alveolus (via an alveolar pore)
• New alveoli continue to develop up to the age of 8 years when there are approximately 300 million.
-Have abundant capillaries
-Are supported by a basketwork of elastic and reticular fibres
-Have a covering composed chiefly of Type I pneumocytes – squamous, covers 90% of surface area and permits gas exchange with capillaries
-Have a scattering of intervening Type II pneumocytes – cuboidal, covers 10% of surface area and contain lamellar bodies which produce surfactant (helps keep alveoli open, reduce surface tension)
-Numerous macrophages line alveolar surface (phagocytose particles)
• Alveoli are surrounded by a basketwork of capillaries and elasti fibres
•Air in alveoli lies as close 0.2μm from blood streaming through capillaries
•Gas exchange occurs across blood-air barrier
What is Emphysema?
•Destruction of alveolar walls and permanent enlargement of air spaces which can result from smoking or alpha 1-antitrypsin deficiency
•Alveolar walls normally hold bronchioles open, allowing air to leave the lungs on exhalation.
•When these walls are damaged, bronchioles collapse, making it difficult for the lungs to empty. Air becomes trapped in the alveoli.
•Hallmark sign: pursed-lip breathing
What happens in Pneumonia?
•Inflammation of the lung caused by bacteria. The lung consolidates as the alveoli fill with inflammatory cells.
•Most common causative bacterium is Streptococcus pneumoniae.
What are the functions of the respiratory tract?
1.Conducting air from atmosphere to the lower respiratory tract
2.Conditioning (warming, humidification and trapping of particles) inspired air
3.Protection of the air during swallowing (larynx)
Other functions include:
•Swallowing (oropharynx & laryngopharynx)
•Smell (olfactory epithelium in nose)
Describe the external nose and cavity
•External nose (made up of bone and cartilage)
•Nasal cavity is divided into right and left cavities by the MEDIAN NASAL SEPTUM (medial wall)
•Each nasal cavity extends from the nostril (or anterior nares) to the posterior nares (posterior choanae), behind, through which the nasal cavity communicates with the NASOPHARYNX situated behind it
•Lined by pseudostratified columnar ciliated epithelium
•The floor of the nasal cavity forms the roof of the mouth, which is the hard and soft palates
What are Conchae and Meatuses?
•The lateral wall of each nasal cavity contains 3 bony projections known as the conchae/turbinates (superior, middle and inferior)
•Space BELOW each concha is known as the meatus (superior, middle and inferior respectively)
•The paranasal sinuses and the naso-lacrimal duct which drains tears from the eye, open into these spaces
•Inferior concha is formed by an independent bone – the inferior concha
•The middle and superior conchae are the medial processes of the ethmoid bone
•The sphenoethmoidal recess, lying superposterior to the superior concha, receives the opening of the sphenoidal sinus
What are the Paranasal Sinuses?
•4 air-filled extensions of the respiratory part of the nasal cavity into cranial bones: the frontal, ethmoidal, maximllary and spheroidal sinuses (named according to the bones into whch they are located)
•They open into the nasal cavity and are lined by respiratory (pseudostratified columnar) epithelium which contain goblet cells and glands
•The Right and Left Frontal Sinuses are between the outer and inner tables of the frontal bone, posterior to the superciliary arches and the root of the nose. They each drain through a frontonasal duct into the ethmoidal infundibulm, which opens into the semi-lunar hiatus of the Middle Nasal Recess
•The Ethmoidal sinuses are small invaginations of the mucous membane of the middle and superior nasal recesses into the ethmoid bone.
-Anterior ethmoidal sinuses drain directly or indirectly into the middle nasal recess through the ethmoidal infundibulum
-The middle ethmoidal sinuses open directly into the middle nasal recess
-The posterior ethmoidal cells open directly into the superior nasal recess
•The Sphenomoidal sinuses are located in the body of sphenoid and may extend into the wings of the bone. The body of the sphenoid is fragile and only thin plates of bone separate the sinuses from several important structures (Optic nerves and chiasm, the pituitary gland, internal carotid arteries). They drain directly into the sphenoethmoidal recess.
•The Maxillary sinuses are the largest of the paranasal sinuses. They occupy the bodies of the Maxillae. They drain by one or more openings, the Maxillary Ostium (ostia), into the middle nasal recess by way of the semilunar hiatus.
Describe the Function of the External Nose and Nasal Cavity
•Structure of the nose is adapted to its important functions of filtering, warming and humidification of inspired air
•The vascular mucosa and the large surface area of the conchae are important for warming and humidification (watery nasal secretions – water evaporates to humidify the air). The conchae also cause turbulence (helps mix the air) and slow down airflow, increasing the time available for warming and humidification
•Each nostril is lined with coarse hairs (cilia) to trap large particles in inhaled air.
•Air is heated approximately to body temperature on passage through the nose
•The humidification is achieved by transudation of fluid through the epithelium and to a lesser extent by mucus secretion
•Vessels just below epithelium - warms air.
•The mucus secreted by the goblet cells traps almost all particles >5μm
•The cilia waft the mucus to oropharynx where it is swallowed
Describe the Function of the Paranasal Sinuses
•The paranasal sinuses also contribute to the warming and humidification of inhaled air
-Extension of nasal cavity
•Secretion of mucus to moisten nasal chambers
•Lightening weight of skull
•Buffer for trauma – protects cranial cavity
•Insulating sensitive structures from temperature variations – dental roots, eyes
Give some clinical applications of the external nose, naval cavity and paranasal sinuses
•The natural airway is through the nose which remains open at all times unlike the mouse
•The lower air passages will dry out and become more vulnerable to infection if nose breathing is impossible
•Mouth breathing during exercise with consequent drying and cooling of the bronchial mucous membranes is thought to be a mechanism of bronchoconstriction which occurs when asthmatics exercise
•Nasal polyps can lead to mouth breathing which is not ideal as there is less humidification etc.
•Infections of the URT involving the nasal cavity (e.g. common cold) can spread to involve:
-Paranasal sinuses (via their openings into the nasal cavity and may result in sinusitis)
-The pharynx and the larynx
-The middle ear
Describe the Pharynx
•Extends from the cranial base to the inferior border of the cricoid cartilage aneriorly and the inferior border of C6 vertebra posteriorly
•Has 3 parts:
-The nasopharynx which lies behind (posterior) the nose, is the part of the pharynx above the level of the soft palate (superior) Respiratory function as it is the posterior extension of the nasal cavities. Lymphoid tissue forms a tonsillar ring around the superior part of the pharynx, which aggregates to form tonsils
-The oropharynx which lies behind the mouth is the part of the pharynx between the soft palate and tip (superior border) of the epiglottis. Digestive function as it is involved in swallowing.
-The laryngopharynx which lies behind the larynx is the part of the pharynx between tip of the epiglottis and lower border of the cricoid cartilage, where it becomes continuous with the oesophagus
• Air and food have a common passage through parts of the pharynx. Hence, mechanisms to prevent aspiration (inhalation) of food particles during swallowing are important. Failure of these mechanisms can cause aspiration leading to potentially life threatening airway obstruction and/or infection (aspiration pneumonia)
•The nasopharynx is connected to the middle ear cavity via the auditory tube (or Eustachian tube). This allows the air pressure in the middle ear cavity to be equalized to atmospheric pressure
What is the Clinical Relevance of the Auditory Tube?
•Upper respiratory tract infections can be spread into the middle ear cavity (tympanic cavity) via the auditory tube resulting in middle ear infection (acute otitis media)
•More likely in children than adults
•The tympanic cavity is the narrow air-filled chamber in the petrous part of the temporal bone and is connected with the nasopharynx anteromedially.
Describe the Larynx
•The larynx links the inferior oropharynx pharynx to the trachea
•It extends from the laryngeal inlet through which it communicates with the laryngopharynx to the level of the inferior border of the cricoid cartilage. Here the laryngeal cavity is continuous with the trachea.
•Its cartilaginous skeleton made up by 3 unpaired cartilages (the epiglottis, thyroid cartilage and cricoid cartilage) and the paired arytenoid cartilages.
•These 5 cartilages are linked by ligaments and muscles
•The larynx contains the vocal cords which guard the entrance to the trachea
•Larynx’s most important function is to guard the air passages, especially during swallowing when it serves as the sphincter/valve of the lower respiratory tract, thus maintaining the airway
What are the Vocal Cords?
•AKA the vocal folds or vocal ligaments
•The vocal cords + the aperture between the cords is together termed the ‘glottis’
•Rima glottides is the term for the aperture between the 2 vocal cords
•Movement of the vocal cords towards the midline is called ADDUCTION and results in closing of the aperture e.g. during swallowing. Also referred to as the closure of the glottis
•The movement of the vocal cords away from the midline is called ABDUCTION – results in opening of the aperture (aka opening of the glottis)
Describe the movement of the Vocal Cords
•Abducted during respiration allows free movement of air through the open aperture
•Partially abducted during speech (phonation). Sound is produced by the vibration of vocal cords due to passage of air through the narrowed aperture
•Adducted during swallowing prevents inhalation of food, protecting the lower respiratory tract. This happens involuntarily and is coordinated by the brain
•Adducted during the initial part of the cough reflex. This is also involuntary and coordinated by the brain
•The vocal cords movements are due to the action of the intrinsic laryngeal muscles (as opposed to the extrinsic laryngeal muscles which moves the entire larynx but not the vocal cords)
What is the relevance of the recurrent laryngeal nerve (branch of the vagus nerve)?
•Supplies all intrinsic muscles except the cricothyroid muscle (supplied by the external laryngeal branch branch of the superior laryngeal nerve of the vagus nerve)
•The recurrent laryngeal nerve on the left has a long course, part of which is inside the thoracic cavity (curves under the aortic arch and then comes up in the groove between trachea and esophagus) . Hence intra thoracic disease compressing / infiltrating the nerve can result in a hoarse voice, due to paralysis of the left vocal cord
•Right recurrent laryngeal nerve curves under subclavian artery
•Lesions may cause hoarseness of voice
•Voice change can be the first sign of serious intra-thoracic disease, due to the involvement of the left recurrent laryngeal nerve in the thorax
•Laryngeal disease can present as airway obstruction and difficulty in breathing
How is the risk of aspiration reduced?
•The vocal cords act as the essential sphincter or valve, guarding the entrance to the trachea, being abducted in respiration and adducted during closing
•If the vocal cords are dysfunctional, there is a risk of aspiration of food/liquid during swallowing into the respiratory tract
•The risk of aspiration is further reduced by the epiglottis folding downwards (like a lid) to cover the laryngeal inlet, which is also narrowed
How are the vocal cords important for the cough reflex?
•Ability to close the vocal cords is necessary for an effective cough reflex
•The cough reflex is a protective mechanism to expel inhaled particles, and also serves as a clearance mechanism for disposing of excessive secretions from the airways
One of the causes of respiratory difficulty can be due to narrowing of the Glottis - why?
Can be due to
•Laryngeal tumours or vocal cord tumours
•Laryngeal oedema (swelling of the mucosal lining of the larynx) due to allergic reactions (e.g. anaphylaxis) or severe infections such as croup or acute epiglottis
•Bilateral vocal cord paralysis causes loss of vocal cord abduction
One of the causes of respiratory difficulty can be pharyngeal obstructrion - why?
•The airway of an unconscious person lying on his back may be obstructed by the tongue falling backward to obstruct the pharynx
•This is the reason for keeping unconscious persons in the recovery position (turned on their side), as this prevents the tongue falling back, and also allow secretions in the mouth to drain out of the mouth (than risk aspiration into the lungs)
•Sleep Apnoea Syndrome: loss of tone in the pharyngeal muscles during sleep causes them to become floppy and obstruct the airways during sleep.
•Remember the importance of airway management in critically ill patients!
Elaborate on the A in the ABC of Care in Emergency Situations
•A – Airway
•In unconscious patients obstruction of the pharynx can occur due to
-The Tongue falling back
-A foreign body (e.g. dislodged denture)
-Vomit (which may also be aspirated into lung)