Describe the structure of the bony thorax
- The bony thorax is made up by the thoracic vertebrae posteriorly, the ribs and the intercostal spaces laterally, and the sternum and costal cartilages anteriorly.
- Above, at the thoracic inlet (aka the superior thoracic aperture – the superior opening the thoracic cavity), it is continuous with the neck
- Below, the diaphragm separates it from the abdominal cavity.
Describe the sternum and the sternal angle
The sternum consists of 3 parts:
- The manubrium which articulates with the 1st and part of the 2nd costal cartilage
- The body which articulates part of the 2nd and the 3rd-7th costal cartilages
- The xiphersternum which remains cartilaginous into adult life.
The junction of the manubrium and body is known as the sternal angle and is felt as a transverse ridge on palpation of the sternum
- The fact that the 2nd costal cartilage articulates with the sternum at the level of the sternal angle enables the 2nd rib to be identified and the other ribs to counted during clinical examination to accurately surface mark intra thoracic structures.
Describe the 12 ribs
The 12 ribs articulate with vertebral column posteriorly via the costal-vertebral joints. Anteriorly,
- Ribs 1-7 are connected to the sternum via costal cartilages
- Ribs 8-10 are connected to the costal cartilage above (connection with sternum is indirect – known as false ribs)
- Ribs 11 and 12 (and sometimes 10th) end free in the posterior abdominal muscles (aka ‘floating ribs’)
NB: Superior Thoracic Aperture is aka Thoracic Inlet (anatomically) and Thoracic Outlet (clinically)
Describe a typical rib
A ‘typical’ rib (ribs 3-9) has a head, a neck, a tubercle and a shaft.
- the head has two articular facets for articulation with the body of the corresponding vertebra and the vertebra above
- the tubercle has one articular facet for articulation with the transverse process of the corresponding thoracic vertebra.
- The shaft is thin, flat and curved. At the angle, the shaft twists forward to form its characteristic curve.
- The shaft has a costal groove close to its lower border which protects the intercostal vessels and nerve
Describe the typical features of thoracic vertebrae
- Typical thoracic vertebra features: independent, have bodies , vertebral arches and seven processes for muscular and articular connections. Characteristics include:
- Bilateral costal facets (demifacets): on the vertebral bodies, usually occurring in inferior and superior pairs for articulation with the heads of the rib inferior to it.
- Costal facets: One the transverse processes for articulation with the tubercles of corresponding ribs (except the inferior two or three thoracic vertebrae – atypical ribs)
- Spinous (processes: long slanting inferiorly and anteriorly. Offers increased protection to the spinal cord, preventing an object like a knife entering the spinal canal through the intervertebral discs.
- Vertebral foramen is circular.
What are costovertebral joints?
there are 2 synovial joints, which connect the ribs with the thoracic vertebrae.
- Joint of the rib head – the head of rib articulates with body of the corresponding vertebra and the vertebra above.
- Cost transverse joint – the articular facet on the tubercle of the rib articulates with the transverse process of the corresponding vertebra.
Describe how ribs 1&2 different to typical ribs.
- The 1st rib is the broadest (i.e. its body is widest and nearly horizontal), shortest and most sharply curved of the 7 true ribs. It has a single facet on its head for articulation with the T1 vertebra only and two transversely directed grooves crossing its superior surface for the sub-clavian vessels. The two grooves are separated by a scalene tubercle and ridge, to which the anterior scalene muscle is attached.
- The 2nd rib has a thinner, less curved body and is substantially longer than the 1st rib. It’s head has two facets fir articulations with the bodies of the T1 and T2 vertebrae. Its main atypical feature is a rough area on its upper surface, the tuberosity for serratus anterior and has a poorly marked costal groove.
Describe how ribs 10, 11 and 12 are atypical ribs
- 10th - 12th ribs have a single facet on their heads and articulate with a single vertebra.
- 11th-12th are short and have no neck or tubercle.
Describe the intercostal muscles
Each intercostal space contains 3 muscles, innervated by the intercostal nerves. From superficial to deep they are:
- The external intercostal muscles: fibres run downwards and anteriorly from the inferior margin of the rib above to the superior margin of the rib below. These muscles are responsible for 30% of chest expansion during quiet respiration. Contraction elevates the upper ribs increasing the anterior-posterior diameter of thorax and elevation of the lower ribs, increasing the lateral (transverse) diameter of the thorax.
- Internal intercostal muscles: fibres run downwards and posteriorly from the rib above to rib below. Hence, their action pulls the ribs down (depress) from the position of chest expansion. They reduce the AP and lateral diameters of the chest. They are active during forced expiration. (quiet expiration is passive).
- Innermost intercostal muscles are similar to the internal costal muscles but are less well-developed. They act along with the internal intercostal muscles during forced expiration.
Describe the structure of the diaphragm
The diaphragm is a dome-shaped muscle which divides the thoracic cavity from the abdominal cavity. The shape of the diaphragm (marked convexity) means the thoracic cavity is much smaller than the bony thorax would suggest.
- The liver, spleen, parts of the stomach and upper kidneys, which lie in the abdominal cavity, are covered by the ribs.
- The right dome of the diaphragm lies at the level of the 5th rib, and left dome is slightly lower at the level of the 5th intercostal space.
- The diaphragm consists of a peripheral muscular part and a central tendon. The muscle fibres insert into central tendon.
What is the peripheral muscular part of the diaphragm? What are the arcuate ligaments and crura?
The peripheral muscular part is made up by:
- A sternal part arising from the deep surface of the xiphisterum
- A costal part arising from the inner aspects of the 7-12 costal cartilages
- A vertebral part arising from the arcuate ligament and crura.
The arcuate ligaments are thickenings of fascia over the muscles of the posterior abdominal wall.
The crura are strong tendons attached to the antero-lateral surfaces of the upper 3 lumbar vertebral bodies The right crus arises from bodies and intervertebral discs of L1, 2 &3.
What openings does the diaphragm have for? What action is it involved in? Innervation?
The diaphragm has openings for:
- Inferior vena cava (T8 level)
- Oesophagus (T10 level)
- Aorta (T12 level – aortic hiatus)
The diaphragm is the main muscle of inspiration being responsible for 70% of chest expansion in quiet respiration. On contraction of the diaphragm, it moves downwards (flattens) to increase vertical diameter of thoracic cavity.
The diaphragm is innervated by phrenic nerve (C3,C4,C5). It provides a motor supply to the diaphragm and sensory supply to both surfaces of the diaphragm, pericardium mediastinal part of parietal pleura and diaphragmatic part of parietal pleura.
- Margins of the diaphragm receive innervation from intercostal nerves.
- It is the mediastinal/diaphragmatic part of the parietal pleura.
Describe the distribution of the intercostal nerves, arteries and veins
VAN: the intercostal vein, artery and nerve (in order from above to below) lie in the intercostal groove of the rib, between the internal and innermost intercostal muscles.
They run along the lower border of the rib – important to remember when carrying out a pleural aspiration or insertion of a chest drain, when the needle (or tube) should be inserted at the upper border of the rib to avoid injury.
Describe the intercostal arteries
- Supply the intercostal muscles, parietal pleura and overlying skin
- Each intercostal space has an anterior intercostal artery (except the last two), which anastomoses with a posterior intercostal artery.
- The anterior intercostal arteries arise from the internal thoracic artery (a branch of the subclavian) and its continuation, the musculophrenic artery.
- Brachiocephalic / Aortic arch -> Subclavian -> Internal thoracic/musculophrenic (later) -> anterior intercostal
- Posterior intercostal arteries arise from the aorta
- Brachiocephalic / Aortic arch -> subclavian -> costocervical trunk - > superior intercostal -> posterior intercostal (1st/2nd spaces).
- Thoracic aorta -> posterior intercostal (other spaces)
Describe the intercostal veins and nerves
- The intercostal veins: each intercostal space has two anterior and one posterior vein accompanying the arteries.
- The anterior veins drain via the internal thoracic vein into the subclavian vein.
- Most posterior intercostal veins drain via the azygos vein on the right and hemiazygos vein on the left, into the superior vena cava.
- The intercostal nerves: the anterior rami (formed as soon as the mixed spinal nerves leave the intervertebral foramina) of thoracic spinal nerves (T1-T12) supply the intercostal muscles, the parietal pleura and the overlying skin.
What are the 3 compartments of the thoracic cavity? Describe the pleura
The thoracic cavity has 3 compartments: 2 lateral pulmonary cavities that contain the lungs and one central compartment known as the mediastinum.
- The pleura is a serous membrane consisting of a single layer of mesothelial cells with a thin layer of underlying connective tissue. Each lung is enclosed in a pleural sac consisting of 2 continuous membranes.
- The parietal pleura line the inside of each hemi-thorax (the bony thoracic cage, diaphragm and mediastinal surface) and is continuous at the hilum of the lung with the visceral pleura which lines the outside of lung, forming the pleural cavity – a pleural space.
- The visceral pleura extend between lobes of the lung into the depths of the oblique and horizontal fissures.
What is the pleural space? What does the pleural fluid allow?
- Pleural space (cavity) is in fact a potential space between the two layers of pleura Both layers of pleura are covered with a common film of fluid produced from the parietal surface and absorbed by the parietal lymphatic vessels.
- The pleural fluid allows parietal and visceral parts to slide on one another. Thus in a healthy person, the pleura allows movement of the lung against the chest wall with breathing. The surface tension of the pleural fluid provides the cohesion that keeps the lung surface in contact with the thoracic wall (forms a pleural seal – which can be broken by leakage of air -> lung collapse). As a result, when the thorax expands in inspiration, the lung expands along with it and fills with air.
The lungs do not occupy all the available space in the pleural cavity, even in deep inspiration.
The costodiaphragmatic recesses are the pleural lined-gutters that surround the upward convexities of the diaphragm.
Describe the supply to the parietal and visceral pleura?
- The parietal pleura are supplied by the intercostal arteries and internal thoracic arteries and is drained by the corresponding veins.
- The visceral pleura is supplied by the bronchial arteries and drained by the bronchial veins.
- Parietal pleura: has somatic (voluntary) innervation including pain fibres from intercostal and phrenic nerves, as well as autonomic innervation.
- Visceral pleura: has no somatic innervation, only autonomic innervation
What are the 4 parts of the parietal pleura?
- Cervical pleura: extends into the root of the neck
- Mediastinal pleura: covers the lateral aspects of mediastinum
- Costal pleura: covers the internal surfaces of the thoracic wall (inside of the rib cage)
- Diaphragmatic pleura: covers the superior/thoracic surface of the diaphragm
What are the lines of pleural reflection?
The lines of pleural reflection are the relatively abrupt lines along which the parietal pleura changes direction (reflects) as it passes from one wall of the pleural cavity to another. There are 3 lines of pleural reflection on each side: sternal, costal and diaphragmatic (vertebral)
What makes the Lower Respiratory Tract? Describe the lungs? What is the hilum of the lung?
- Lower Respiratory Tract: trachea, primary bronchi and lungs
- Each lung has an apex (extends above the level of 1st rib into the neck), a base (concave inferior surface, resting on the diaphragm), three surfaces (costal, mediastinal and diaphragmatic) and three borders (anterior, inferior and posterior)
- The convex costal surface and the concave diaphragmatic surface are separated by a sharp inferior border; the posterior border is rounded to fit the paravertebral gutter, the anterior border is thin and on the left side shows the cardiac notch.
- Lingula – is an area on the left upper lobe which corresponds to the right middle lobe.
- The hilum of the lung is a wedge-shaped area on the mediastinal surface of each lung through which the structures forming the roots of the lung pass to enter or exit. The roots of the lung consist of the bronchi, pulmonary arteries, superior and inferior pulmonary veins, the pulmonary plexus of nerves and lymphatics.
Describe the structure of the trachea
The trachea commences at the lower border of the cricoid cartilage in the neck and terminates by dividing into the right and left main bronchi (at the level of the sternal angle/ Angle of Louis).
- The angle between the right and left main bronchi is known as the carina.
- The trachea is held open by 18-22 U/C-shaped cartilages.
- Posteriorly, where the cartilage is deficient, its wall contains the trachealis muscle
- The trachea is lined by pseudo-stratified columnar ciliated epithelium (with goblet cells)
- It descends anterior to the oesophagus and enters the superior mediastinum, inclining a little to the right of the median plane. The posterior surface of the trachea is flat where it is applied to the oesophagus.
Describe the bronchi
The right main bronchus is wider, shorter and more vertical (parallel to the trachea) than the left. It is 2.5cm long and before reaching the hilum of the lung, gives off its upper lobar branch. This means that any inspired objects are more likely to fall into the right main bronchus.
The left main bronchus is 5cm long and passes below the arch of the aorta, anterior the descending aorta and oesophagus
Describe the bronchial tree. What's a bronchopulmonary segment?
The bronchial tree: the primary (left and right main)bronchi divide into lobar bronchi for each lobe (3 on right – upper, middle and lower lobar bronchi; 2 on left – upper and lower lobar bronchi). The lobar bronchi in turn divide into segmental bronchi, each destined for a bronchopulmonary segment.
- The part of the lung supplied by a segmental bronchus is known as a bronchopulmonary segment.
- A bronchopulmonary segment is a pyramid shaped area of lung with its apex facing towards hilum and base toward lung surface. Each is supplied by a segmental bronchus and by its own segmental branch of the pulmonary artery and pulmonary vein. Branches of the bronchial arteries also accompany the bronchi, supplying them.
- Knowledge of bronchopulmonary segments is surgically important because they can be isolated and removed without much bleeding, air leakage or interfering with other bronchopulmonary segments.
- Segmental bronchi divide further -> subsegmental bronchi -> which further divide on and on until they become terminal bronchioles -> respiratory bronchioles -> alveolar ducts -> alveoli.
What's a Bronchoscopy? Why's it done?
Bronchoscopy: the whole of the inner trachea, the carina, the main bronchi, the lobar bronchi and the origin of the segmental bronchi can be visualized at bronchoscopy. Bronchoscopy is use in the diagnosis of bronchial carcinoma to visualize the tumour and obtain a tissue sample for histology.
Describe the bronchial arteries. What do they supply? How does the blood return?
Bronchial arteries supply the bronchial tree from the carina up to the respiratory bronchioles, visceral pleura and connective tissue.
They arise from the thoracic aorta on the left, and the 3rd intercostal artery (branch of the thoracic aorta) on the right.
The two left Bronchial arteries arise directly from the thoracic aorta
The single right bronchial arterial arises from the 3rd intercostal artery.
Most of the blood supplied by the bronchial arteries is returned via the pulmonary veins rather than the bronchial veins The superficial group of veins drain the visceral pleura and the bronchi in the hilar region to azygous vein on the right, and the accessory hemiazygous on the left.
The deep group of veins drain rest of the bronchi (deep in lung) into the main pulmonary vein or directly into left atrium.
Describe the pulmonary arteries and veins
- The pulmonary arteries carry mixed venous blood from the right ventricle for gas exchange at the alveoli. At the level of the sternal angle, the main pulmonary trunk divides into -> the right and left pulmonary arteries which enter the lung with the right and left main bronchi. The pulmonary arteries divide within the bronchial tree (each bronchus is accompanied by a branch of the pulmonary artery) and eventually form the rich capillary network surrounding the alveoli.
- The pulmonary arteries do not supply the bronchi (which are supplied by the bronchial arteries) but do supply the alveoli, giving them all they need other than oxygen, which they get directly from alveolar air.
- There are some anastomoses between the bronchial and pulmonary arteries at pre-capillary level and capillary level (these maintain blood supply to lung parenchyma after pulmonary embolism).
- The pulmonary veins drain the alveoli; they do not closely follow the bronchi but tend to run alonh the intersegmental septa. Two pulmonary veins leave each hilum (draining the upper and lower lobes).
- Two pulmonary veins (superior and inferior) on each side carry
- The pulmonary veins run independently of the arteries and bronchi
- In the right lung, the Middle Lobe Vein is a tributary of the Right Superior Pulmonary Vein
Describe the nerve supply of the lung
The lung receives fibers from the vagi and the sympathetic trunk via the pulmonary plexuses, which are situated at each hilum.
The parasympathetic fibres from the vagus are motor to the bronchial smooth muscle (bronchoconstrictor), inhibitory to pulmonary vessels (vasodilator) and increase activity of the mucous glands (increased mucus production)
The vagal afferent fibres are those for the cough reflex and some subserving pain.
The sympathetic efferents are bronchodilator and vasoconstrictor.
Describe the lymphatic drainage of the lung
The superficial sub-pleural lymphatic plexus lies deep to the visceral pleura and drains the lung parenchyma and visceral pleura. They drain along the surface to the hilar lymph nodes situated in the hilum of each lung.
The deep bronchopulmonary lymphatic plexus lies in the submucosa of the bronchi and peribronchial tissue. They also drain eventually to the hilar nodes (aka the bronchopulmonary nodes
Efferents from these nodes run to the tracheobronchial nodes. (Enlarged tracheobronchial nodes can cause widening of the angle of the carina).
Why is knowledge of the surface marking of the pleural cavity and lung important?
- Stab injuries to the upper abdomen can involve the pleural cavity and lungs
- The apex of the lung extends to the root of the neck and is closely related to the sympathetic trunk, subclavian vessels and the brachial plexus, especially median cord. Tumours of the apex of the lung can involve these structures.
- Stab wounds of the lower neck may injure the pleura and lungs and cannulation of the subclavian vein may puncture the pleura to cause a pneumothorax (air in the pleural cavity). This is the reason a chest x-ray is usually done after insertion of a subclavian line.
Describe the Mediastinum and how it's divided into its 4 compartments
The mediastinum (mass of tissue between the two pulmonary cavities) is covered on each side by mediastinal pleura and contains all the thoracic viscera and structures except the lungs.
- The mediastinum extends from the superior thoracic aperture to the diaphragm inferiorly
- From the sternum and costal cartilages anteriorly to the bodies of the thoracic vertebrae posteriorly.
- Some structures such as the oesophagus, pass vertically through the mediastinum and therefore is in more than one mediastinal component.
The mediastinum is subdivided into 4 compartments – the superior, middle, anterior and posterior mediastinum.
NB: the middle, anterior and posterior mediastina make up the inferior mediastinum (after the sternal angle anteriorly) – between the transverse thoracic plane and the diaphragm.
Describe the superior and middle compartments of the mediastinum
- Superior: upper border is superior thoracic apertute and lower border is the transverse thoracic plane which runs from the sternal angle anteriorly to the junction (IV disc) of T4 and T5 vertebrae posteriorly,. From anterior to posterior, the contents are:
[*] Thymus (a primary lymphoid organ)
[*] Great vessels, with the veins (brachiocephlic veins and SVC) anterior to the arteries (arch of aorta and roots of its major branches – the brachiocephalic trunk, left common carotid and left subclavian) and related nerves (vagus and phrenic nerves and the cardiac plexus of nerves)
[*] Inferior continuation of the cervical viscera (tracheal anteriorly, oesophagus posteriorly) and related nerves (left recurrent laryngeal nerve)
[*] Thoracic duct and lymphatic trunks
- The middle mediastinum includes the pericardium and its contents (heart and roots of its great vessels).
Describe the posterior and anterior parts of the mediastinum
- Posterior: located inferior to the transverse thoracic plane, anterior to the T5-T12 vertebrae, posterior to pericardium and diaphragm and between the parietal pleura of the two lungs.
[*] Contains the thoracic aorta, thoracic duct and lymphatic trunks, posterior mediastinal lymph nodes, azygos and hemiazygos veins, and esophagus and esophageal nerve plexus.
- Anterior: the smallest subdivision of the mediastinum lies between the body of sternum and the transversus thoracis muscles anteriorly and the pericardium posteriorly. It is continuous with the superior mediastinum at the sternal angle and is limited inferiorly by the diaphragm.
[*] Consists of loose connective tissue (sternopericardial ligaments), fat, lymphatic vessels, a few lymph nodes and branches of the internal thoracic vessels
Describe in general terms the structure of the pulmonary circulation and the characteristics that distinguish it from the systemic circulation
The pulmonary circulation must accept the entire cardiac output and works 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.
Describe the properties of the mechanical system comprising the lungs, chest and diaphragm.
- Lungs: bronchioles dilate, increasing their volume and lowering the pressure inside the lungs, moving air in.
- Chest wall:
Parietal pleura secretes fluid, the surface tension of which adheres the two pleural layers together.
So when the chest wall expands, the parietal pleura (attached to the chest wall) moves with it as does the visceral pleura, which is attached to the lung, causing it to expand.
External intercostals elevate the ribs in a ‘bucket-handle’ type movement – increasing the anterior-posterior and transverse diameter of the thoracic cavity
- Diaphragm: contracts and descends to increase the vertical diameter of the thoracic cavity. Accounts for 70% of chest expansion during quiet respiration. Also helps in elevating the lower ribs.
Describe the roles of the muscles involved in inspiration and expiration from the resting expiratory level
Quiet Breathing: inhalation (diaphragm and external intercostals); exhalation: none due to elastic recoil of the chest wall and lungs.
Describe the roles of the diaphragm and accessory respiratory muscles in forced breathing
Forced Breathing Inspiration
Scalene (elevate and fix the upper ribs)
Sternocleidomastoid (elevates the sternum)
Forced Breathing Expiration: these cause a decrease in the AP and transverse diameters and a decrease in vertical dimension (opposite to inspiration!)
Abdominals: this muscle of active expiration depress the lower ribs and compress abdominal contents, thus pushing up the diaphragm
How are alveolar pO2 and pCO2 determined?
- The partial pressure of inspired air is further changed by gas exchange occurring in the alveoli.
- The blood flowing through the alveolar capillaries picks up O2 and loses CO2 by diffusion of these gases across the alveolar wall.
- At the same time there is diffusion of O2 and CO2 between alveolar gas and fresh atmospheric air in the terminal and respiratory bronchioles brought in by ventilation
- Alveolar pO2 is determined by a balance between rate of removal of O2 by the blood and the rate of replenishment of O2 by alveolar ventilation.
- Alveolar pCO2 is similarly determined by the balance between the rate at which CO2 enters the alveoli from blood and the rate at which it is removed from alveolar gas by ventilation.
- The balance between perfusion and ventilation, keeps the partial pressure of oxygen and carbon dioxide in the alveolar gas stable:
- pO2: 13.3kPa
- pCO2: 5.3 kPa
- Mixed venous blood returns to the lungs from the body: pO2 typically 6.0kPa and pCO2 typically 6.5kPa but varies with metabolism.
What 3 factors affect the rate of gas exchange?
Blood flowing through the alveolar capillaries picks up oxygen and loses carbon dioxide by diffusion of those gases across the alveolar wall. The rate at which gases exchange is determined by three factors:
- area available for exchange: the area of the alveolar surfaces is large because there are a huge number of alveoli, generating in a normal lung an exchange area of around 80m2. In normal lungs, therefore the area available is not a limiting factor on gas exchange.
- Resistance to diffusion
- Gradient of partial pressure: large
pO2 in alveolar gas > pO2 in returning (mixed venous) blood
pCO2 in alveolar gas < pCO2 in returning blood
Despite all of this the overall barrier is less than 1 micron – 0.6μm thick
Explain about diffusion resistance
Resistance to diffusion: The diffusion pathway from alveolar gas to alveolar capillary blood is short, but there are several structures between the two. First gas must diffuse through the gas in the alveoli then through: (liquid diffusion)
- The alveolar epithelial cell
- Interstitial fluid
- Capillary endothelial cell
- Red cell membrane (to get to Hb to which O2 binds)
Despite all of this the overall barrier is less than 1 micron – 0.6μm thick
Does O2 or CO2 diffuse faster?
Two gases have to diffuse, oxygen into blood and carbon dioxide out of it. The resistance is not the same for the two gases. For most of the barrier (during the liquid diffusion part), the rate of diffusion is affected by the solubility of the gas in water, and carbon dioxide diffuses much faster, because it is more soluble.
- During the gas diffusion part in the alveoli, gases diffuse through gases at rate inversely proportional to molecular weight. Big molecules diffuse slower so CO2 diffuses slower than oxygen.
Overall, CO2 diffuses 21 times as fast as oxygen for a given gradient. This means that anything affecting diffusion will only change oxygen transport, as that is limiting. (IF there is problem affecting the exchange of gases, O2 will be affected first).
Describe the rate of exchange and what it means
The rate of exchange of oxygen is normally very rapid, so that within half a second of blood arriving in the alveolar capillary, it has equilibrated with the gas in the alveoli. In a normal individual this means the partial pressures of O2 and CO2 in arterial blood will be the same as the partial pressures in the alveolar gas.
- at rest blood cells spend about 1s in capillary so gas diffusion is not limiting on the lung
The partial pressures of O2 and CO2 in the alveolar gas must therefore be kept very close to their normal values of 13.3kPa and 5.3kPA respectively if the tissues of the body are to be properly supplied with oxygen and lose their carbon dioxide. This is achieved by exchange of gas between alveolar gas and atmospheric gas brought close to it through the airways of the lung by the process of ventilation.
- Exchange between alveolar gas and mixed venous blood will tend to lower pO2 and raise pCO2 but this is prevented by diffusion of oxygen into and carbon dioxide out of alveolar air from atmospheric air brought next to the alveoli by ventilation.
What happens during ventilation? And describe the movement of air
- Ventilation: air is driven through the airways of the lungs by pressure changes produced by increases and decreases in the volume of the air spaces next to the alveoli. The movements of breathing lower pressure in the terminal and respiratory bronchioles during inspiration, so air flows down the airways to them, and then increase pressure during expiration so air flows back out again.
- Fresh atmospheric air does not enter the alveoli and exchange of oxygen and carbon dioxide occurs via diffusion between alveolar gas and atmospheric air in the terminal and respiratory bronchioles.
- There are no valves in the respiratory system – movement of air is tidal.
What is Spirometry? And define the Tidal Volume, the Inspiratory Reserve Volume and the Expiratory Reserve Volume
- The movement of air during breathing can be measured by Spirometry. The subject breathes from a closed chamber (has constant pressure) whose volume changes with ventilation.
- A certain volume enters and leaves the lungs with each breath – the Tidal Volume. [The lung volume that represents the amount of air that is displaced between normal inspiration and expiration, when extra effort is not applied].
- During normal respiration the increase in lung volume is not maximal – it can be increased to the extent of the Inspiratory Reserve Volume (the extra volume that can be breathed in when extra effort is applied). We can also breathe out more than at the rest, by using the Expiratory Reserve Volume (the extra volume that can be breathed out when extra effort is applied).
What is the Residual Volume?
We cannot however empty our lungs completely, so even after a forced expiration, a RESIDUAL VOLUME will remain.
- The volume left in the lungs at maximal expiration, the residual volume, cannot be measured with a spirometer. It is measured by helium dilution.
Lung Volumes change. What are better measurements?
Lung volumes change with changes in tidal volume (changes in breathing pattern) but LUNG CAPACITIES do not change with tidal volume, as they are measured from fixed points in the breathing cycle. These are:
- Maximum Inspiration
- Maximum Expiration
- End of a quiet expiration
Inspiratory Capacity is: end of quiet expiration to maximum inspiration (i.e. inspiratory reserve + tidal volumes). [The biggest breath that can be taken from resting expiratory level – lung volume at end of quiet expiration – and is often ~3L].
Functional residual capacity is the volume of air in the lungs at the end of a quiet expiration (At resting expiratory level). [Expiratory reserve volume + residual volume]. ~2L
Vital Capacity = Inspiratory Capacity + Expiratory Reserve OR Inspiratory Reserve Volume + Tidal Volume + Expiratory Reserve Volume. The biggest breath that can be taken in, measured from the max inspiration to max expiration. It often changes in disease and is ~5L.
Total Lung Volume = Vital Capacity + Residual Volume
What are typical values?
- Tidal Volume: 0.5L
- Inspiratory Reserve Volume: 3.3L
- Expiratory Reserve Volume: 1.2L
- Residual Volume: 0.8L
- Functional Residual Capacity: 2.0L
- Inspiratory Capacity: 3.8L
- Vital Capacity: 2L
- Total Lung Capacity: 5.8L
What is Serial Dead Space?
- The air moved in the tidal volume, enters both the conducting pathways and the terminal and respiratory bronchioles. Air enters and leaves the lungs by the same airways so the last air in is the first air out and does not reach the alveoli – not available for gas exchange.
- Only part of the inspired air is therefore available for gas exchange – the remainder is contained in Dead Space and undergoes no interaction with the blood (ineffective, wasted breathing)
- The volume of the conducting airways (up to and including the terminal bronchiole) is known as the Anatomical Dead Space (or Serial Dead Space) and is normally ~150ml. This may be measured by the nitrogen washout test.
Describe the nitrogen washout test
- The patient takes a maximum inspiration of 100% oxygen
- The oxygen that reaches the alveoli will mix with alveolar air and the resulting mix will contain Nitrogen (there is 79% Nitrogen in air)
- However the air in the conducting airways (dead space) will still be filled with pure oxygen.
- The person exhales through a one-way valve that measures the percentage of Nitrogen in and volume of air expired.
- Nitrogen concentration is initially zero as the patient exhales the dead space oxygen.
- As alveolar air begins to move out and mix with dead space air, nitrogen concentration gradually climbs until it reaches a plateau where only alveolar gas is being expired.
- A graph can be drawn to determine the dead space, plotting Nitrogen % against Expired Volume.
What is the alveolar dead space and physiological dead space?
- The air contained in the conducting airways is not the only air, which fails to equilibrate with alveolar capillary blood. Some alveoli receive an insufficient blood supply; others are damaged by accident or disease so that even in the air, which reaches the alveolar boundary, there is a certain proportion, which fails to exchange. The volume of air in alveoli not taking part in gas exchange is known as the alveolar dead space (or distributive dead space).
- The sum of the Anatomical dead space + Alveolar dead space is known as the Physiological dead space (~0.17L). This can be determined by measuring pCO2 or PO2 of expired and alveolar air. Alveolar air is diluted from by dead space air to form the expired air, and the degree of dilution is a measurement of the physiological dead space.
What is the Ventilation Rate?
Ventilation rate = tidal volume (volume moved per breath) and respiratory rate
- Typically 8L/ min at rest
- Can exceed 80L/min in exercise
The total rate of movement of air into and out of the lungs – the Pulmonary Ventilation Rate – is therefore composed of two components – movement of air into and out of the dead space – Dead Space Ventilation – which has no effect upon the blood, and Alveolar Ventilation. The ideal parameter to measure is therefore Alveolar Ventilation Rate (the amount of air that actually reaches the alveoli)
- At each breath the dead space is fully ventilated. The deeper the breath, the higher the proportion of air available for gas exchange.
- All other things being equal, deeper breathing is more effective breathing.
- In practice, the ideal depth of respiration is limited by mechanical factors.
How would you calculate the alveolar ventilation rate?
Alveolar Ventilation Rate = Pulmonary Ventilation Rate – Dead Space Ventilation Rate = (Tidal Volume x RR) – (Dead Space Volume x RR)
- At normal breathing, this can be about ~1/3 of inspired air being wasted
- In rapid shallow (excited) breathing, this can rise to almost 2/3s of inspired air being wasted.
- In slow deep breathing, much less is wasted.
- Therefore slow deep breathing gets most air to alveoli but is hard work so at rest we adopt an intermediate rate and depth