SGT3A - Anatomy of the Thoracic Cavity PDF

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SGT3A - Anatomy of the Thoracic Cavity...

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SGT3A: Anatomy of the Thoracic Cavity LO: Describe the functional anatomy of the thoracic cage, muscles, pleura and lungs during inspiration and expiration, and to relate this to functional abnormalities, disease and trauma. Components of The Bony Thorax  Manubrium of sternum[1]  Body of Sternum – flat bone [2]  Xiphoid Process [3]  Coastal cartilages [4]  12 pairs of ribs [5]  12 Thoracic Vertebrae (T1-T12) [8] The Boundaries of the Thoracic Cavity  Superior: Superior Thoracic Aperture (thoracic inlet)  Inferior: Inferior Thoracic Aperture  Antero-Lateral: Ribs  Anterior: Manubrium, Sternum & Xiphoid Process  Posterior: Thoracic vertebrae (T1 -T12) Superior Thoracic Aperture 

The boundaries of the superior thoracic aperture: o

Body of T1

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1st rib and its cartilage

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Manubrium of sternum

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Suprasternal Notch

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The structures that pass through the thoracic aperture are:

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Left:

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Left phrenic and left vagus nerve

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Left common carotid and left subclavian artery

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Left brachiocephalic vein (left subclavian vein and left internal jugular vein)

Right o

Right phrenic and vagus nerve

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Right Brachiocephalic trunk (right common carotid and

subclavian artery) o 

Right brachiocephalic vein (right subclavian vein and right internal jugular vein)

Middle o

Oesophagus

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Trachea

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Recurrent laryngeal nerve

Inferior Thoracic Aperture  The boundaries of the inferior thoracic aperture: o Body of T12 (posteriorly) o Ribs 11&12 (postero-laterally) o Cartilaginous ends of ribs 7-10 (Anterolaterally) o Xiphoid process (anteriorly)  The inferior thoracic aperture is closed by the diaphragm Gross Anatomy of The Thoracic Cavity  The thoracic cavity is divided into 3 separate anatomical compartments by the fibrous pericardium: o Left Pulmonary Cavity  Left of the mediastinum  Occupied by the left lung o The Mediastinum  Occupies the centre of the thoracic cavity  Its boundaries are created by the fibrous pericardium  It contains the heart and great vessels, oesophagus and trachea.  It sits on the superior surface of the diaphragm o Right Pulmonary Cavity  Right of the mediastinum  Occupied by the right lung

Openings of the Diaphragm

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Openings: caval hiatus, oesophageal hiatus, and aortic hiatus. An easy way to remember the location and structures passing through the diaphragm is by using this mnemonic: 'I 8 10 EGGs AAT 12' (read: I ate ten eggs at twelve). o I 8 - IVC crosses the diaphragm at the level of T8  Right Phrenic nerve o 10 EGGs - EsophaGus + vaGus cross the diaphragm at the level of T10  Oesophageal branches of left gastric artery/vein o AAT 12 - Aorta + Azygos vein + Thoracic duct cross the diaphragm at the level of T12 The left phrenic nerve pierces the diaphragm separately.

Attachments of Diaphragm  Anteriorly to the xiphoid process and costal margin (costal cartilages ribs 7-10)  Laterally to ribs 11 & 12  Posteriorly to the lumbar vertebrae via crura (tendonous band) (see above) o Right crus – Arises from L1-L3 and their intervertebral discs. o Left crus – Arises from L1-L2 and their intervertebral discs.  The muscle fibres of the diaphragm extending from their bony attachments to converge on a central tendon. Innervation of Diaphragm  Motor innervation: comes from the phrenic nerves (C3-C5). o C3, 4, 5 keeps the diaphragm alive!

Left half of the diaphragm is innervated by left phrenic nerve and vice versa Sensory innervation: (pain and proprioception) o Central tendon: innervated by the phrenic nerves, o Peripheral muscular portions: innervated by 6th to 11th intercostal nerves. (T6-T11) o

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Anatomical Landmarks Sternal Angle  Sternal angle: angle formed between the manubrium of the sternum and the body of the sternum. This level marks the position of the intervertebral disc between T4-T5  Importance of sternal angle: o Beginning and end of aortic arch o Bifurcation of the trachea o Azygos veins drains into SVC o Level of the second rib o Bifurcation of the pulmonary trunk into pulmonary arteries o Thoracic duct cross into left side of the thorax The suprasternal notch (Jugular Notch)  It is a large, visible dip at the midline of the neck and the two clavicles.  It occurs at the superior border of the manubrium of the sternum  It is at the level of the T2 and T3 vertebrae.  The trachea lies just behind it, rising about 5 cm above it in adults.

Ribs  Posteriorly a rib makes contact with the vertebrae three times: o Costovertebral joint  The head of a rib has two facets  One facet connects to superior vertebrae and the other connects to the directly inferior vertebrae.  This forms a costovertebral joint o Costotransverse joint  The tubercle of the rib connects to transvers column  This forms a costotransverse joint  This helps stabilise the rib and rib cage  Anteriorly the rib connects to the sternum via costal cartilag o Costal cartilage provides flexibility to the rib cage w breathing.

Anterior Articulations of Ribs

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Rib 1 (true rib) joins to the manubrium via a fibrous joint which allows no movement Ribs 2 – 7 are true ribs o True rib: a direct connection between the rib, its costal cartilage and sternum/manubrium. o Synovial joints between rib and sternum Ribs 8-10 are false ribs o False Ribs: ribs that articulate their costal cartilage with the costal cartilage of rib 7 to join to the sternum Ribs 11-12 are false & floating ribs (do not attach to sternum)  costal cartilage not long enough

LO: Describe the distribution of the visceral and parietal pleural membranes and pleural cavity (and contents) in health and disease. Structure of the Pleurae  Pleura: a layer of simple squamous cells supported by connective tissue.  The pleura can be divided into two parts: o Visceral pleura – covers outer surface of the lungs and extends into the interlobular fissures o Parietal pleura – covers the internal surface of the thoracic cavity. Parietal Pleura  The parietal pleura covers the internal surface of the thoracic cavity: o Cervical pleura o Mediastinal pleura – Covers lateral aspect of the mediastinum o Costal pleura – Covers inner aspect of the ribs, costal cartilages, and intercostal muscles. o Diaphragmatic pleura – Covers thoracic surface of the diaphragm.

Pleural Cavity  Pleural cavity (intrapleural space): Potential space between the viscera and parietal pleura.  It contains a small volume of serous fluid, which has two major functions: o It lubricates the surfaces of the pleurae, allowing them to slide over each other. o Produces a surface tension, pulling the parietal and visceral pleura together, which couples the lung to the thorax. (Note: if air enters the pleural cavity, this surface tension is lost – a condition known as pneumothorax)

Pleural Recesses  Anteriorly and posteroinferiorly, each pleural cavity is not completely filled by the lungs.  This gives rise to two recesses in each cavity. o Costodiaphragmatic (costophrenic) Recess  Located between the costal pleurae and the diaphragmatic pleura.  It is at the most inferior aspect of the lung. o Costomediastinal recess  Located between the costal pleurae and the mediastinal pleurae  Found behind the sternum.  The Costodiaphragmatic recess if often blunted when there is pathology at the base of the lungs: o Pleural effusion – fluid collects here o Pneumonia of the lower lobes

Angle is h

Left and i ht l

Innervation  Parietal Pleura o Innervated by sensory division of somatic nervous system:  Innervated by the phrenic and intercostal nerve (T1-T11) o Sensitive to pressure, pain, and temperature. o It produces a well localised pain  Visceral Pleura o Innervated by sensory division of ANS  Innervated by pulmonary plexus o Not sensitive to pain, temperature or touch. o Its sensory fibres only detect stretch.

What are the two mechanical models used for the movement of the ribs and sternum?  During breathing, the dimensions of the thorax change in the vertical, lateral, and anteroposterior directions.  Change in Vertical Dimension: o Due to depression (contraction) and elevation (relax) of the diaphragm  Changes in the anteroposterior dimensions: o Pump Handle Movement:  Because the anterior ends of the ribs are inferior to the posterior ends:  when the ribs are elevated, they move the sternum upward and forwards  when the ribs are depressed, the sternum moves downward and backward.  Changes in the Lateral dimensions: o Bucket Handle Movement  The middle shafts of the ribs tend to be lower than the anterior and posterior ends:  When the ribs are elevated, the middles of the shafts move laterally  increases the lateral dimensions of the thorax.  Any muscles attaching to the ribs can potentially move one rib relative to another and therefore act as accessory respiratory muscles.

Development of the lungs  The lower respiratory system begins to develop in the fourth week of development.  It starts off with the median laryngotracheal tube in the floor of the primitive pharynx.  This tube deepens and is separated from the developing oesophagus by the growth of the tracheo-oesophageal septum.  The resulting laryngotracheal tube develops a lung bud at its distal end which divides into two bronchial buds by the middle of the fifth week.  These grow laterally into the primitive pleural cavities.  They then divide to form the primary bronchi and continue dividing until they form terminal bronchioles.

Lung development is divided into four stages:  The pseudoglandular period (weeks 5 - 17): o The lung has a distinct glandular appearance. o The major elements divide as far as the terminal bronchioles  The canalicular period (weeks 16-25)  When the: o Lumina of the bronchi and terminal bronchioles enlarge, o Respiratory bronchioles and alveolar ducts develop o The lung tissue becomes highly vascularised.  The terminal sac period (weeks 24 - birth) o When the alveolar ducts develop terminal sacs  primitive alveoli.

Initially these are lined with a simple cuboidal epithelium, but this matures into squamous epithelium at about 26 weeks. o Two cell types are present:  Type I squamous epithelial cells (capillaries lie in close proximity to these cells)  Type II squamous epithelia  more rounded cells which secrete surfactant. o At this stage the lungs are sufficiently developed to permit survival of the foetus if it is born prematurely. The alveolar period (Late foetal - 8 years) o The epithelium of the terminal sacs continues to thin so that adjacent capillaries will bulge into them, forming immature alveoli. o At the beginning of the alveolar period, each respiratory bronchiole terminates in a cluster of thin-walled sacs, separated from each other by loose connective tissue. o These terminal sacs represent future alveolar ducts. o Characteristic mature alveoli do not develop until after birth. o Until 3 years of age the lungs grow by formation of more immature alveoli, o From 3 - 8 years these alveoli increase in size. o About 95% of alveoli will develop after birth. o

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