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BDS 1st Year Anatomy - Respiratory System

Kathmandu University Affiliated

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LONG QUESTIONS

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Q1. Name the laryngeal cartilages and its intrinsic muscles with their nerve supply. [3+2+2=7]

A. Laryngeal Cartilages [3 marks]

1. Thyroid cartilage - largest, shield-shaped, two laminae meeting at 90° (males) or 120° (females); forms the laryngeal prominence (Adam's apple)
2. Cricoid cartilage - signet ring-shaped; only complete cartilaginous ring in the airway; lies at C6 level
3. Epiglottis - leaf/spoon-shaped elastic fibrocartilage; attached to inner angle of thyroid by thyroepiglottic ligament

B. Intrinsic Muscles of Larynx [2 marks]

C. Nerve Supply [2 marks]

• Superior Laryngeal Nerve (SLN):
  • External branch - motor to cricothyroid only
  • Internal branch - sensory to laryngeal mucosa above vocal cords
• Recurrent Laryngeal Nerve (RLN):
  • Motor to all intrinsic muscles except cricothyroid
  • Sensory to mucosa below vocal cords
  • Right RLN loops around subclavian artery; Left RLN loops around arch of aorta

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Q2. Describe the components of a bronchopulmonary segment and mention its applied importance. [5+2=7]

A. Components [5 marks]

1. Segmental bronchus - tertiary bronchus; enters at the hilum of the segment
2. Segmental artery - branch of pulmonary artery; accompanies bronchus centrally
3. Segmental vein - drains via intersegmental veins (in the intersegmental septum) to pulmonary veins
4. Lymphatics - drain toward hilum
5. Autonomic nerves - sympathetic and parasympathetic fibers along bronchus
6. Connective tissue septum - thin intersegmental septa separate adjacent segments
7. Lung parenchyma - alveoli, alveolar ducts, respiratory bronchioles

• Right lung: 10 segments (upper 3, middle 2, lower 5)
• Left lung: 8-10 segments (upper lobe 4, lower lobe 4/5; apical and posterior often fused; medial basal often absent)

• Upper lobe: Apical, Posterior, Anterior
• Middle lobe: Lateral, Medial
• Lower lobe: Superior, Medial basal, Anterior basal, Lateral basal, Posterior basal

• Upper lobe: Apico-posterior, Anterior, Superior lingular, Inferior lingular
• Lower lobe: Superior, Anteromedial basal, Lateral basal, Posterior basal

B. Applied Importance [2 marks]

1. Surgical resection (Segmentectomy): Each segment is an independent unit - diseased segments (TB cavities, bronchiectasis, tumors) can be surgically removed individually, preserving maximum healthy lung tissue.
2. Postural drainage: Specific positions are used to drain each segment by gravity - important in bronchiectasis and cystic fibrosis management.
3. Bronchoscopy: Knowledge of segment positions helps localize foreign bodies or identify the source of hemoptysis bronchoscopically.
4. Collapse patterns: Collapse of individual segments produces characteristic radiological patterns used in diagnosis.
5. Intersegmental veins as guides: During surgery, intersegmental veins lie in the connective tissue septa and mark boundaries between segments, helping surgeons identify correct resection planes.

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Q3. Enumerate the paranasal sinuses and mention their drainage. [4+3=7]

A. Enumeration of Paranasal Sinuses [4 marks]

1. Maxillary sinus (Antrum of Highmore)
   • Largest paranasal sinus
   • Lies within the body of the maxilla
   • Apex points toward zygomatic bone; base is nasal wall
   • Floor related to roots of upper molar/premolar teeth (clinically significant)
2. Frontal sinus
   • Within frontal bone, above medial part of orbit
   • Separated by bony septum (often deviated)
   • Develops from anterior ethmoidal air cells
3. Ethmoidal air sinuses
   • Multiple small cells within ethmoid bone
   • Divided into anterior, middle, and posterior groups
4. Sphenoidal sinus
   • Within body of sphenoid bone
   • Related to pituitary fossa superiorly, optic nerves, cavernous sinus
   • Trans-sphenoidal surgery approach to pituitary

B. Drainage [3 marks]

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SHORT NOTES

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1. Surfactant

• Definition: A surface-active lipoprotein complex produced by Type II pneumocytes (Type II alveolar cells / great alveolar cells)
• Composition: ~90% lipids (mainly dipalmitoyl phosphatidylcholine - DPPC) + 10% proteins (SP-A, SP-B, SP-C, SP-D)
• Function: Reduces surface tension at the air-alveolar interface; prevents alveolar collapse at end-expiration (prevents atelectasis)
• LaPlace's Law: Pressure = 2T/r - without surfactant, smaller alveoli would collapse into larger ones
• Development: Appears at 20 weeks gestation; adequate amounts by 35-36 weeks
• Clinical importance:
  • Respiratory Distress Syndrome (RDS) / Hyaline Membrane Disease: Deficiency in premature neonates (< 34 weeks); treated with exogenous surfactant (beractant/poractant alfa) and antenatal corticosteroids (dexamethasone/betamethasone accelerate maturation)
  • Lecithin:Sphingomyelin (L:S) ratio > 2:1 in amniotic fluid indicates lung maturity

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2. Lanugo

• Definition: Fine, soft, unpigmented hair that covers the fetal body from about 16 weeks of gestation
• Derived from: Hair follicles that develop from the epidermis
• Function: Helps hold vernix caseosa (the protective white waxy coating) on the skin surface; thermoregulation
• Timeline: Appears at 16-20 weeks; shed by 36-40 weeks (mostly shed by birth in term infants)
• Clinical significance:
  • Presence at birth indicates prematurity
  • Persistence after birth is a marker of preterm birth
  • Also reappears in severe malnutrition and anorexia nervosa (as a compensatory thermoregulatory mechanism)

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3. Sternal Angle (Angle of Louis)

• Definition: The transverse ridge at the junction of the manubrium and body of the sternum (manubriosternal joint / secondary cartilaginous joint)
• Level: T4-T5 intervertebral disc level
• Structures at this level:
  • 2nd costal cartilage articulates laterally (landmark for counting ribs)
  • Aortic arch begins and ends here
  • Trachea bifurcates (carina - into right and left main bronchi)
  • Azygos vein joins the SVC
  • Ligamentum arteriosum (remnant of ductus arteriosus)
  • Thoracic duct crosses from right to left
  • Upper limit of pericardium
• Clinical importance:
  • Primary landmark for rib counting in clinical practice
  • Marks entry into superior mediastinum
  • Reference point in chest radiograph reading

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4. Maxillary Air Sinus

• Location: Within the body of the maxilla
• Shape: Pyramidal
• Capacity: Approximately 15 mL (largest paranasal sinus)
• Relations:
  • Roof: Floor of orbit (infraorbital nerve and vessels run here)
  • Floor: Alveolar process - roots of upper 2nd premolar, 1st and 2nd molar project into floor
  • Medial wall: Lateral wall of nasal cavity
  • Posterior wall: Infratemporal fossa
• Drainage: Into middle meatus via hiatus semilunaris (ostium is high on medial wall - poor drainage in upright position)
• Nerve supply: Anterior, middle, and posterior superior alveolar nerves (branches of maxillary nerve V2)
• Blood supply: Infraorbital and superior alveolar branches of maxillary artery
• Applied:
  • Maxillary sinusitis: most common sinusitis; referred toothache from molar region
  • Oro-antral fistula: after upper molar extraction
  • Drainage: Caldwell-Luc procedure

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5. Palatine Tonsil

• Location: In the tonsillar fossa (between palatoglossal and palatopharyngeal folds) - within the oropharynx
• Capsule: Fibrous capsule derived from pharyngobasilar fascia; loosely attached to superior pharyngeal constrictor
• Surfaces: Medial (covered with stratified squamous epithelium with 10-20 crypts) and lateral (attached to pharyngeal wall)
• Blood supply:
  • Main: Tonsillar branch of facial artery (from below)
  • Also: Ascending pharyngeal, lingual (dorsal lingual), and descending palatine arteries
  • Veins: Tonsillar vein → pharyngeal plexus
• Nerve supply: Glossopharyngeal nerve (CN IX) and lesser palatine nerve
• Lymphatic drainage: Jugulodigastric (tonsillar) node (most important - first to enlarge in tonsillitis)
• Applied:
  • Peritonsillar abscess (quinsy): pus between capsule and superior constrictor; uvula deviated to opposite side
  • Tonsillectomy: scissor and snare technique; bleeding point - tonsillar artery from below
  • Tonsillitis - common in children; can cause rheumatic fever (Group A streptococcus)

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6. Recesses of Pleura

• Between costal and diaphragmatic pleura
• Deepest part: in the mid-axillary line at the level of 10th rib
• Extent: 8th rib (mid-clavicular line) → 10th rib (mid-axillary) → 12th rib (paravertebral)
• First to fill with fluid in pleural effusion
• Used for thoracocentesis (pleural tap) - needle inserted in 9th intercostal space, mid-axillary line, above upper border of lower rib (to avoid neurovascular bundle)

• Between costal and mediastinal pleura
• Located behind the sternum/costal cartilages anteriorly
• Left side is larger (cardiac notch of left lung - less lung to fill this space)
• Used in emergency cardiac procedures (cardiac massage without thoracotomy)

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7. Dangerous Area of Face

• Definition: The area drained by veins that communicate with the cavernous sinus via valveless veins
• Boundaries: Roughly triangular - from the corners of the mouth to the bridge of the nose ("danger triangle of face")
• Veins involved: Facial vein → communicates via:
  • Ophthalmic veins (superior and inferior) → Cavernous sinus
  • Deep facial vein → Pterygoid plexus → Emissary veins → Cavernous sinus
• Why dangerous: Facial vein has NO valves - infection/thrombophlebitis from a furuncle (boil) or squeezed pimple can spread retrogradely to the cavernous sinus
• Consequence: Cavernous sinus thrombosis:
  • Proptosis, chemosis, orbital pain
  • Involvement of CN III, IV, V1, V2, VI (all pass through cavernous sinus)
  • Septic meningitis and brain abscess
  • High mortality
• Lesson: Never squeeze a pimple or boil in this region!

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8. Cartilage of Larynx (Short note)

1. Hyaline cartilage: Thyroid, cricoid, lower part of arytenoids, and most laryngeal cartilages
   • May ossify with age (calcification starts after 25 years; visible on X-ray)
   • First to ossify: cricoid
2. Elastic fibrocartilage: Epiglottis, corniculate, cuneiform, apex and vocal process of arytenoid
   • Does NOT calcify

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SHORT ANSWER QUESTIONS

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Q4. What is a typical intercostal space? Write down its boundaries, contents and clinical importance. [1+2+1+1=5]

Definition [1]:

Boundaries [2]:

• Above: Lower border of upper rib + costal groove
• Below: Upper border of lower rib
• Anterior: Sternum or costal cartilage
• Posterior: Vertebral column (head and neck of rib)
• Outer: Skin, superficial fascia, serratus anterior/external oblique
• Inner: Parietal pleura (innermost intercostal muscle)

Contents [2]:

1. External intercostal - fibers directed downward and forward; active in inspiration; fills the space posterolaterally
2. Internal intercostal - fibers directed downward and backward; active in expiration; fills entire space
3. Innermost intercostal - incomplete layer; crossed by intercostal neurovascular bundle

• Order from above to below: VAN (Vein, Artery, Nerve)
• Intercostal vein (most superior, in costal groove)
• Intercostal artery (from aorta posteriorly, anterior intercostal from internal thoracic)
• Intercostal nerve (ventral ramus of thoracic spinal nerve - T1-T11)

Clinical Importance [1+1]:

1. Intercostal nerve block: Injection given just below the rib (costal groove) - analgesia for rib fractures, thoracic surgery, post-herpetic neuralgia
2. Thoracocentesis/chest drain (intercostal drainage): Needle/tube inserted along the upper border of the lower rib to avoid neurovascular bundle (VAN is just below the upper rib). Site: 5th-6th intercostal space, mid-axillary line for fluid; 2nd intercostal space, mid-clavicular line for tension pneumothorax
3. Herpes zoster (shingles): Follows dermatomal distribution of intercostal nerve

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Q5. Mention briefly the different stages of maturation of lungs. [5]

1. Embryonic Stage (0-7 weeks)

• Lung bud appears at day 26 from the foregut endoderm as a ventral diverticulum
• Single lung bud → trachea → 2 bronchial buds → right (3 lobar buds) and left (2 lobar buds)
• Tracheoesophageal septum separates trachea from esophagus
• TEF (tracheoesophageal fistula) - failure of this separation

2. Pseudoglandular Stage (6-16 weeks)

• Bronchi and bronchioles form by repeated branching (branching morphogenesis)
• Up to terminal bronchioles by 16 weeks
• Lung resembles an exocrine gland histologically (tubules lined by cuboidal/columnar epithelium)
• No respiratory exchange possible - not viable

3. Canalicular Stage (16-26 weeks)

• Formation of respiratory bronchioles and alveolar ducts
• Vascularization begins - capillaries approach epithelium
• Type I and Type II pneumocytes differentiate
• Surfactant production begins (~20-24 weeks)
• Limited viability possible from 24-26 weeks with intensive care

4. Saccular (Terminal Sac) Stage (26-36 weeks)

• Terminal saccules (primitive alveoli) form
• Thin blood-air barrier established
• Surfactant production increases
• Reasonable viability from 26-28 weeks

5. Alveolar Stage (36 weeks - 8 years)

• Mature alveoli develop by thinning of walls and subdivision of saccules
• At birth: ~50 million alveoli; adult: ~300-500 million alveoli
• Most alveolar development occurs postnatally (up to 3-8 years of age)

• RDS: insufficient surfactant before 34-35 weeks
• Pulmonary hypoplasia: associated with congenital diaphragmatic hernia (CDH)
• Lung agenesis, sequestration as developmental anomalies

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Q6. Mention the boundary and structure of nasopharynx with its applied anatomy. [2+2+1=5]

Boundaries [2]:

• Above (Roof): Body of sphenoid + basilar part of occipital bone (basiocciput)
• Below: Continuous with oropharynx at the level of soft palate / pharyngeal isthmus
• Anteriorly: Opens into nasal cavity via choanae (posterior nasal apertures)
• Posteriorly: Posterior pharyngeal wall (C1 vertebra/atlas)
• Laterally: Eustachian (pharyngotympanic) tube opening and salpingopharyngeal fold

Structure [2]:

• Mucosa: Pseudostratified columnar ciliated (respiratory) epithelium in children → stratified squamous in adults (due to repeated trauma from air)
• Pharyngeal tonsil (adenoid): Mass of lymphoid tissue in the roof/posterior wall; present in children; normally involutes by puberty
• Eustachian tube opening: Lies at the level of inferior concha on the lateral wall; surrounded by cartilaginous ridge (torus tubarius)
• Pharyngeal recess (Fossa of Rosenmüller): Depression behind the torus tubarius - important site for nasopharyngeal carcinoma
• Salpingopharyngeal fold: From torus tubarius downward
• Tubal tonsil: Around Eustachian tube opening
• Waldeyer's ring: Pharyngeal tonsil (adenoid) + tubal tonsils + palatine tonsils + lingual tonsil form a ring of lymphoid tissue

Applied Anatomy [1]:

1. Nasopharyngeal carcinoma (NPC): Most common site is fossa of Rosenmüller; associated with EBV infection; common in Southeast Asians; presents with epistaxis, cervical lymphadenopathy, conductive deafness
2. Adenoid hypertrophy: In children; causes nasal obstruction, snoring, mouth breathing, "adenoid facies," and conductive hearing loss (Eustachian tube obstruction → otitis media with effusion)
3. Otitis media with effusion (Glue ear): Eustachian tube dysfunction due to adenoids or nasopharyngeal infection

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Q7. Describe the formation and features of the lateral wall of the nasal cavity. [3+2=5]

Formation [3]:

1. Nasal bone - anterosuperior
2. Frontal process of maxilla - anterior part of lateral wall
3. Lacrimal bone - small area; anterior part (contains nasolacrimal groove)
4. Labyrinth of ethmoid bone - largest contributor; central part; contains middle and superior conchae and ethmoidal air cells
5. Inferior concha (turbinate) - separate bone; largest concha; occupies inferior part of lateral wall
6. Perpendicular plate of palatine bone - posterior part
7. Medial pterygoid plate of sphenoid - most posterior part

Features [2]:

• Bulla ethmoidalis - rounded elevation - middle ethmoidal cells open here
• Hiatus semilunaris - semilunar groove below the bulla
• Infundibulum - funnel-shaped passage from hiatus into which frontal and anterior ethmoidal sinuses open
• Uncinate process - hook-like process below bulla

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Q8. Describe Bronchopulmonary segment with its applied anatomy. [3+2=5]

Bronchopulmonary Segment [3]:

• Defined as a wedge-shaped unit of lung tissue supplied by its own segmental (3rd order) bronchus with its accompanying segmental artery; surrounded by connective tissue septa
• Each segment is an independent anatomical and functional unit
• The apex of each segment points toward the hilum; base lies at the lung surface
• Intersegmental veins drain along the septa (unlike arteries/bronchi which run centrally)

• Right lung: 10 segments (3 upper, 2 middle, 5 lower)
• Left lung: 8-9 segments (apical + posterior fused in upper; no medial basal in some descriptions)

Applied Anatomy [2]:

1. Segmentectomy: Independent blood supply and lymphatics allow safe surgical removal of individual diseased segments (bronchiectasis, TB cavities, carcinoid, small peripheral lung cancers) without disturbing adjacent segments
2. Postural drainage: Each segment can be selectively drained by appropriate patient positioning + physiotherapy; essential in bronchiectasis and cystic fibrosis
3. Bronchoscopic localization: Each segmental bronchus can be identified endoscopically; foreign body removal, endobronchial biopsy, BAL
4. Collapse/consolidation patterns: Radiological shadows conform to segmental boundaries
5. Intersegmental veins as surgical landmarks: Guide resection planes

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Q9. Explain nerve supply and applied anatomy of pleura. [3+1+1=5]

Nerve Supply [3]:

• Costal pleura and peripheral diaphragmatic pleura: Intercostal nerves (T1-T11) - pain referred to thoracic wall and abdominal wall
• Mediastinal pleura and central diaphragmatic pleura: Phrenic nerve (C3,4,5) - pain referred to shoulder tip (C4 dermatome - referred to neck and shoulder)
• Cervical pleura (dome/apex): Intercostal nerves + cervical plexus (C3, C4)

• Autonomic nerves only (vagal and sympathetic)
• No somatic pain sensation - hence lung carcinoma/pneumonia does not cause pain until parietal pleura is involved
• Only responds to stretch

Applied 1 - Diaphragmatic pleura referred pain [1]:

• Central diaphragmatic pleura (phrenic nerve C3,4,5): pain referred to shoulder tip and neck
• Seen in: subphrenic abscess, diaphragmatic pleurisy, hepatic pathology
• Clinical example: Kehr's sign - left shoulder tip pain in splenic rupture due to irritation of diaphragmatic pleura

Applied 2 - Thoracocentesis [1]:

• Pleural tap performed at lower border of intercostal space, along upper border of lower rib to avoid intercostal neurovascular bundle
• Site: 9th ICS, mid-axillary line (costodiaphragmatic recess)
• Complication: intercostal nerve damage causes intercostal neuralgia

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Q10. Describe gross anatomy and development of diaphragm. [3+2=5]

Gross Anatomy [3]:

• Central tendon: Clover/trefoil-shaped fibrous center; no muscle; blends with pericardium above
• Muscular part: Three portions:
  • Sternal part: Two slips from posterior xiphisternum
  • Costal part: Inner surfaces of lower 6 ribs and costal cartilages (7-12)
  • Lumbar (vertebral) part: Right and left crura from lumbar vertebrae

• Right crus: L1-L3 vertebral bodies + discs
• Left crus: L1-L2
• Medial arcuate ligament: over psoas major (L1)
• Lateral arcuate ligament: over quadratus lumborum

Development [2]:

1. Septum transversum - central tendon (incomplete partition at C3-C5 level initially); major contributor
2. Pleuroperitoneal membranes - paired membranes from lateral body wall; close pleuroperitoneal canals (fuse with septum transversum and mesoesophagus by 7th week)
3. Mesentery of esophagus (dorsal mesentery) - contributes to the crura
4. Ingrowth of body wall muscle (myoblasts) - peripheral muscular rim; migrates with phrenic nerve (C3,4,5 - explains why phrenic nerve travels from neck)

• Failure of pleuroperitoneal membranes to fuse with septum transversum
• Most common: Left posterolateral (Bochdalek hernia) - 85-90% of CDH
• Abdominal contents herniate into thorax → hypoplastic lung (major cause of morbidity/mortality)
• Presents at birth with respiratory distress, barrel chest, scaphoid abdomen, bowel sounds in thorax

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Q11. Sketch the framework of thoracic cage. [5]

• Sternum (anteriorly): Manubrium (T3-T4) + Body (T5-T9) + Xiphisternum; joins at sternal angle (T4/5) - articulates with 2nd costal cartilage
• 12 pairs of ribs: True ribs (1-7, articulate directly with sternum), False ribs (8-10, articulate via shared costal cartilage), Floating ribs (11-12, no anterior attachment)
• 12 thoracic vertebrae (posteriorly)

• Sternal angle (T4/5) - 2nd rib level
• Jugular notch (T2) - articulates with clavicle
• Xiphisternal joint (T9)
• Infrasternal angle between costal margins
• Costal margin (7th-10th ribs anteriorly)
• Vertebrochondral ribs (8-10)
• Subcostal angle (normally ~70°)

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Q12. Describe the development of lung with its anomalies. [2]

Development:

• Day 22-26: Respiratory diverticulum (lung bud) from ventral wall of foregut endoderm (caudal to 4th pharyngeal pouch)
• Splanchnic mesoderm surrounds the bud and forms connective tissue, cartilage, smooth muscle, blood vessels
• The bud elongates → trachea; bifurcates → 2 bronchial buds → right (3) and left (2) secondary buds → repeated branching
• Development continues through 5 stages (embryonic → pseudoglandular → canalicular → saccular → alveolar) as described in Q5

Anomalies:

1. Tracheoesophageal Fistula (TEF): Most common type (type C): blind esophagus (esophageal atresia) + lower segment connected to trachea; presents with polyhydramnios, excessive secretions, aspiration; associated with VACTERL
2. Pulmonary Agenesis/Aplasia/Hypoplasia: Complete absence or incomplete development; associated with CDH, oligohydramnios
3. Pulmonary Sequestration: Mass of non-functioning lung tissue with systemic blood supply (not pulmonary artery); intralobar or extralobar; presents with recurrent pneumonia
4. Congenital Lobar Emphysema: Over-inflation of lobe (usually upper lobe); causes respiratory distress in neonates
5. Congenital Cystic Adenomatoid Malformation (CCAM/CPAM): Hamartomatous lung lesion; multiple cysts; may cause respiratory distress
6. Accessory lobes: Azygos lobe (most common - right upper lobe separated by azygos vein creating its own mesentery of pleura)

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Q13. Describe the histological features of lung with a well-labelled diagram. [3+2]

Histological Features [3]:

• Trachea → primary bronchi → lobar bronchi → segmental bronchi → bronchioles → terminal bronchioles
• Lined by pseudostratified ciliated columnar epithelium with goblet cells
• Wall contains: hyaline cartilage (C-rings in trachea; irregular plates in bronchi; absent in bronchioles), smooth muscle, seromucous glands

• Respiratory bronchioles: epithelium changes from ciliated columnar to cuboidal; walls interrupted by alveoli (first site of gas exchange); Clara cells (club cells) - non-ciliated, secretory

• Alveolar ducts → alveolar sacs → alveoli
• Lined by:
  • Type I pneumocytes (squamous alveolar cells): Cover 95% of alveolar surface; thin (0.2 µm); gas exchange
  • Type II pneumocytes (great alveolar cells): Cuboidal; 5% of surface area but 60% of cells; produce surfactant; progenitor cells for Type I
• Alveolar macrophages (dust cells): in alveolar spaces; phagocytose particles; contain carbon in urban lungs
• Blood-air barrier: Type I cell + basement membrane + capillary endothelium

Diagram Labels:

Alveolus - cross section:
[Type I pneumocyte | Alveolar macrophage | Type II pneumocyte | 
Capillary with RBCs | Elastic fibers | Interstitium |
Blood-air barrier (3 layers)]

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Q14. Describe briefly the parts of bronchial tree with their named histological structural changes. [3+2=5]

Parts of Bronchial Tree [3]:

1. Trachea (C6 to T4/T5): 16-20 C-shaped hyaline cartilage rings; trachealis muscle posteriorly; 10-12 cm long
2. Main (Primary) bronchi - Right (shorter, wider, more vertical - foreign bodies tend to go here) and Left
3. Lobar (Secondary) bronchi - Right: 3 (upper, middle, lower); Left: 2 (upper, lower)
4. Segmental (Tertiary) bronchi - 10 right, 8-10 left
5. Bronchioles (< 1mm diameter; no cartilage)
6. Terminal bronchioles - last purely conducting airways (generations ~16)
7. Respiratory bronchioles - occasional alveoli in walls (generations 17-19)
8. Alveolar ducts (generations 20-22)
9. Alveolar sacs (generation 23)
10. Alveoli - ~300 million in adult

Histological Structural Changes [2]:

• Epithelium: pseudostratified columnar → columnar → cuboidal → squamous
• Goblet cells: decrease and disappear at bronchioles
• Cartilage: C-rings → irregular plates → absent (at bronchioles)
• Smooth muscle: relative increase at bronchioles → disappears at alveoli
• Glands: present to level of bronchi → absent in bronchioles
• Clara cells appear in terminal/respiratory bronchioles

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