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The lungs occupy the major part of the thoracic cavity, leaving only a small space for the heart.
The left lung is slightly smaller because the heart excavates more space on that side, forming the cardiac notch.
Together, both lungs enclose the heart, offering protection and cushioning.
Each lung is a spongy, elastic organ that functions as the primary organ of respiration.
The right lung is typically heavier (≈700 g) and has three lobes, while the left lung is lighter (≈600 g) and has two lobes.
In the young, lungs are brownish-grey, but with age, they turn mottled black due to deposition of carbon particles from polluted air.
Each lung is enclosed by its own pleural sac and separated from the other by the mediastinum.
There are ten bronchopulmonary segments in each lung, each functioning as an independent respiratory unit
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.Identification of lungs:
Each lung is conical, with:
A blunt apex projecting into the neck above the first rib.
A concave base resting on the diaphragm.
Three borders — anterior (thin), posterior (thick), and inferior.
Two surfaces — costal and medial (facing the ribs and mediastinum respectively).
The right lung has three lobes (upper, middle, lower), separated by two fissures.
The left lung has two lobes (upper and lower) separated by one fissure
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.Right lung dissection highlights:
Two bronchi at the hilum — eparterial (upper) and hyparterial (lower).
Pulmonary artery lies between the two bronchi.
Pulmonary veins lie anterior and below the bronchi.
Impressions on the medial surface:
Anterior to root → Superior vena cava, inferior vena cava, right atrium, right ventricle.
Posterior to root → Azygos vein and oesophagus.
Left lung dissection highlights:
Single bronchus located posteriorly.
Pulmonary artery lies above the bronchus.
Pulmonary veins lie anterior and below it.
Impressions:
Heart (left ventricle), ascending aorta, arch of aorta, and descending thoracic aorta.
Oesophageal groove in the lower mediastinal part
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.In summary, both lungs are elastic, lobulated, conical organs vital for gaseous exchange. The right lung is larger and divided into three lobes, while the left accommodates the heart, showing a cardiac notch and lingula. The dissection reveals distinct bronchial, vascular, and surface impressions, important for understanding their anatomical relations and clinical correlations.
Each lung is divided into lobes by fissures, which represent deep invaginations of visceral pleura.
These fissures separate the lungs into independent lobes, each ventilated by a secondary (lobar) bronchus.
The lobes function semi-independently — an important feature in localized pathology (e.g. lobar pneumonia, segmental resection).
The right lung is larger, shorter, and wider than the left.
It has three lobes:
Upper (Superior) lobe
Middle lobe
Lower (Inferior) lobe
These lobes are separated by two fissures:
Oblique fissure
Horizontal fissure
Begins posteriorly about 2.5 cm lateral to the spine at the level of the T4 (4th thoracic vertebra).
It runs downward and forward, crossing the 5th intercostal space and following the 6th rib anteriorly to end near the 6th costochondral junction.
Functionally, it separates the lower lobe from the upper and middle lobes.
Extends from the oblique fissure along the 4th costal cartilage to meet the anterior border of the lung.
It separates the upper lobe from the middle lobe.
The fissure corresponds to the 4th intercostal space anteriorly.
Upper lobe: above horizontal fissure.
Middle lobe: between horizontal and oblique fissures.
Lower lobe: below oblique fissure.
The left lung is smaller and narrower due to the cardiac impression and notch.
It has two lobes:
Upper (Superior) lobe
Lower (Inferior) lobe
These are separated by a single oblique fissure similar to that on the right side.
Begins posteriorly at the T4 level, about 2.5 cm lateral to the midline.
Runs downward and forward, crossing the 5th intercostal space and ending near the 6th costal cartilage anteriorly.
It separates the upper lobe from the lower lobe.
The anterior border of the left lung shows a deep cardiac notch below the 4th costal cartilage, accommodating the heart.
Below the notch lies a tongue-like projection of the upper lobe known as the lingula, which corresponds to the middle lobe of the right lung both anatomically and functionally.
Each lobe acts as a distinct respiratory unit, ventilated by its own lobar bronchus and pulmonary artery branch.
The arrangement allows segmental resection in localized disease without impairing overall lung function.
The fissures facilitate lung expansion during inspiration but may also serve as planes of spread for infection or effusion.
Incomplete fissures are common, leading to fusion between lobes, which can alter the spread of infection.
Accessory fissures may occur, such as:
Superior accessory fissure (separating superior segment of lower lobe).
Inferior accessory fissure (isolating medial basal segment).
On chest X-ray, fissures appear as thin linear shadows; knowledge of their normal position helps identify lobar collapse, effusion, and tumours.
| Lung | Lobes | Fissures | Remarks |
|---|---|---|---|
| Right Lung | 3 (Upper, Middle, Lower) | Oblique, Horizontal | Larger, shorter, wider |
| Left Lung | 2 (Upper, Lower) | Oblique | Has cardiac notch and lingula |
The fissures and lobes are not only anatomical boundaries but also surgical and radiological landmarks — understanding them is essential for interpreting lung imaging, resections, and pathology localization.
The root of the lung is a short, broad pedicle connecting the medial surface of the lung to the mediastinum.
It contains all the structures entering or leaving the lung at the hilum (the depression on the medial surface).
It lies opposite the bodies of the 5th to 7th thoracic vertebrae
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.Contents of the Root:
Bronchus:
Right side — two bronchi (eparterial and hyparterial)
Left side — one principal bronchus
Pulmonary artery — middle part of the root
Two pulmonary veins — superior and inferior, in the anterior part
Bronchial arteries — one on right, two on left
Bronchial veins
Lymphatics of the lung
Bronchopulmonary lymph nodes
Anterior and posterior pulmonary nerve plexuses
Areolar tissue
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Vertebral Level:
Roots lie opposite T5–T7 vertebrae.
Relations at the Hilum:
Right lung: impressions of SVC, IVC, right atrium, azygos vein, oesophagus
Left lung: impressions of left ventricle, arch of aorta, descending thoracic aorta, thoracic duct, oesophagus
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Vascular Supply:
Right lung: one bronchial artery (from 3rd posterior intercostal artery).
Left lung: two bronchial arteries (from descending thoracic aorta).
Deoxygenated blood enters via pulmonary arteries; oxygenated blood exits via pulmonary veins
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.Bronchial and pulmonary arteries anastomose precapillarily; the anastomosis enlarges in disease states
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.The trachea divides at the lower border of the T4 vertebra into two primary (principal) bronchi.
Right principal bronchus:
2.5 cm long, shorter, wider, and more vertical (angle 25°).
Hence, foreign bodies tend to lodge here.
Left principal bronchus:
5 cm long, narrower, more oblique (angle 45°)
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.Branching Pattern:
Each principal bronchus → Lobar bronchi (secondary) → one for each lobe.
Right lung → 3 lobar bronchi.
Left lung → 2 lobar bronchi.
Lobar bronchi → Segmental bronchi (tertiary) → one for each bronchopulmonary segment (10 on each side).
Segmental bronchi → Terminal bronchioles → Respiratory bronchioles → Alveolar ducts → Atria → Air saccules → Alveoli
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.Functional Unit:
Each respiratory bronchiole aerates a pulmonary unit, the smallest functional area for gaseous exchange.
Identify lungs by thin anterior border, thick posterior border, apex, and concave base.
Right lung → 3 lobes (upper, middle, lower).
Left lung → 2 lobes (upper, lower).
At the hilum of the right lung:
Two bronchi — eparterial (upper) and hyparterial (lower).
Pulmonary artery — between bronchi.
Pulmonary veins — anterior and inferior.
At the hilum of the left lung:
One bronchus (posterior).
Pulmonary artery — above bronchus.
Pulmonary veins — anterior and inferior
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.Impressions on Medial Surface:
Right lung: SVC, IVC, right atrium, azygos vein, oesophagus.
Left lung: Left ventricle, aortic arch, descending aorta, thoracic duct, oesophagus
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.| Structure | Right Lung | Left Lung |
|---|---|---|
| Bronchus | Two (eparterial & hyparterial) | One principal |
| Pulmonary Artery | Between bronchi | Above bronchus |
| Pulmonary Veins | Anterior & below bronchi | Anterior & below bronchus |
| Bronchial Arteries | One (from 3rd right posterior intercostal) | Two (from thoracic aorta) |
| Impressions | SVC, azygos, right atrium | Aortic arch, left ventricle |
| Vertebral Level | T5–T7 | T5–T7 |
Together, the root of the lung, bronchial tree, and hilum anatomy form the basis for understanding bronchopulmonary segments, pulmonary surgeries, and radiological orientation.
The respiratory system begins to develop in the 3rd week of intrauterine life as an outgrowth (respiratory diverticulum) from the ventral wall of the foregut (primitive pharynx caudal to the hypobranchial eminence).
The epithelium of the larynx, trachea, bronchi, and alveoli is derived from the endoderm of the foregut, while the cartilage, muscle, and connective tissue arise from splanchnic mesoderm surrounding the foregut
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.The respiratory diverticulum separates from the foregut by the tracheo-oesophageal septum, except at the opening of the larynx.
Sequence of Lung Development:
The respiratory diverticulum grows caudally to form the trachea, which bifurcates into two lateral lung buds.
During the 5th week, the proximal parts of each lung bud form the principal bronchi.
The bronchi invaginate into the pericardioperitoneal canals (primitive pleural cavities).
The right bronchus divides into three secondary bronchi and the left into two — corresponding to the lobes of each lung.
These bronchi branch repeatedly to form tertiary bronchi, each with surrounding mesenchyme forming a bronchopulmonary segment.
By the 24th week, there are around 17 generations of branches, and the parenchyma develops through four overlapping stages
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.Pseudoglandular Stage (5–17 weeks):
Lung resembles a gland; formation of bronchi and terminal bronchioles.
Canalicular Stage (16–25 weeks):
Enlargement of bronchi and bronchioles; vascularization increases.
Terminal Sac Stage (24 weeks to birth):
Terminal sacs (primitive alveoli) form; capillaries approach epithelium for gas exchange.
Alveolar Stage (late fetal period to ~8 years postnatal):
Alveoli mature and multiply; alveolocapillary membrane allows efficient gas exchange.
Cranial segments mature earlier than caudal ones.
By 28–32 weeks, type II alveolar epithelial cells begin secreting pulmonary surfactant, which reduces surface tension and maintains alveolar patency
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.
TBX4 transcription factor (in the endoderm of the gut tube) initiates lung bud formation and regulates growth and differentiation of the lungs.
FGF10 (Fibroblast Growth Factor 10) from the splanchnic mesenchyme induces outgrowth of the tracheal bud.
Sonic Hedgehog (SHH-Gli) signaling coordinates epithelial-mesenchymal interactions, controlling branching and proliferation of the tracheobronchial tree
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.The lung is a lacework of alveoli separated by thin-walled septa, traversed by bronchi, bronchioles, and alveolar ducts.
The visceral pleura may be seen as a mesothelial covering.
1. Intrapulmonary Bronchus:
Epithelium: Pseudostratified ciliated columnar with goblet cells.
Lamina propria: Reticular and elastic fibers.
Submucosa: Contains serous and mucous glands.
Smooth muscle layer: Circular; responsible for mucosal infoldings.
Cartilage: Irregular plates of hyaline cartilage.
2. Terminal Bronchiole:
Diameter <1 mm.
Epithelium: Simple columnar.
Lamina propria: Elastic and smooth muscle fibers.
No glands or cartilage present
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.3. Alveoli:
Lined by Type I pneumocytes (flat, squamous) and Type II pneumocytes (large, secretory).
Type II cells produce surfactant, preventing alveolar collapse.
The interalveolar septum contains capillaries lined by non-fenestrated endothelial cells, facilitating gas exchange
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.| Structure | Developed From |
|---|---|
| Epithelium of larynx, trachea, bronchi, alveoli | Endoderm of foregut |
| Muscles of larynx | Mesoderm of 4th and 6th pharyngeal arches |
| Cartilages of larynx | 4th & 6th arch cartilages |
| Epiglottis | Dorsal part of hypobranchial eminence |
| Glands of respiratory tract | Endoderm |
| Connective tissue, smooth muscle, cartilage of trachea & bronchi | Splanchnic mesoderm
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Tracheo-oesophageal fistula: Failure of septum formation between trachea and oesophagus.
Tracheal stenosis: Abnormal narrowing of the trachea.
Azygos lobe: Accessory lobe formed by an aberrant azygos vein.
Hyaline membrane disease (neonatal respiratory distress): Caused by deficiency of surfactant.
Agenesis of lung: Failure of lung bud development
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