External Features of lungs

Introduction

• Lungs or pulmones are the vital organs of respiration.
• Lungs (right and left) are situated in the thoracic cavity.
• Each lung is covered by the pleural sac.
• The space between the right and left pleural sacs is called mediastinum, which is mainly occupied by the heart, pericardium, major vessels, trachea, and oesophagus.
• The main function of the lung is oxygenation of blood (gaseous exchange).

Features of Lungs

• There are two lungs: Right and left.
• Shape: Pyramidal (conical)
• Weight: Right lung (~700 g) is slightly heavier than the left lung (~650 g).
• Color: Normally, lungs are rosy pink. On exposureto pollution and smoke, lungs become brown to black.
• Texture: Lungs are spongy because of the presence of air in the lung alveoli.

External Features of Lung

Morphologically, each lung has the following features:

1. Apex (upper end)
2. Base (lower concave surface)
3. Three borders: Anterior, posterior, and inferior
4. Two surfaces: Costal and mediastinal
5. Fissures: Horizontal and oblique on the right and only oblique on the left.
6. Lobes: On the right: Superior, middle, and inferior, and on the left: Superior and inferior.

Anatomical Position and Side Determination

• Hold the lung in such a way so that Practical guide
• Its pointed apex is directed upward and broad base is directed downward.
• Its thin anterior border should face anteriorly, and thick rounded posterior border should face posteriorly.
• Its convex costal surface is directed laterally (toward the side of the lung – for right lung on the right side and for left lung on the left side) and mediastinal surface with hilum is directed medially.
• As number of lobes and fissures are not constant, do not determine the side of the lung using lobes and fissures.

Apex

• Apex is rounded (blunt) upper part of the lung.
• Apex is directed toward the root of the neck.
• Apex lies 2.5 cm above the medial end of clavicle and 3–4 cm above the first costal cartilage.
• Apex is covered by cervical pleura and suprapleural membrane.

Relations

Anterior: Subclavian artery, scalenus anterior muscle. Superolateral part of apex is grooved by the subclavian artery.

Posterior: Stellate (cervicothoracic) sympathetic ganglion, ventral ramus of the first thoracic nerve, superior thoracic artery, neck of first rib.

Superior: Cervical pleura, suprapleural membrane.

Base

• Base is concave semilunar surface of the lungs.
• It is also called diaphragmatic surface of lung, as it rests on the diaphragm

Relations

• Base is separated from diaphragm by diaphragmatic pleura and costodiaphragmatic recesses.
• Inferiorly diaphragm separates
  • Right lung from liver
  • Left lung from fundus of stomach and spleen
• Base of the right lung is more concave (deeper) because of higher position of the right dome of diaphragm and liver.

Borders

• Lung has three borders: Anterior, posterior, and inferior.

Anterior border

• It is thinner and shorter than the posterior border.

Identification feature

• Anterior border of right lung is vertical.
• Anterior border of left lung has a cardiac notch.

Posterior border

• It is thick, rounded, and ill-defined.

Inferior border

• It is sharp, semilunar. It separates the base from the costal and mediastinal surfaces.

Surfaces of lungs

• Lung has two surfaces: Costal and medial.

Costal surface

• It is large, convex, and smooth.
• It is separated from the thoracic wall by costal pleura and endothoracic fascia.
• It shows impressions (grooves) of ribs.

Medial surface

• It is divided in to two parts: Large anterior mediastinal part and small posterior vertebral part.
• Mediastinal part
• Mediastinal surface is concave and related to mediastinal structures.
• It shows hilum of the lung.

Vertebral part

• Vertebral part is related to the bodies of thoracic vertebrae, intervertebral discs, posterior intercostal vessels, and splanchnic nerves.

Relations of Mediastinal Surface of Lungs

• Mediastinal surfaces of both the lungs are covered by mediastinal pleura.
• Mediastinal surface has hilum of lung, through which vessels, nerves and principal bronchus enter/exit the lung.
• Relations of mediastinal surface are listed in Table 8.1.

Lobes and Fissures

• Right lung is divided into three lobes: Superior, middle, and inferior by two fissures: Oblique and horizontal fissure (Flowchart 8.2).
• Left lung is divided into two lobes: Superior and inferior by an oblique fissures.

Table 8.1: Relations of mediastinal surfaces of right and left lungs

Mediastinal surface of right lung	Mediastinal surface of left lung
1. Cardiac impression:	1. Cardiac impression:
• Right atrium and auricle	• Left ventricle
• Small part of right ventricle	• Left auricle
	• Infundibular surface, and adjoining part of right ventricle
2. Vessels	2. Vessel
• Superior vena cava	• Ascending aorta
• Inferior vena cava	• Arch and descending aorta
• Azygos vein aorta	• Pulmonary trunk
• Right brachiocephalic vein	• Left subclavian artery
	• Left common carotid artery
3. Nerves	3. Nerves
• Right phrenic nerve	• Left phrenic nerve
• Right vagus nerve	• Left vagus nerve
• Right sympathetic chain	• Left sympathetic chain
	• Left recurrent laryngeal nerve
4. Viscera	4. Viscera
• Esophagus	• Esophagus
• Trachea	• Thoracic duct

Oblique fissure (major)

• It is the deep fissure that separates superior and middle lobes from inferior lobe.
• It passes obliquely downward and forward.
• It begins on mediastinal surface above and behind the hilum.
• Due to the course of oblique fissure, the inferior lobe of lung lies posteroinferior to the superior and middle lobe.
• It crosses posterior border 6 cm below the apex and inferior border anteriorly about 7.5 cm lateral to midline.
• Oblique fissure of the left lung is more vertical.

Horizontal fissure (minor)

• It is present only in right lung.
• It is a short fissure that separates middle lobe from superior lobe.
• It runs horizontally from oblique fissure at midaxillary line up to the anterior border of lung (at 4^th costal cartilage).

Lingula

• Left lung shows tongue-shaped projection below the cardiac notch called lingula.
• It corresponds to the middle lobe of the right lung.

Root of the Lung

• Root or pedicle of the lung connects the medial surface of lung to the mediastinum.
• Root of the lung is a wedge-shaped passage that permits the structures entering or leaving the lung.
• Root lies opposite to the bodies of 5th, 6th, and 7^th thoracic vertebrae.

VAB: vein, artery, bronchus.

Contents

• The root of the lung has the following structures:
  1. Bronchus: Principal bronchus on left, eparterial and hyparterial bronchi on right
  2. Pulmonary artery
  3. Two pulmonary veins
  4. Bronchial arteries: One on right, two on left
  5. Bronchial veins
  6. Lymphatics of lung and bronchopulmonary lymph nodes
  7. Pulmonary plexuses of nerves (anterior and posterior).
• Pleural sleeve (mesopneumonium): It is a sleeve-like covering of pleura around the hilum of the lung. It is the site of continuation of mediastinal parietal pleura with visceral pleura. Inferiorly pleural sleeve continues as pulmonary ligament.

Arrangement of Structure in the Root

• The structures at the root of the lung can be studied as follows:
  1. From anterior to posterior (similar on both sides):

VAB

1. Superior pulmonary vein
2. Pulmonary artery
3. Bronchus (principal bronchus on left eparterial and hyparterial bronchi on right).
   1. From above downward (different on two sides)

Right side	Left side
Eparterial bronchus	—
Pulmonary artery	Pulmonary artery
Hyparterial bronchus	Left principal bronchus
Inferior pulmonary vein	Inferior pulmonary vein

Note: In the Figure 8.10, the pulmonary veins are shown in red as they carry oxygenated blood, whereas pulmonary arteries in blue as they carry deoxygenated blood.

Practical guide: How to differentiate pulmonary artery, pulmonary vein, and bronchus in a cadaveric lung?

Bronchus: Hard due to cartilage, circular lumen, do not have clotted blood. Pulmonary artery: Whitish, circular structure, wall is thicker than pulmonary vein.

Pulmonary vein: Collapsed lumen, mostly shows clotted blood and have thinner wall than pulmonary artery.

Table 8.2: Differences between right and left lungs

	Right lung	Left lung
Size and shape	It is larger, shorter, and broader than the left lung	It is smaller, longer, and narrower than the right lung
Weight	It is heavier (~700 g)	It is lighter (~650 g)
Fissures	It has 2 fissures: Oblique, horizontal	It has 1 fissure: Oblique
Lobes	It has 3 lobes: Superior, middle, and inferior	It has 2 lobes: Superior, inferior
Anterior border	It is straight	It has cardiac notch
Hilum	It has 2 bronchi: Eparterial, hyparterial	It has 1 bronchus: Left principal bronchus
Lingula	It does not have lingula	It has lingula
Cardiac impression	Shallow	Deep
Bronchial artery	One	Two

Relations of Root of Lung

Anterior: Phrenic nerve, anterior pulmonary plexus, superior vena cava (on right only)

Posterior: Vagus nerve, posterior pulmonary plexus, descending thoracic aorta only on the left

Superior: On right – arch of azygos vein

On left – arch of aorta

Inferior: Pulmonary ligament.

The differences between the right and left lungs are listed in Table 8.2.

Surface Marking of Lung

Apex

• Apex of lung coincide with cervical pleura.
• It can be represented by convex line joining the following points:

A: Junction of middle and medial one-third of clavicle

B: Point 2.5 cm above medial end of clavicle

C: Midpoint of sternoclavicular joint

Borders (Margins)

Anterior border

• On right: It runs vertically downward and marked by joining the following points:

C: Midpoint of sternoclavicular joint

D: Median plane of sternal angle

E: Median point of xiphisternal joint.

• On left: It has cardiac notch. It is marked by joining

C: Midpoint of sternoclavicular joint

D: Median plane of sternal angle

D1: Median plane at the level of 4th costal cartilage and then forms convex lateral notch.

E-1: At 6th costal cartilage (4 cm from median plane)

Lower border

• It crosses

F: 6th rib in midclavicular line

G: 8th rib in midaxillary line

H: 10th rib in paravertebral line (2 cm lateral to 10^th thoracic spine).

Posterior border

• It extends from

H: 10th rib in paravertebral line to

I: 7th cervical spine level in paravertebral line.

Fissures

Oblique fissure

• It can be marked joining

J: Point 2 cm lateral to third thoracic spine

K: Point on 5th rib in midaxillary line

L: 6th costal cartilage 7.5 cm lateral to median plane.

Horizontal fissure

• It is present only in the right lung.
• It can be marked by a horizontal line along the 4^th costal cartilage. This line extends from anterior border (4th costal cartilage level) to the oblique fissure (at 5th rib in midaxillary line).

Percussion is a method of clinical examination that involves tapping with fingers and hands. It gives resonating sound over air containing structures (lungs) and dull note over solid structures (liver).

Blood Supply of Lungs

Arterial Supply

• Each lung is supplied by two arteries: Bronchial and pulmonary arteries.

Bronchial arteries (right and left)

• Bronchial arteries supply oxygenated blood and nutrition to the lungs.

• Right bronchial artery: There is only one right bronchial artery. It is the branch of right third posterior intercostal artery (sometimes branch of upper left bronchial artery)
• Left bronchial arteries: There are two left bronchial arteries. They are branches of descending thoracic aorta
• Bronchial arteries supply respiratory bronchioles

Pulmonary arteries (right and left)

• Each lung receives deoxygenated blood from the right ventricle via one pulmonary artery.
• They are the branches of pulmonary trunk.
• Pulmonary artery divides in lobar and then segmental branches.
• Each bronchopulmonary segment is supplied by a separate branch of pulmonary artery.

Venous Drainage

• Blood from the lungs is drained by two sets of veins: Bronchial vein and pulmonary vein.

Bronchial veins (right and left)

• There are two bronchial veins for each lung. These veins carry deoxygenated blood.
• Right bronchial veins drain into azygos vein, whereas left bronchial veins drain into accessory hemiazygos vein or left superior intercostal vein.

Pulmonary veins (superior and inferior)

• Oxygenated blood from each lung is drained by two pulmonary veins (superior and inferior) that open into the left atrium.
• Tributaries of pulmonary veins are intersegmental. (They drain adjacent bronchopulmonary segments of lungs.)

LYMPHATIC DRAINAGE

• Lymphatics from lungs is drained by two sets of lymph vessels:
• Superficial vessels
• Deep vessels
• Superficial vessels (subpleural plexus)
  • These vessels drain a small peripheral part of lung that lies just beneath the visceral pleura.
  • These vessels form the subpleural plexus and carry lymph to the bronchopulmonary nodes.
• Deep lymph vessels
  • These vessels drain the deeper part of the lungs.
  • These vessels drain the lymph to pulmonary lymph nodes (located within the substance of lungs) and finally to bronchopulmonary lymph nodes.
  • Bronchopulmonary lymph nodes drain into superior and inferior tracheobronchial lymph nodes and further into paratracheal and pretracheal lymph nodes.

Note: The alveoli do not have lymph vessels.

Nerve Supply

• Lungs are supplied by autonomic nerves through anterior and posterior pulmonary plexuses

• These plexuses are formed by parasympathetic and sympathetic fibers as follows:

Parasympathetic fibers

• These are derived from vagus nerve.
• Parasympathetic fibers cause bronchoconstriction (contraction of bronchial muscles), increased mucus secretion in bronchi, secretomotor to mucous glands, carry sensation from the lung, and vasodilatation.

Sympathetic fibers

• Sympathetic fibers produce bronchodilatation (inhibitory for bronchial smooth muscles), decreased mucus secretion, and vasoconstriction.
• Sympathetic fibers arise from T2 to T5 segments of spinal cord.

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