Lung Anatomy_ Overview, Gross Anatomy, Microscopic Anatomy

4/29/2016 Lung Anatomy: Overview, Gross Anatomy, Microscopic Anatomy

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Lung Anatomy

Author: Eduardo A Celis, MD; Chief Editor: Zab Mosenifar, MD, FACP, FCCP more...

Updated: Feb 20, 2013

Overview

The anatomy of the respiratory system can be divided into 2 major parts, airway

anatomy and lung anatomy.

Airway anatomy can be further subdivided into the following 2 segments:

The extrathoracic (superior) airway, which includes the supraglottic, glottic,

and infraglottic regions

The intrathoracic (inferior) airway, which includes the trachea, the mainstem

bronchi, and multiple bronchial generations (which have as their main

function the conduction of air to the alveolar surface)

Lung anatomy includes the lung parenchyma, which carries part of the conduction

system but is mainly involved in the gas exchange at the alveolar level. The lung

parenchyma is further subdivided into lobes and segments.

The purpose of this chapter is to provide a better understanding of the anatomy of

the airways and lungs, which will help the health provider to recognize and manage

different respiratory abnormalities.

Gross Anatomy

Trachea

The trachea is a cartilaginous and fibromuscular tube that extends from the inferior

aspect of the cricoid cartilage (sixth cervical vertebra level) to the main carina (fifth

thoracic vertebra level). Its length is 3 cm at birth and 10-12 cm in adults (of which

2-4 cm is extrathoracic and 6-9 cm intrathoracic). Tracheal diameters vary widely,

ranging from 13 to 25 mm (coronal plane) in men. In women, the variability is still

noted, with a range of 10-21 mm (coronal plane). The shape of the intrathoracic

trachea changes during expiration as a result of invagination of the posterior wall,

causing as much as a 30% reduction of the anteroposterior diameter as seen on

dynamic computed tomography (CT) scanning (see the images below).[1]

Dynamic CT scan of chest during inspiration in normal patient.

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Dynamic CT scan of chest during expiration in normal patient. See how anteroposterior diameter ofthe trachea decreases because of collapse of posterior w all.

The tracheal wall has 4 different layers: mucosa, submucosa, cartilage or muscle,

and adventitia. The posterior tracheal wall lacks cartilage and instead is supported

by a thin band of smooth muscle.

Bronchi

The airways divide by dichotomous branching, with approximately 23 generations

of branches from the trachea to the alveoli (see the images below).

Bronchial tree w ith nomenclature.

CT scan of chest (coronal view ). Trachea, main carina, and right mainstem bronchus w ith upper,middle, and low er lobe airw ays can be seen. Left mainstem bronchus is also seen w ith upper lobeairw ay. Left low er lobe airw ay cannot be seen.

Bronchi are composed of cartilaginous and fibromuscular elements; however, the

distinction between these elements is less clear-cut in the bronchi than in the

trachea, especially on the more distal airways. The wall thickness is

approximately proportional to the airway diameter on airways distal to the

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segmental branches. For airways less than 5 mm in diameter, the wall shouldmeasure 1/6 to 1/10 of the diameter.

Different systems of nomenclature have been applied to the bronchial tree over the

years.[2] In general usage, there are 2 mainstem bronchi (right and left) and 3 lobar

bronchi (right), with a total of 10 segmental bronchi; 2 lobar bronchi are found on

the left, with a total of 8 segmental bronchi. No accepted terminology for

subsegmental bronchi exists. The terminal bronchioles, including respiratory

bronchioles, alveolar ducts, and alveolar sacs, are discussed elsewhere (see

Microscopic Anatomy section). Generally, the length and diameter of the central

airways vary from right to left.

The vascular supply of the trachea and bronchial tree depends on branches from

the inferior thyroid arteries, intercostal arteries, and bronchial arteries (aortic

branches). These arteries (except the thyroid artery) form a peribronchial plexus

that follows the bronchial tree deep into the lung parenchyma to supply blood also

to the visceral pleura and the walls of the pulmonary arteries and veins (vasa

vasorum).

Lungs

Some symmetry exists between the right and the left lungs. Both lungs are divided

into lobes (see the image below). The gross functional subunits of each lung are

called segments and have a close relation with the segmental bronchi described

above. The right lung comprises 10 segments: 3 in the right upper lobe (apical,

anterior and medial), 2 in the right middle lobe (medial and lateral), and 5 in the

right lower lobe (superior, medial, anterior, lateral, and posterior). The left lung

comprises 8 segments: 4 in the left upper lobe (apicoposterior, anterior, superior

lingula, and inferior lingula) and 4 in the left lower lobe (superior, anteromedial,

lateral, and posterior).

Lung anatomy: lobes and segments.

The lungs are covered by the visceral pleura, which is contiguous with the parietal

pleura as it reflects from the lateral surfaces of the mediastinum. The visceral

pleura forms invaginations into both lungs, which are called fissures. There are 2

complete fissures in the right lung and 1 complete fissure with an incomplete

fissure in the left (see the image below); these separate the different lung lobes.

The pleura also forms the pulmonary ligament, which is a double layer of pleura

that extends caudad along the mediastinum from the inferior pulmonary vein to the

diaphragm.

CT scan of chest (coronal view ). Blue arrow points at minor fissure in right lung. Red arrow sshow both major fissures.

Pulmonary vasculature

A close relation exists between the bronchial tree and the anatomy of the

4/29/2016 Lung Anatomy: Overview, Gross Anatomy, Microscopic Anatomy

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pulmonary vasculature, composed mainly of the pulmonary arteries and veins (see

the image below). The main pulmonary artery originates in the right ventricle and

divides into 2 branches. The right pulmonary artery passes posterior to the aorta

and the superior vena cava, emerging lateral to the atria and anterior and slightly

inferior to the right mainstem bronchus. In contrast, the origin of the left pulmonary

artery is situated anterior to the left mainstem bronchus. The arborization of the

pulmonary arteries varies from right to left but mainly divides into truncal, lobar,

segmental, and subsegmental arteries, which generally follow the branches of thebronchial tree.

Pulmonary artery and vein in relation to airw ays and lungs.

The pulmonary veins originate in the alveoli and also receive drainage from the

bronchial and pleural branches. After the confluence of the small branches into

bigger ones, 2 pulmonary veins, superior and inferior, are formed on each side.

These 4 veins typically join at or near their junction with the left atrium, and usually

this common area is intrapericardial.

Pulmonary lymphatic system

The lymphatic drainage of the lungs start with lymphatic vessels that first drain into

intraparenchymal lymphatics and lymph nodes, then move to peribronchial (hilar)

lymph nodes, and subsequently move to subcarinal, tracheobronchial, and

paratracheal lymph nodes (see the images below). The lymphatics eventually

communicate with the venous system via the bronchomediastinal lymphatic trunk

and the thoracic duct or via the inferior deep cervical (scalene) lymph nodes.

However, some variants of the lymphatic drainage are very important to consider

overall in the dissemination of pulmonary neoplasms (see Pathophysiologic

Variants).

Mediastinal and hilar lymph nodes. Terms reflect nomenclature used for staging of lung cancer.

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CT scan of chest (coronal view ) show ing different mediastinal and hilar lymph nodes. (Red arrow :station 4 left; green arrow : station 7; yellow arrow : station 11 right.) Terms based on lung cancernomenclature.

Microscopic Anatomy

The trachea has multiple layers (see the image below). The mucosa is composed

of a ciliated pseudostratified columnar epithelium and numerous mucus-secreting

goblet cells that rest on a basement membrane with a thin lamina propria (mainly

collagenous). The submucosa contains seromucous glands. The adventitia

contains cartilaginous rings interconnected by connective tissue. The hyaline

cartilage rings have the form of the letter C and are opened posteriorly. The open

ends are connected by fibroelastic tissue and a band of smooth muscle (the

trachealis).

Microscopic picture of trachea show ing different layers: mucosa, submucosa, cartilage.

The epithelium of the bronchus is pseudostratified columnar ciliated epithelium,

also with numerous goblet cells. This epithelium transitions first into a simple

columnar ciliated epithelium and then into a cuboidal epithelium as it continues

branching into smaller bronchioles. The cartilage support is eventually lost at the

bronchiolar level (0.5-1.0 mm diameter). The muscle layer becomes the dominant

structure and is composed of smooth muscle and elastic fibers (see the image

below). At this level, the mucosa may be highly folded because of the loss of

supporting structure.

Microscopic picture of bronchus (hematoxylin and eosin stain). Note mucosal layer w ith multiplegoblet cells. Smooth muscle layer is in periphery. Image courtesy of Dr. Chad Stone.

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The terminal bronchioles are considered the respiratory zone of the lungs (ie, the

area where gas exchange occurs). They divide into respiratory bronchioles, which

continue downstream as alveolar ducts that are completely lined with alveoli and

alveolar sacs (see the image below). Over 300 million alveoli exist in the human

lung, all of them covered by an extensive network of capillaries (branches from the

pulmonary arteries). The respiratory zone constitutes most of the lung (2.5-3 L).

Microscopic picture (hematoxylin and eosin stain) of alveolar sacs and alveoli. Image courtesy ofDr. Chad Stone.

The epithelium of the respiratory bronchiole is primary cuboidal and may be

ciliated; goblet cells are absent. The supporting thin layer is formed by collagenous

and smooth muscle. Alveoli appear as small pockets that interrupt the main wall.

The terminal portion of the respiratory duct gives rise to the alveolar sacs

(composed of a variable number of alveoli). The alveoli are the smallest and most

numerous subdivisions of the respiratory system. The interalveolar septum often

contains 10-15 μ m openings between neighboring alveoli that help equalize air

pressures among them.

The alveolar wall is very thin (25 nm) and formed by squamous epithelium (type I

cells) covered by a thin film of surfactant fluid rich in hydrophilic phospholipid

produced by type II cells (septal cells). This surfactant fluid keeps the alveoli open

by reducing the surface tension of the interface between opposing alveolar

surfaces, which reflects into reduced inspiratory work.

The respiratory epithelium is composed mainly of type I cells (98%), along with

some type II cells. The basal lamina is in intimate contact with the capillaries from

the pulmonary vascular system, favoring the transfer of oxygen to the red blood

cells and the release and transfer of carbon dioxide to the alveolar airway.

Natural Variants

As a human being develops from a fetus to a fully developed adult, several changes

take place. Some of these changes follow regular patterns, and others either

compensate for certain conditions or occur for unknown reasons.

Congenital anatomic variants of the lungs are present in the following forms:

Agenesis - A congenital complete absence of one or both lungs, the latter

being incompatible with life; the condition is associated with other

congenital abnormalities and is rare

Aplasia or hypoplasia - The presence of a rudimentary bronchus that ends

in a blind pouch with no evidence of pulmonary vasculature or lung

parenchyma

Accessory lobes, and fusion of lobes - Variations over the lung lobes that

are mainly caused by the incomplete obliteration of the visceral pleural

folds, result from the presence of abnormal vessels (creating extra lobes), or

occur secondary to (completely or partially) fused lobes from obliteration of

the normal lung fissures (see the images below) [3]

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CT scan of chest in patient w ith congenital third fissure in right lung. Abnormal fissure isseen below major fissure in right.

Azygous lobe. This lobe w as created by azygous vein as it descended into thorax duringembryonic development.

Congenital anatomic variants of the airway are present in the following forms:

Bronchial variations - Variations in the patterns of the bronchial tree are

predominantly due to displacement of segmental and subsegmental bronchi

(reduction migration and selection theories) [4] ; anatomic abnormalities of

the bronchi may be the favored locales for deformities, chronic

inflammations (see the image below), and bronchial neoplasms

CT scan of chest in patient w ith acquired right middle lobe bronchiectasis due to chronicnontuberculosis mycobacterial infection.

Congenital anatomic variants of the diaphragm are present in the following forms:

Normal variations in the diaphragm are mainly consistent with different sites

of insertion of the muscle that form the diaphragm, or due to congenital

defect leading to communication from the abdominal to the chest cavity (eg,

Bochdalek hernia, Morgani hernia, diaphragmatic eventration) (see the

image below)

4/29/2016 Lung Anatomy: Overview, Gross Anatomy, Microscopic Anatomy

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Congenital diaphragmatic hernia, w ith displacement of liver and intestines into chest cavity.

Pathophysiologic Variants

Pathologic variants are related to changes in the structure of the airways, the lung

parenchyma, or adjacent structures that lead to disruption of the normal anatomy

of the respiratory system. The most common such variants are as follows.

Emphysema/chronic obstructive pulmonary disease

Emphysema and chronic obstructive pulmonary disease are caused by an

accumulation of inflammatory mucus that gives rise to a loss of elastic recoil

resulting from lung tissue destruction or to an increase in the resistance of the

conducting airways leading to an abnormal permanent enlargement of air spaces

distal to the terminal bronchioles (see the images below).

CT scan of chest in patient w ith emphysema from smoking. Note formation of bullae in upper lobes.

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Microscopic picture of emphysematous lung (hematoxylin and eosin stain). Upper part of pictureshow s destruction of alveolar septa. Low er part of picture show s normal alveoli. Image courtesyof Dr. Chad Stone.

Pneumothorax, hemothorax, and hydrothorax

Pneumothorax, hemothorax, and hydrothorax are caused by a decrease in lung

volume secondary to the presence of air, blood, or fluid between the visceral and

parietal components of the pleura (see the images below).

Chest radiography of patient w ith spontaneous right pneumothorax. Red arrow s delineate lungedge.

CT scan of chest in same patient w ith spontaneous right pneumothorax.

Chest radiography of patient w ith right pleural effusion.

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CT scan of chest in same patient w ith right pleural effusion. Red arrow s point at effusion. Left lungis normal.

Diaphragmatic hernias

Diaphragmatic hernia occurs when a defect in the diaphragm allows the abdominal

contents to move into the chest cavity (see the image below). This could be

congenital, traumatic, iatrogenic, or due to a weakness over the muscles forming

the diaphragm.

CT scan of chest of patient w ith large left diaphragmatic hernia. Note bow el loops and mesenteriuminside left chest cavity.

Bronchial and tracheal stenosis

Bronchial and tracheal stenosis occurs as a result of secondary and numerous

malignant and benign processes and is also a consequence of surgical procedures

and trauma (see the images and video below). The main defect is the obstruction

or collapse of the airways at any level, which leads to changes in air flow that

result in hypoxemia.

Chest radiography of female patient w ith tracheal stenosis due to previous endotracheal intubation.Red arrow s show area of narrow ing in trachea.

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Bronchoscopic picture of trachea show ing area of stenosis (cicatricial stenosis) from previousendotracheal intubation.

CT scan of chest in patient w ith tracheal stenosis due to lung cancer. Yellow arrow s show tumorinvading lateral w all and grow ing into trachea, causing stenosis. (T=tumor).

CT scan of chest of patient w ith endobronchial carcinoid tumor of right mainstem bronchus (redarrow s) causing complete collapse of right lung.

This video demonstrates the results of rigid direct laryngoscopy and flexible tracheal endoscopy ina patient w ith significant tracheal stenosis.

Vocal cord paralysis/dysfunction

In most cases, paralysis or dysfunction of the vocal cords is caused by

dysfunction of the recurrent laryngeal or vagus nerve innervating the larynx. Even

when no real alteration of the anatomy is present, this condition may cause many

of the same problems associated with bronchial and tracheal stenosis.

Infectious processes (eg, bacterial pneumonia, tuberculosis)

Infectious etiologies (eg, bacterial pneumonia and tuberculosis) include viral,

fungal, and bacterial infections. They are characterized by consolidation of the

affected part of the lung and filling of the alveolar air spaces with exudate,

inflammatory cells, and fibrin, leading to a decrease in oxygen exchange

(ventilation mismatch) and, in severe cases, to destruction of the lung parenchyma

(see the images below).

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Chest radiography of patient w ith miliary tuberculosis.

CT scan of chest of patient w ith chronic invasive aspergillosis. Note bilateral nodular infiltrate w ithcavitations.

Micrographic picture of lung parenchyma show ing areas of necrosis w ith organizing pneumonia inpatient w ith Nocardia (hematoxylin and eosin). Image courtesy of Dr. Chad Stone.

Interstitial lung diseases

Interstitial lung diseases include conditions caused by drugs, autoimmune

processes, fibrotic diseases, organic and inorganic dust exposure, sarcoidosis,

lymphangioleiomyomatosis (LAM), histiocytosis X, vasculitis, pulmonary alveolar

proteinosis, and any other process that will cause reduced lung volumes due to an

alteration in lung parenchyma leading to ventilation-perfusion mismatch (see the

images below).

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CT scan of chest in patient w ith pulmonary alveolar proteinosis. Note classic "crazy pavement"pattern of lungs.

CT scan of patient w ith usual interstitial pneumonia. Note interstitial infiltrates and honeycombing inperiphery of lungs.

Micrographic picture (hematoxylin and eosin stain) of lung w ith usual interstitial pneumonia. Notealternating areas of normal lung, interstitial inflammation, fibrosis, and honeycomb change(patchw ork appearance). Image courtesy of Dr. Chad Stone.

Malignancy

Malignancy is an uncontrolled cell growth of the tissue in the lungs or airways.

Depending on the location and severity of the malignancy, it may lead to any of the

above described anatomic changes.

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Chest radiography of patient w ith metastatic melanoma to lungs. Note bilateral nodules(metastasis).

CT scan of chest in the same patient as preceding image w ith metastatic melanoma of lungs.

Contributor Information and Disclosures

Author

Eduardo A Celis, MD Physician in Pulmonary and Critical Care Medicine, Interventional Pulmonology

Eduardo A Celis, MD is a member of the following medical societies: American Association for Bronchology

and Interventional Pulmonology, American College of Chest Physicians, American College of Physicians,

American Thoracic Society, Society of Critical Care Medicine

Disclosure: Nothing to disclose.

Coauthor(s)

Javier I Diaz-Mendoza, MD Senior Staff Physician, Interventional Pulmonology, Division of Pulmonary and

Critical Care Medicine, Henry Ford Hospital

Javier I Diaz-Mendoza, MD is a member of the following medical societies: American College of Chest

Physicians, American Thoracic Society, American Association for Bronchology and Interventional Pulmonology

Disclosure: Nothing to disclose.

Chief Editor

Zab Mosenifar, MD, FACP, FCCP Geri and Richard Brawerman Chair in Pulmonary and Critical Care

Medicine, Professor and Executive Vice Chairman, Department of Medicine, Medical Director, Women's Guild

Lung Institute, Cedars Sinai Medical Center, University of California, Los Angeles, David Geffen School of

Medicine

Zab Mosenifar, MD, FACP, FCCP is a member of the following medical societies: American College of Chest

Physicians, American College of Physicians, American Federation for Medical Research, American Thoracic

Society

Disclosure: Nothing to disclose.

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