Chapter 2-2 RESPIRATORY SYSTEM - second.histo.space

Part II – Chapter 2- 1

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RESPIRATORY TRACT

• The respiratory system, includes the lungs and a sequence of airways leading to the external environment,

• Its main function is providing oxygen (O2) to and eliminating carbon dioxide (CO2) from the cells of the body.

• The process of gas exchange requires the fulfillment

of the following four events, collectively known as respiration:

1. Movement of air in and out of the lungs (breathing or ventilation)

2. Exchange of O2 in the inspired air for carbon dioxide in the blood (external respiration)

3. Transportation of O2 and CO2 to and from the cells (transport of gases)

4. Exchange of CO2 for O2 at the level of the cells (internal respiration)

• The first two of these events, ventilation and external

respiration, occur within the respiratory system,

whereas the transport of gases is performed by the

circulatory system and internal respiration occurs in

the tissues throughout the body.

• The respiratory system is subdivided into two major components:

1. The conducting portion

2. The respiratory portion

• The conducting portion, located both outside and within the lungs, conveys air

from the external environment to the lungs.

• The respiratory portion, located strictly within the lungs, functions in the actual

exchange of oxygen for carbon dioxide (external respiration).

Chapter 2-2 RESPIRATORY SYSTEM


CONDUCTING PORTION OF THE RESPIRATORY SYSTEM • The conducting portion of the respiratory system, listed in order from the

exterior to the inside of the lung, is composed of the nasal cavity, mouth, nasopharynx, pharynx, larynx, trachea, primary bronchi, secondary bronchi (lobar bronchi), tertiary bronchi (segmental bronchi), bronchioles, and terminal bronchioles.

• These structures are not only transporting but also filtering, moistening, and warming the inspired air before it reaches the respiratory portion of the lungs.

• The patency of the conducting airways is maintained by a combination of bone, cartilage, and fibrous elements.

• As the air passes along the airway during inspiration, it encounters a branching system of tubules.

• The velocity of air flow for a given volume of inhaled air decreases as the air proceeds toward the respiratory portion.

Nasal Cavity • The nasal cavity is divided into right and left halves by the cartilaginous and bony

nasal septum. • Each half of the nasal cavity is bounded laterally by a bony wall and a

cartilaginous ala (wing) of the nose; • It communicates with the outside, anteriorly, via the naris (nostril) and with the

nasopharynx by way of the conchae (turbinate). • Projecting from the bony lateral wall are three thin scroll-like bony shelves,

situated one above the other: the superior, middle, and inferior nasal conchae.


The anterior portion of the nasal cavity:

• It is dilated and is known as the vestibule.

• This region is lined with thin skin and has vibrissae-short, stiff hairs that prevent

larger dust particles from entering the nasal cavity.

• The dermis of the vestibule houses numerous sebaceous and sweat glands. The

dermis is anchored by numerous collagen bundles to the perichondria of the

hyaline cartilage segments that form the supporting skeleton of the ala.

Posterior Aspect of the Nasal Cavity

• Except for the vestibule and the olfactory region, the nasal cavity is lined by

pseudostratified ciliated columnar epithelium, frequently called the respiratory

epithelium, which is rich with goblet cells in the more posterior regions of the

nasal cavity.

• The subepithelial connective tissue (lamina propria) is richly vascularized,

especially in the region of the conchae and the anterior aspect of the nasal septum,

housing large arterial plexuses and venous sinuses.

• The lamina propria has many seromucous glands and abundant lymphoid

elements, including occasional lymphoid nodules, mast cells, and plasma cells.

• Antibodies produced by plasma cells (immunoglobulin IgA, IgE, and IgG) protect

the nasal mucosa against inhaled antigens as well as against microbial invasion.

The roof of the nasal cavity

• The superior aspect of the nasal septum, and the superior concha are covered by an

olfactory epithelium 60 µm thick.

• The underlying lamina propria houses serous fluid-secreting Bowman's glands, a

rich vascular plexus, and collections of axons that arise from the olfactory cells of

the olfactory epithelium.

• The olfactory epithelium, which is yellow in the living person, is composed of

three types of cells: olfactory, supporting (sustentacular), and basal.


Olfactory Region of the Nasal Cavity

• The olfactory region comprises the olfactory epithelium and the underlying lamina

propria that houses Bowman's glands and a rich vascular plexus.

Olfactory cells

• Olfactory cells are bipolar neurons whose apical aspect, the distal terminus of its

slender dendrite, is modified to form a bulb, the olfactory vesicle, which projects

above the surface of the supporting cells .

• The nucleus of the cell is spherical and is closer to the basal lamina than to the

olfactory vesicle.

• SEM demonstrates that six to eight long, nonmotile olfactory cilia extend from the

olfactory vesicle and lie on the free surface of the epithelium.

• TEM of these cilia display an unusual axoneme pattern that begins as a typical

peripheral ring of nine doublet microtubules surrounding two central singlets (9 +

2 configuration) but without the characteristic dynein arms.


• The axoneme changes distally so that it is composed of nine singlets surrounding

the two central singlets, and near the end of the cilium only the central singlets are

present.

• The basal region of the olfactory cell is its axon, which penetrates the basal lamina

and joins similar axons to form bundles of nerve fibers.

• Each axon, although unmyelinated, has a sheath composed of Schwann cell-like

olfactory ensheathing (glial) cells.

• The nerve fibers pass through the cribriform plate in the roof of the nasal cavity to

synapse with secondary neurons in the olfactory bulb.

Supporting And Basal Cells:

• Supporting cells are columnar cells, 50 to 60 µm tall, whose apical aspects have a

striated border composed of microvilli.

• Their oval nuclei are in the apical third of the cell, somewhat superficial to the

location of the olfactory cell nuclei.

• The apical cytoplasm of these cells has secretory granules housing a yellow

pigment whose color is characteristic of the olfactory mucosa.

• EM of sustentacular cells demonstrates that they form junctional complexes with

the olfactory vesicle regions of olfactory cells as well as with contiguous

sustentacular cells.

• These cells are believed to provide physical support, nourishment, and electrical

insulation for the olfactory cells.

Basal Cells:

• Basal cells are of two types, horizontal and globose (pyramid-shaped).

• Horizontal cells are flat and lie against the basement membrane, whereas globose

cells are short, basophilic, pyramid-shaped cells whose apical aspects do not reach

the epithelial surface.

• Their nuclei are centrally located, but because these are short cells, the nuclei

occupy the basal third of the epithelium.


• The globose type of basal cells has considerable proliferative capacity and can

replace both sustentacular and olfactory cells.

• In a healthy person, the olfactory cells live for less than three months and

sustentacular cells have a life span of less than a year. The horizontal basal cells

replicate to replace the globose basal cells.

LAMINA PROPRIA

• The lamina propria of the olfactory mucosa is composed of a richly vascularized,

loose to dense, irregular collagenous connective tissue that is firmly attached to the

underlying periosteum.

• It houses numerous lymphoid elements as well as the collection of axons of the

olfactory cells, which form fascicles of unmyelinated nerve fibers.

• Bowman's glands (olfactory glands), which produce a serous secretory product,

are also present and are indicative of the olfactory mucosa.

• These glands release IgA, lactoferrin, lysozyme, and odorant-binding protein, a

molecule that prevents the odorant from leaving the region of the olfactory

epithelium, thus enhancing a person's ability to detect odors.

• The nasal mucosa is protected from dehydration by alternating blood flow to the

venous sinuses of the lamina propria overlying the conchae of the right and left

nasal cavities.

• The erectile tissue-like region (swell bodies) of one side expands when its venous

sinuses become engorged with blood, reducing the flow of air through that side.

• Plasma from the sinuses and seromucous secretions from the glands rehydrates the

mucosa approximately every half hour. Paranasal Sinuses

• The ethmoid, sphenoid, frontal, and maxilla bones of the skull house large,

mucoperiosteum-lined spaces, the paranasal sinuses (named after their location),

which communicate with the nasal cavity.


• The mucosa of each sinus comprises a vascular connective tissue lamina propria

fused with the periosteum.

• The thin lamina propria resembles that of the nasal cavity, in that it houses

seromucous glands as well as lymphoid elements.

• The respiratory epithelial lining of the paranasal sinuses, similar to that of the

nasal cavity, has numerous ciliated columnar cells whose cilia sweep the mucus

layer toward the nasal cavity.

Nasopharynx

• The pharynx begins at the choana and extends to the opening of the larynx.

This continuous cavity is subdivided into three regions:

(1) The superior nasopharynx,

(2) The middle oral pharynx, and

(3) The inferior laryngeal pharynx.

• The nasopharynx is lined by a respiratory epithelium, whereas the oral and

laryngeal regions are lined by a stratified squamous epithelium.

• The lamina propria is composed of a loose to dense, irregular type of

vascularized connective tissue housing seromucous glands and lymphoid

elements.

• It is fused with the epimysium of the skeletal muscle components of the

pharynx.

• The lamina propria of the posterior aspect of the nasopharynx houses the

pharyngeal tonsil, an unencapsulated collection of lymphoid tissue.


Larynx

• The larynx, or voice box, is responsible for phonation and for preventing the entry

of food and fluids into the respiratory system.

• The larynx, situated between the pharynx and the trachea, is a rigid, short,

cylindrical tube 4 cm in length and approximately 4 cm in diameter.

• The wall of the larynx is reinforced by several hyaline cartilages (the unpaired

thyroid and cricoid cartilages and the inferior aspect of the paired arytenoids) and

elastic cartilages (the unpaired epiglottis, the paired corniculate and cuneiform

cartilages, and the superior aspect of the arytenoids).

• These cartilages are connected to one another by ligaments, and their movements

with respect to one another are controlled by intrinsic and extrinsic skeletal

muscles.

• The thyroid and cricoid cartilages form the cylindrical support for the larynx,

whereas the epiglottis provides a cover over the laryngeal opening.


• During respiration, the epiglottis is in the vertical position, permitting the flow of

air.

• During swallowing of food, fluids, or saliva, however, it is positioned horizontally,

closing the laryngeal opening.

• The arytenoid and corniculate cartilages are occasionally fused to each other, and

most of the intrinsic muscles of the larynx move the two arytenoids with respect to

each other and to the cricoid cartilage.

• The lumen of the larynx is characterized by the presence of two pairs of shelf-like

folds, the superiorly positioned vestibular folds and the inferiorly placed vocal

folds.

• The vestibular folds are immovable. Their lamina propria, composed of loose

connective tissue, houses seromucous glands, adipose cells, and lymphoid

elements.

• The free edge of each vocal fold is reinforced by dense, regular elastic connective

tissue, the vocal ligament.

• The vocalis muscle, attached to the vocal ligament, assists the other intrinsic

muscles of the larynx in altering the tension on the vocal folds. These muscles also

regulate the width of the space between the vocal folds, thus permitting precisely

regulated vibrations of their free edges by the exhaled air.

• During silent respiration, the vocal folds are partly abducted (pulled apart), and

during forced inspiration, they are fully abducted. During phonation, however, the

vocal folds are strongly adducted (drawn together), forming a narrow interval

between them.

• The movement of air against the edges of the strongly adducted vocal folds

produces and modulates sound (but not speech, which is formed by movements of

pharynx, soft palate, tongue, and lips). The longer and more relaxed the vocal

folds, the deeper the pitch of the sound.

• Because the larynx is prominent in male than that of a female, men tend to have

deeper voices than women.


• The larynx is lined by respiratory epithelium, except on the superior surfaces of

the epiglottis and vocal folds, which are covered by stratified squamous non-

keratinized epithelium.

• The cilia of the larynx beat toward the pharynx, transporting mucus and trapped

particulate matter toward the mouth to be expectorated or swallowed.

Trachea

• The trachea has three layers: mucosa, submucosa, and adventitia. C-rings are

located in the adventitia.

• The trachea is a tube, 12 cm in length and 2 cm in diameter, that begins at the

cricoid cartilage of the larynx and ends when it bifurcates to form the primary

bronchi.

• The wall of the trachea is reinforced by 10 to 12 horseshoe-shaped hyaline

cartilage rings (C-rings).

• The open ends of these rings face posteriorly and are connected to each other by

smooth muscle, the trachealis muscle. Because of this arrangement of the C-rings,

the trachea is rounded anteriorly but flattened posteriorly.

• The perichondrium of each C-ring is connected to the perichondria lying directly

above and below it by fibroelastic connective tissue, which provides flexibility to

the trachea and permits its elongation during inspiration.

• Contraction of the trachealis muscle decreases the diameter of the tracheal lumen,

resulting in faster air flow, which assists in the dislodging of foreign material (or

mucus or other irritants) from the larynx by coughing.

Mucosa of Trachea

• The mucosal lining of the trachea is composed of respiratory epithelium, the

subepithelial connective tissue (lamina propria), and a relatively thick bundle of

elastic fibers separating the mucosa from the submucosa.


• The respiratory epithelium is a pseudostratified ciliated columnar epithelium

composed of six cell types; goblet cells, ciliated columnar cells, basal cells, brush

cells, serous cells, and cells of the diffuse neuroendocrine system (DNES).

• All of these cells come into contact with the basement membrane, but they do not

all reach the lumen.

• Goblet cells constitute about 30% of the total cell population of the respiratory

epithelium.

• They produce mucinogen, which becomes hydrated and is known as mucin when

released into an aqueous environment.

• Like goblet cells elsewhere, goblet cells in the respiratory epithelium have a

narrow, basally positioned stem and an expanded theca containing secretory

granules.

• EM demonstrates that the nucleus and most organelles are located in the stem.

This region displays a rich network of rough endoplasmic reticulum (RER), a

well-developed Golgi complex, numerous mitochondria, and an abundance of

ribosomes. The theca is filled with numerous mucinogen-containing secretory

granules of varied diameters. The apical plasmalemma has a few short, blunt

microvilli.

• Ciliated columnar cells constitute approximately 30% of the total cell

population.

• These tall, slender cells have a basally located nucleus and possess cilia and

microvilli on their apical cell membrane.

• The cytoplasm just below these structures is rich in mitochondria and has a Golgi

complex. The remainder of the cytoplasm possesses some RER and a few

ribosomes.

• These cells move the mucus and its trapped particulate matter, via ciliary action,

toward the nasopharynx for elimination.

• The short basal cells constitute about 30% of the total cell population.


• They are located on the basement membrane, but their apical surfaces do not reach

the lumen.

• These relatively undifferentiated cells are considered to be stem cells that

proliferate to replace defunct goblet, ciliated columnar, and brush cells.

• Brush cells (small-granule mucous cells) constitute about 3% of the total cell

population.

• They are narrow, columnar cells with tall microvilli. Their function is unknown,

but they have been associated with nerve endings; thus, some investigators suggest

that they may have a sensory role. Other investigators believe that brush cells are

merely goblet cells that have released their mucinogen.

• Serous cells, which make up about 3% of the total cell population of the

respiratory epithelium, are columnar cells.

• They have apical microvilli and apical granules containing an electron-dense

secretory product, a serous fluid of unknown composition.

• DNES cells, also known as small-granule cells, constitute about 3% to 4% of

the total cell population.

• Many of these cells possess long, slender processes that extend into the lumen, and

it is believed that they have the ability to monitor the oxygen and carbon dioxide

levels in the lumen of the airway.

• These cells are closely associated with naked sensory nerve endings with which

they make synaptic contact, and together with these nerve fibers they are referred

to as pulmonary neuroepithelial bodies.

• DNES cells contain numerous granules in their basal cytoplasm that house

pharmacological agents such as amines, peptides, acetylcholine.

• Under hypoxic conditions, these agents are released not only into the synaptic

clefts but also into the connective tissue spaces of the lamina propria, where they

act as paracrine hormones or may enter the vascular supply to act as hormones.

• Therefore, it has been suggested that these pulmonary neuroepithelial bodies can exert local effects to alleviate localized hypoxic conditions by regulating perfusion


and ventilation in their vicinity or they may have generalized effects via the efferent nerve fibers that relay information about hypoxic conditions to the respiratory regulators located in the medulla oblongata.

Lamina Propria and Elastic Fibers

• The lamina propria of the trachea is composed of a loose, fibroelastic connective

tissue.

• It contains lymphoid elements (e.g., lymphoid nodules, lymphocytes, and

neutrophils) as well as mucous and seromucous glands, whose ducts open onto the

epithelial surface.

• A dense layer of elastic fibers, the elastic lamina, separates the lamina propria from the underlying submucosa.

Submucosa

• The tracheal submucosa is composed of a dense, irregular fibroelastic connective tissue housing numerous mucous and seromucous glands.

• The short ducts of these glands pierce the elastic lamina and the lamina propria to

open onto the epithelial surface.

• Lymphoid elements are also present in the submucosa. Moreover, this region has a

rich blood and lymph supply, the smaller branches of which reach the lamina

propria.

Adventitia

• The adventitia of the trachea is composed of a fibroelastic connective tissue.

• The most prominent features of the adventitia are the hyaline cartilage C-rings and

the intervening fibrous connective tissue.

• The adventitia also is responsible for anchoring the trachea to the adjacent

structures (i.e., esophagus and connective tissues of the neck).


Bronchial Tree

• The bronchial tree begins at the bifurcation of the trachea, as the right and left

primary bronchi, which form branches that gradually decrease in size.

• The bronchial tree is composed of airways located outside the lungs (primary

bronchi, extrapulmonary bronchi) and airways located inside the lungs:

intrapulmonary bronchi (secondary and tertiary bronchi), bronchioles, terminal

bronchioles, and respiratory bronchioles.

• The bronchial tree divides 15 to 20 times before reaching the level of the terminal

bronchioles.

• As the airways progressively decrease in size, there are several changes, including

a decrease in the amount of cartilage, the numbers of glands and goblet

cells, and the height of epithelial cells and an increase in smooth muscle and

elastic tissue (in relation to thickness of the wall).


Primary (Extrapulmonary) Bronchi

• The structure of the primary bronchi is identical to that of the trachea, except that

primary bronchi are smaller in diameter and their walls are thinner.

• Each primary bronchus, accompanied by the pulmonary arteries, veins, and lymph

vessels, pierces the root of the lung.

• The right bronchus is straighter than the left bronchus. The right bronchus

trifurcates to lead to the three lobes of the right lung, and the left bronchus

bifurcates, sending branches to the two lobes of the left lung. These branches then

enter the substance of the lungs as intrapulmonary bronchi.

Secondary and Tertiary (Intrapulmonary) Bronchi

• Each intrapulmonary bronchus serves a lobe of the lung; tertiary bronchi serve

bronchopulmonary segments.

• Each intrapulmonary bronchus is the airway to a lobe of the lung. These airways

are similar to primary bronchi, with the following exceptions.

Ø The cartilage C-rings are replaced by irregular plates of hyaline cartilage

that completely surround the lumina of the intrapulmonary bronchi;

Ø These airways do not have a flattened region but are completely round.

Ø The smooth muscle is located at the interface of the fibroelastic lamina

propria and submucosa as two distinct smooth muscle layers spiraling in

opposite directions.

Ø Elastic fibers, which radiate from the adventitia, connect to elastic fibers

arising from the adventitia of other parts of the bronchial tree.

• As in the primary bronchi and in the trachea, seromucous glands and lymphoid

elements are present in the lamina propria and the submucosa of the

intrapulmonary bronchi.

• Ducts of these glands deliver their secretory products onto the surface of the

pseudostratified, ciliated epithelial lining of the lumen.


• Lymphoid nodules are particularly evident where these airways branch to form

increasingly smaller intrapulmonary bronchi.

• The smaller intrapulmonary bronchi have thinner walls, decreasing amounts of

hyaline cartilage plates, and shorter epithelium-lining cells.

• Secondary bronchi, direct branches of the primary bronchi leading to the lobes of

the lung, are also known as lobar bronchi.

• The left lung has two lobes and thus has two secondary bronchi; the right lung has

three lobes and thus has three secondary bronchi.

• As secondary bronchi enter the lobes of the lung, they subdivide into smaller

branches, tertiary (segmental) bronchi.

• Each tertiary bronchus arborizes but leads to a specific area of lung tissue known

as a bronchopulmonary segment.

• Each lung has 10 bronchopulmonary segments that are completely separated from

one another by connective tissue elements and are clinically important in surgical

procedures involving the lungs.

• As the arborized branches of intrapulmonary bronchi decrease in diameter, they

eventually lead to bronchioles.

Bronchioles

• Bronchioles possess no cartilage in their walls, are less than 1 mm in diameter, and

have Clara cells in their epithelial lining.

• Each bronchiole (or primary bronchiole) supplies air to a pulmonary lobule.

• Bronchioles are considered the 10th to 15th generation of branching of the

bronchial tree.

• Their diameter commonly is described as less than 1 mm (0.3 mm – 5mm).

• The epithelial lining of bronchioles ranges from ciliated simple columnar with

occasional goblet cells in larger bronchioles to simple cuboidal (many with cilia)

with occasional Clara cells and no goblet cells in smaller bronchioles.


Clara cells

Ø They are columnar cells with dome-shaped apices that have short, blunt

microvilli.

Ø Their apical cytoplasm houses numerous secretory granules containing

glycoproteins manufactured on their abundant RER.

Ø Clara cells are believed to protect the bronchiolar epithelium by lining it

with their secretory product.

Ø Additionally, these cells degrade toxins in the inhaled air via cytochrome

P-450 enzymes in their smooth endoplasmic reticulum.

Ø Some investigators suggest that Clara cells produce a surfactant-like

material that reduces the surface tension of bronchioles and facilitates the

maintenance of their patency.

Ø Moreover, Clara cells divide to regenerate the bronchiolar epithelium.

• The lamina propria of bronchioles has no glands; it is surrounded by a loose

meshwork of helically oriented smooth muscle layers.

• The walls of bronchioles and their branches have no cartilage.

• Elastic fibers radiate from the fibroelastic connective tissue that surrounds the

smooth muscle coats of bronchioles.

• These elastic fibers connect to elastic fibers ramifying from other branches of the

bronchial tree.

• During inhalation, as the lung expands in volume, the elastic fibers exert tension

on the bronchiolar walls; by pulling uniformly in all directions, the elastic fibers

help maintain the patency of the bronchioles.


RESPIRATORY PORTION OF THE RESPIRATORY SYSTEM

• The respiratory portion of the respiratory system is composed of respiratory

bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Respiratory Bronchioles

• Respiratory bronchioles are the first region of the respiratory system where

exchange of gases can occur.

• Respiratory bronchioles are similar in structure to terminal bronchioles, but their

wall is interrupted by the presence of thin-walled, pouch-like structures known as

alveoli, where gaseous exchange (O2 for CO2) can occur.

• As respiratory bronchioles branch, they become narrower in diameter and their

population of alveoli increases. Subsequent to several branching, each respiratory

bronchiole terminates in an alveolar duct

Alveolar Duct, Atrium, and Alveolar Sac

• Alveolar ducts, atria, and alveoli are supplied by a rich capillary network.

• Alveolar ducts do not have walls of their own; they are merely linear arrangements

of alveoli.

• An alveolar duct that arises from a respiratory bronchiole branches, and each of

the resultant alveolar ducts usually ends as a blind end composed of two or more

small clusters of alveoli, in which each cluster is known as an alveolar sac.

• These alveolar sacs thus open into a common space, which some investigators call

the atrium.

• Slender connective tissue elements between alveoli, the interalveolar septa,

reinforce the alveolar duct, stabilizing it somewhat.


• Additionally, the opening of each alveolus to the alveolar duct is controlled by a

single smooth muscle cell, embedded in type III collagen, which forms a delicate

sphincter regulating the diameter of the opening.

• Fine elastic fibers ramify from the periphery of alveolar ducts and sacs radiating

from other intrapulmonary elements.

• This network of elastic fibers not only maintains the patency of these delicate

structures during inhalation but also protects them against damage during

distention and is responsible for nonforced exhalation.

Alveoli

• Alveoli are small air sacs composed of highly attenuated type I pneumocytes and

larger type II pneumocytes.


• Each alveolus is a small outpouching, about 200 µm in diameter, of respiratory

bronchioles, alveolar ducts, and alveolar

• Alveoli form the primary structural and functional unit of the respiratory system,

because their thin walls permit exchange of CO2 for O2 between the air in their

lumina and blood in adjacent capillaries.

• Although each alveolus is a small structure, about 0.002mm3, their total number

approximates 300 million, conferring on the lung its sponge-like consistency.

• It has been estimated that the total surface area of all the alveoli available for gas

exchange exceeds 140 m2.

• Because of their large number, alveoli are frequently pressed against each other,

eliminating the connective tissue interstitium between them.


• In such areas of contact, the air spaces of the two alveoli may communicate with

each other through an alveolar pore (pore of Kohn), whose diameter varies from 8

to 60µm.

• These pores presumably function to equilibrate air pressure within pulmonary

segments.

• The region between adjacent alveoli is known as the interalveolar septum.

• It is occupied by an extensive capillary bed composed of continuous capillaries,

supplied by the pulmonary artery and drained by the pulmonary vein.

• The connective tissue of the interalveolar septum is rich in elastic fibers and type

III collagen (reticular) fibers.

• Because alveoli and capillaries are composed of epithelial cells, they are invested

by a prominent basal lamina.

• The openings of alveoli associated with alveolar sacs, unlike those of respiratory

bronchioles and alveolar ducts, are devoid of smooth muscle cells. Instead, their

orifices are circumscribed by elastic and, especially, reticular fibers.

• Walls of alveoli are composed of two types of cells: type I pneumocytes and

type II pneumocytes.

Type I Pneumocytes

• Approximately 95% of the alveolar surface is composed of simple squamous

epithelium, whose cells are known as type I pneumocytes (also called type I

alveolar cells and squamous alveolar cells).

• Because the cells of this epithelium are highly attenuated, their cytoplasm may be

as thin as 80 nm in width.

• The region of the nucleus is, as expected, wider, and it houses much of the cell's

organelle population, composed of a small number of mitochondria, a few profiles

of RER, and a modest Golgi apparatus.


• Type I pneumocytes form occluding junctions with each other, thus preventing the

seepage of extracellular fluid (tissue fluid) into the alveolar lumen.

• The adluminal aspect of these cells is covered by a well-developed basal lamina,

which extends almost to the rim of the alveolar pores.

• The rim of each alveolar pore is formed by the fusion of the cell membranes of

two closely apposed type I pneumocytes that belong to two discrete alveoli. The

luminal aspect of type I pneumocytes is lined by surfactant.

Type II Pneumocytes

• Although type II pneumocytes (also known as great alveolar cells, septal cells,

and type II alveolar cells) are more numerous than type I pneumocytes, they

occupy only about 5% of the alveolar surface.

• These cuboidal cells are interspersed among, and form occluding junctions with,

type I pneumocytes.

• Their dome-shaped apical surface juts into the lumen of the alveolus.

• Type II pneumocytes are usually located in regions where adjacent alveoli are

separated from each other by a septum (hence the name septal cells), and their

adluminal surface is covered by basal lamina.

• Electron micrographs of type II pneumocytes display short, apical microvilli.

• They have a centrally placed nucleus, an abundance of RER profiles, a well-

developed Golgi apparatus, and mitochondria.

• The most distinguishing feature of these cells is the presence of membrane-bound

lamellar bodies that contain pulmonary surfactant, the secretory product of

these cells.

• In addition to producing and phagocytosing surfactant, type II pneumocytes

undergo mitosis to regenerate themselves as well as type I pneumocytes.

•

• The surfactant is released by exocytosis into the lumen of the alveolus.


• it separated into lipid and protein portions. The lipid is inserted into a

monomolecular phospholipid film, forming an interface with air, and the protein

enters an aqueous layer between the pneumocytes and the phospholipid film.

• The surfactant decreases surface tension, thus preventing atelectasis, namely the

collapse of the alveolus.

• It is continuously manufactured by type II pneumocytes and is phagocytosed and

recycled by type II pneumocytes and, less frequently, by alveolar macrophages.

Alveolar Macrophages (Dust Cells)

• Alveolar macrophages phagocytose particulate matter in the lumen of the alveolus

as well as in the interalveolar spaces.

• Monocytes gain access to the pulmonary interstitium, become alveolar

macrophages (dust cells), migrate between type I pneumocytes, and enter the

lumen of the alveolus.

• These cells phagocytose particulate matter, such as dust and bacteria, and thus

maintain a sterile environment within the lungs.

• Dust cells also assist type II pneumocytes in the uptake of surfactant.

Approximately 100 million macrophages migrate to the bronchi each day and are

transported from there by ciliary action to the pharynx to be eliminated by being

swallowed or expectorated.

• Some alveolar macrophages, however, reenter the pulmonary interstitium and migrate into lymph vessels to exit the lungs.

Interalveolar Septum

• The region between two adjacent alveoli, known as an interalveolar septum, is

lined on both sides by alveolar epithelium.

• The interalveolar septum may be extremely narrow, housing only a continuous

capillary and its basal lamina, or it may be somewhat wider, including connective

tissue elements, such as type III collagen and elastic fibers, macrophages,

fibroblasts (and myofibroblasts), mast cells, and lymphoid elements.


Blood-Gas Barrier

• The blood-gas barrier is that region of the interalveolar septum that is traversed by

O2 and CO2 as these gases go from the lumen of the blood vessel to the lumen of

the alveolus, and vice versa.

• The thinnest regions of the interalveolar septum where gases can be exchanged are

called the blood-gas barriers.

• The narrowest blood-gas barrier, where type I pneumocytes are in intimate contact

with the endothelial lining of the capillary and where the basal laminae of the two

epithelia become fused, is most efficient for the exchange of O2 (in the alveolar

lumen) for CO2 (in the blood). These regions are composed of the following

structures:

1. Surfactant and type I pneumocytes 2. Fused basal laminae of type I pneumocytes and endothelial cells of the

capillary 3. Endothelial cells of the continuous capillary 4. Exchange of Gases between the Tissues and Lungs 5. In the lungs, O2 is exchanged for CO2 carried by blood; in the tissues of the

body, CO2 is exchanged for O2 carried by blood. Pleural Cavities

• Alteration of the volume of the pleural cavities by muscle action is responsible for the movement of gases into and out of the respiratory system.

• The thoracic cage is separated into three regions: the left and right thoracic cavities

and the centrally located mediastinum.

• Each thoracic cavity is lined by a serous membrane, the pleura, composed of

simple squamous epithelium and subserous connective tissue.

• The pleura may be imagined as an inflated balloon; as the lung develops, it pushes

against the serous membrane, as if a fist were pushing against the outer surface of

a balloon.

• In this fashion, a portion of the pleura, the visceral pleura, covers and adheres to

the lung, and the remainder of the pleura, the parietal pleura, lines and adheres to

the walls of the thoracic cavity.

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Chapter 2-2 RESPIRATORY SYSTEM - second.histo.space