Respiratory System Objectives

- Describe the components and functions of the conducting and

respiratory portions of the respiratory system.

- Describe the embryologic steps in the development of the

respiratory system.

- Describe and compare the characteristic microscopic features and

functions of the different regions and airways of the respiratory

system, including lining epithelium, glands, cartilage, smooth

muscle and elastic fibers.

Respiratory System

* Introduction

* Conducting portion

* Respiratory portion

* Pleura

Respiratory System

• Introduction

– Anatomy

Anatomy of the

respiratory system

Two Functional Parts of the Respiratory Tract

* Conducting portion

- Conduct air to and from alveoli

- Condition the air (warm, humidify, filter)

* Respiratory portion

- Gas exchange

Trachea

Right lung Left lung

Primary bronchus

Secondary bronchi

Bronchiole (1 mm or less in diameter)

Terminal bronchiole

Respiratory bronchiole

Alveolar duct

Alveolar sac

Conducting

portion

Respiratory

portion

Respiratory System

* Embryology

Pharyn

x

Lungs

Esophagus

Gallbladder

Stomach

Liver

Stuff that develops from the foregut

Duodenum (½)

Pancreas

Development of respiratory diverticulum (bud)

Endoderm gives rise

to the epithelium and glands

of the larynx, trachea,

bronchi and the pulmonary

epithelium.

Mesoderm gives rise

to the connective tissue,

cartilage and smooth muscle

of

the respiratory tract.

- The lung bud divides into two

bronchial buds, which divide

and form secondary and tertiary

bronchi.

- As bronchi develop, the

surrounding mesenchyme forms

cartilage, smooth muscle,

connective tissue and

capillaries.

Development of bronchi

and bronchioles

- By week 24, respiratory bronchioles are present and lungs are vascularized.

Respiration is possible – but chances of survival outside placenta are slim.

- Weeks 24-26: terminal alveolar sacs develop and type II pneumocytes secrete

surfactant (decreases surface tension)

- By week 26-28: there are enough alveolar sacs and surfactant to allow a

prematurely born infant to survive without medical intervention.

When can the baby breathe?

- 95% of mature alveoli develop after birth!

- The newborn infant has only 1/6 – 1/8 the number of alveoli as adults.

- Most alveolar growth is done by age 8.

- At birth, the lungs are half-filled with amniotic fluid.

- This fluid is cleared through the mouth and nose by pressure on thorax during

delivery.

- It also is reabsorbed into pulmonary blood vessels and lymphatics.

Lungs at Birth

Respiratory System

* Introduction

* Conducting portion

- Nasal cavity

- Larynx

- Trachea

- Bronchi

- Bronchioles

* The walls of the conducting system change in thickness and composition from

region to region.

* Components include:

- Epithelium (“respiratory epithelium”)

- Lamina propria

- Mucous and serous glands

- Cartilage

- Smooth muscle

- Adventitia

Walls of the Conducting System

Respiratory System

* Introduction

* Conducting portion

- Nasal cavity

* Respiratory epithelium everywhere except at the top (which has

specialized olfactory epithelium).

* Serous and mucous glands and numerous blood vessels in lamina

propria.

* Nasal septum: midline structure consisting of bone and hyaline

cartilage.

* Nasal fossa: chambers on each side of septum.

Nasal Cavity

* NASAL CAVITIES- The nasal cavities are paired chambers separated by a bony and

cartilaginous septum.

• They are elongated spaces with a wide base and a narrow apex .

• The skeletal framework is formed by bones and cartilages.

• Each cavity communicates anteriorly with the external environment

through the anterior nares (nostrils);

- posteriorly with the nasopharynx through the choanae; and

laterally with the paranasal sinuses and nasolacrimal duct.

- The chambers are divided into three regions:

• Nasal vestibule, is lined by skin

• Respiratory region, (inferior two-thirds) of the nasal cavities and is

lined by respiratory mucosa

• Olfactory region, at the apex (upper one-third) of each nasal cavity

and is lined by specialized olfactory mucosa

* Respiratory Region of the Nasal Cavity- It is lined by the respiratory mucosa that contains a ciliated,

pseudostratified columnar epithelium.

- lamina propria is firmly attached to the periosteum and perichondrium .

- The medial wall (the nasal septum) , is smooth, but the lateral walls are

thrown into folds by the presence of three shelf-like, bony projections

called conchae or turbinates.

• The conchae play a dual role:

- They increase surface area and

- cause turbulence in airflow to allow conditioning of inspired air.

- The ciliated, pseudostratified columnar epithelium is composed of five

cell types:

• Ciliated cells, tall columnar cells with cilia.

• Goblet cells that synthesize and secrete mucus

• Brush cells, bear short, blunt microvilli.

• Small granule cells (Kulchitsky cells) that contain secretory

granules (diffuse neuroendocrine system (DNES)

• Basal cells, stem cells from which the other cell types arise

* The mucosa of the respiratory region warms, moistens, and

filters inspired air.- The lamina propria has a rich, a complex set of capillary loops.

- The capillaries that reside near the surface are arranged in rows;

. the blood flows perpendicular ( a mechanical heat exchange system).

. These same vessels may become engorged and leaky during allergic

reactions or viral infections such as the common cold.

- Th e lamina propria contains mucous glands, many exhibiting serous

demilunes.

- the conchae (turbinates) increase the efficiency with which the inspired

air is warmed.

- The turbinates increase the efficiency of filtration through the process of

turbulent precipitation.

- The air stream is broken into eddies by the turbinates.

* Olfactory Region of the Nasal Cavity- The olfactory region is located on part of the dome of each nasal cavity.

- It is lined with a specialized olfactory mucosa.

. Its slight yellowish brown color caused by pigment in the olfactory

epithelium and the associated olfactory glands.

. Its total surface area is only about 10 cm2 ;

- in certain dog species have more than 150 cm2.

- The olfactory epithelium, is also pseudostratified, contains very different

cell types.

. it lacks goblet cells.

- The lamina propria is directly contiguous with the periosteum .

. It contains numerous blood and lymphatic vessels, unmyelinated

olfactory nerves, myelinated nerves, and olfactory glands.

- The olfactory epithelium is composed of the following cell types:

• Olfactory receptor cells are bipolar olfactory neurons that span the

thickness of the epithelium and enter the central nervous system.

• Supporting or sustentacular cells are columnar cells that are similar to

neuroglia cells and provide mechanical and metabolic support to the

olfactory receptor cells.

- They synthesize and secrete odorant-binding proteins.

• Basal cells are stem cells from which new olfactory receptor cells and

supporting cells differentiate.

• Brush cells are the same cell type that occurs in the respiratory

epithelium.

- The olfactory epithelium is composed of the following cell types:

• Olfactory receptor cells are bipolar olfactory neurons that span the

thickness of the epithelium and enter the central nervous system.

• Supporting or sustentacular cells are columnar cells that are similar to

neuroglia cells and provide mechanical and metabolic support to the

olfactory receptor cells.

- They synthesize and secrete odorant-binding proteins.

• Basal cells are stem cells from which new olfactory receptor cells and

supporting cells differentiate.

• Brush cells are the same cell type that occurs in the respiratory

epithelium.

* Olfactory receptor cells are bipolar neurons that possess an

apical projection bearing cilia.- The apical domain of each olfactory receptor cell has a single dendritic

process as a knob-like structure called the olfactory vesicle.

.Thin cilia (10 to 23) with typical basal bodies arise from the

olfactory vesicle and extend radially parallel to the surface.

. up to 200 µm long and may overlap with cilia of adjacent .

. The cilia are nonmotile, or have limited motility.

- The basal domain of the cell gives rise to an unmyelinated axonal process

. Collections of axons grouped into bundles that pass through a

thin cribriform plate of the ethmoid bone, and finally entering the

olfactory bulb of the brain.

. The collections of axons from olfactory receptor cells form the

olfactory nerve (cranial nerve I).

- The olfactory axons are very fragile and can be harmed during

traumatic head injury.

. They can be permanently severed, resulting in anosmia

(loss of the sense of smell).

- The olfactory receptor cells have a life span of about 1 month.

. If injured, they are quickly replaced.

- Olfactory receptor cells (and some neurons of the enteric division ) are

readily replaced during postnatal life.

Olfactory mucosa of the nasal cavity. a. This diagram shows the three major cell types located within the

olfactory epithelium: the olfactory cell, supporting (sustentacular) cell, and basal cell. The olfactory cell is the receptor cell; it has an

apical expansion, the olfactory vesicle, from which long, nonmotile cilia extend. At its basal surface, it extends an axon into the

connective tissue that joins with axons of other olfactory cells to form an olfactory nerve. The basal cells are small and cuboidal. They are

restricted to the basal part of the epithelium. The supporting cells, in contrast, are columnar and extend the full thickness of the

epithelium; their nuclei are located in the upper portion of the cell. Note the olfactory (Bowman’s) gland and its duct that empties on the

surface of the mucosa. b. Photomicrograph of the olfactory mucosa. The olfactory epithelium exhibits nuclei through much of its

thickness, but the individual cell types to which they belong are not discernible. The underlying connective tissue is largely occupied by

numerous olfactory (Bowman’s) glands, olfactory nerves, and blood vessels. Note that the ducts of the olfactory glands extend from the

secretory portion of the gland to the epithelial surface. 240.

Diagram of olfactory transduction

pathway.

This diagram shows interactions of the

odorant molecules with proteins

associated with olfactory receptor cell.

Incoming inhaled air odorant

molecules are solubilized in the

olfactory mucus and bind to olfactory

binding proteins, which deliver them

to the olfactory receptors. Note that

different odorant molecules bind with

different affinity to the olfactory

receptors. Strong signal (see green G

protein–coupled olfactory receptor) is

produced by high affinity binding

where odorant molecule (green)

matches perfectly the binging site on

the receptor. Other olfactory receptors

(yellow and pink) show less affinity

binding, thus producing weaker

signals. Stimulated by odorant

molecules, olfactory receptors activate

enzyme adenylyl cyclase and initiate

the cAMP cascade of events leading

to the opening of specific Na+ and Ca2+

channels. Influx of Na+ and Ca2+ is

responsible for cell depolarization.

Generated action potential travels

on axons of olfactory receptor cells

from the nasal cavity, passing through

the ethmoid bone and surrounding

brain coverings to the olfactory

bulb of the brain.

* Paranasal Sinuses Paranasal sinuses are air-filled spaces in the

bones of the walls of the nasal cavity.- The paranasal sinuses are extensions of the respiratory region of the

nasal cavity and are lined by respiratory epithelium.

- The sinuses are named for the bone in which they are found (i.e., the

ethmoid, frontal, sphenoid, and maxillary bones).

- The sinuses communicate with the nasal cavities via narrow openings

onto the respiratory mucosa.

- The mucosal surface of the sinuses is a thin, ciliated, pseudostratified

columnar epithelium with numerous goblet cells.

- Mucus produced in the sinuses is swept into the nasal cavities by

coordinated ciliary movements.

- The sinuses are often subject to acute infection after viral infection of the

upper respiratory tract.

Respiratory System

* Introduction

* Conducting portion

- Nasal cavity

- Larynx

* LARYNX- The passageway for air between the oropharynx and trachea is the larynx.

- This complex tubular region of the respiratory system is formed by

irregularly shaped plates of hyaline and elastic cartilage (the epiglottis and

the vocal processes of the arytenoid cartilages).

- The larynx serves as a conduit for air, organ for producing sounds.

Epiglottis covers

laryngeal opening during

swallowing.

Core of elastic cartilage.

Superior surface:

nonkeratinized stratified

squamous epithelium.

Inferior surface:

respiratory epithelium

Laryngeal cartilages

support the wall of the

larynx and serve as

attachments for vocalis

muscles.

Vocal folds are covered

by nonkeratinized

stratified squamous

epithelium

False vocal folds are

covered by respiratory

epithelium

Larynx and epiglottis

False vocal folds

True vocal folds

Vocalis muscle

Vestibule

Larynx, coronal section

Larynx: true and false vocal cords

* The trachea is a short, flexible, air tube about 2.5 cm in diameter

and about 10 cm long.

* Extends from larynx and divides into two primary bronchi.

* Contains 16-20 C-shaped hyaline cartilage rings with the dorsal

opening bridged by smooth muscle (trachealis muscle).

* Lined by respiratory epithelium.

* Seromucous glands in lamina propria and submucosa.

Trachea

Trachea

Trachea divides into 2 primary bronchi.

- Ciliated columnar cells: most abundant cell type. Cilia beat in unison and move

mucus and trapped particles to oropharynx, where it is swallowed or

expectorated.

- Goblet cells: produce mucus.

- Basal cells: stem cells that replenish epithelium. Hard to see.

- Brush cells: Columnar cells. No cilia but have apical microvilli. Hard to see.

- Neuroendocrine cells: Small granule cells (Kulchitsky cells) epithelial cells

containing hormones. Hard to see.

Respiratory Epithelium Cell Types

Photomicrograph of tracheal epithelium.

Three major cell types are evident in the tracheal

epithelium (Ep): ciliated columnar cells; mucus-

secreting goblet cells (G) interspersed between

the ciliated cells; and basal cells, which are close to

the basement membrane (BM). The ciliated columnar

cells extend from the basement membrane to the

surface. At their free surface, they contain numerous

cilia that, together, give the surface a brush-like

appearance. At the base of the cilia is a dense

eosinophilic line. This is owing to the linear

aggregation of structures referred to as basal bodies,

located at the proximal end of each cilium. Although

basement membranes are not ordinarily seen in H&E

preparations, a structure identified as such is seen

regularly under the epithelium in the human trachea.

The underlying lamina propria (LP) consists of loose

connective tissue. The more deeply located

submucosa (SM) contains dense irregular connective

tissue with blood and lymphatic vessels, nerves, and

numerous mucus-secreting tracheal glands. X 400.

Trachea: respiratory epithelium

Electron micrograph of human

trachea. This electron micrograph

shows the three main cell types of

this respiratory epithelium. They

are represented by ciliated

epithelial cells extending to the

surface, where they possess cilia;

goblet cells with mucinogen

granules; and basal cells, which

are confined to the basal portion

of the epithelial layer near the

connective tissue. X1,800.

(Courtesy of Dr. Johannes A. G.

Rhodin.)

* BRONCHI- The trachea divides into two main (primary) bronchi.

- Anatomically, these divisions are more frequently described

as simply the right and left main bronchi.

. The right bronchus is wider and significantly shorter

than the left.

- On entering the hilum of the lung, each main bronchus

divides into the lobar bronchi (secondary bronchi).

- The right bronchus divides into three lobar bronchial

branches and the left into two lobar bronchial branches, with

each branch supplying one lobe.

- The lobar bronchi give rise to 10 segmental bronchi (

tertiary bronchi);

- The lobar bronchi of the left lung give rise to 8 segmental

bronchi.

* As branching progresses:

- Connective tissue decreases in thickness

- Relative amount of smooth muscle and elastic tissue increases

- Cartilage disappears (gone by bronchioles)

Morphologic Changes as Bronchi Branch

Lung super low-power view

Bronchus

Wall of Bronchus

- Respiratory epithelium (E)

- Lamina propria (LP)

- Smooth muscle (SM)

- Submucosa

- Hyaline cartilage (C)

- Nerves (N) and blood vessels (V)

Note the order of structures:

E → LP → SM → C

Bronchus: respiratory epithelium

Goblet cellsPseudostratified

columnar

epithelium

Cilia

* NO glands or cartilage.

* Larger bronchioles have respiratory epithelium.

* Smaller bronchioles have low columnar epithelium.

* In asthma, the smooth muscle in the bronchioles constricts, causing

difficulty breathing.

Bronchioles

Bronchiole

Smooth

muscle

Respiratory

epithelium is

folded

Fibrous

connective

tissue is

present but

no cartilage

or glands!

Smaller bronchioles have simple columnar epithelium

* Simple cuboidal epithelium with cilia.

* Also: Clara cells (non-ciliated epithelial cells with secretory

granules).

* No goblet cells.

* As you go down the respiratory tract, goblet cells are lost before

cilia.

Terminal Bronchioles

Photomicrograph showing the

respiratory portion of the bronchial

tree. In this photomicrograph, a terminal

bronchiole (TB) is shown longitudinally

sectioned as it branches into two respiratory

bronchioles (RB). The terminal bronchiole is the

most distal part of the conducting portion of the

respiratory system and is not engaged in gas

exchange. The respiratory bronchiole engages in

gas exchange and is the beginning of the

respiratory portion of the bronchial tree.

Respiratory bronchioles give rise to alveolar

ducts (AD), which are elongate airways that have

almost no walls, only alveoli surrounding the

duct space. Alveolar sacs (AS) are spaces at the

termination of the alveolar ducts that, likewise,

are surrounded by alveoli. X120.

Light micrograph of a 1 14m thick plastic section, stained with toluidine blue, showing a respiratory

bronchiole with non-ciliated, dense-granulated cells (arrows). Serous-like cells containing

relatively large dense granules are indicated by the "solid" arrows.

The "open" arrow indicates a typical Clara cell containing fewer dense secretory granules at its

protruding apex. (Scale bar•S 14m)

Clara cells

Make surfactant components, break down mucus, detoxify harmful substances, transfer

IgA, fight bacteria

produce a 16 kDa protein known as Clara cell secretory protein (CC16).

Diagram of a terminal bronchiolar epithelium. This diagram shows several cells found in the respiratory

epithelium. The Clara cell, as illustrated here, is interposed between the brush cell and the small granule cell. The

Clara cell is a nonciliated cell that has rounded apical surface, well-developed basal rER and Golgi apparatus and

contains secretory vesicles filled with a surface-active agent. Adjacent, is the brush cell containing blunt microvilli,

which are distinctive features of this cell. Cytoplasm of the brush cell shows a Golgi apparatus, lysosomes,

mitochondria, and glycogen inclusions. A small granule cell is shown located between the Clara cell and ciliated cell.

This cell contains small secretory vesicles, most of which are in the basal portion of the cell. In addition to the

vesicles, the most conspicuous organelles of this cell are rER, a Golgi apparatus, and mitochondria. A nerve terminal

is shown within the epithelium.

Scanning electron micrograph of a terminal

bronchiole. This scanning photomicrograph

shows a longitudinal

section throughout the terminal bronchiole and

surrounding alveoli (A).

Note that the apical surfaces of the Clara cells

possess no cilia and have a characteristic dome-

shaped appearance. 150. The inset shows some

of the Clara cells at a higher magnifi cation and

the cilia of a neighboring ciliated cell, which are

present in very small numbers at this level. Note

the relatively few cilia present on these small

cells. X1,200.

* Respiratory bronchioles

- As you go distally along the respiratory bronchioles, alveoli

increase in number.

- Cilia are gone by the end of the respiratory bronchiole.

* Alveolar ducts

- Back-to-back alveolar openings along wall

- Smooth muscle between alveolar openings looks like knobs

Airways Preceding Alveoli

Respiratory bronchiole

Alveolar duct

Alveolar duct (AD), alveolar sac (AS) and alveolus (A)

* Alveoli .- About 150 to 250 million alveoli are found in each adult lung;

- their combined internal surface area is approximately 75 m2.

* Sac-like structures with super-thin walls so O2 and CO2 can diffuse

between air and blood.

* Separated by interalveolar septae, which contain capillaries.

* Cells lining interalveolar septae:

- Type I cells (thin, flat squamous cells)

- Type II cells (pneumocytes): produce surfactant

- Brush cells are also present in the alveolar wall(few in

number).

Alveoli

Alveolus

* Cover 95% of alveolar surface, comprise only 40% of the entire

alveolar lining cells

* Simple squamous cells with thin cytoplasm

* Blood-air barrier includes (from air to blood):

- Type I cells

- Fused basal laminae of type I cells and capillary endothelial cells

- Capillary endothelial cells

Type I Cells

* Cover 5% of alveolar surface, account for 60% of the alveolar

lining cells

* Large cuboidal cells with round nuclei.

* Typical secretory cell structure.

- Lamellar bodies in cytoplasm rich in a mixture of phospholipids, neutral

lipids, and proteins that form and store surfactant.

* Surfactant decreases surface tension in alveoli and prevents

collapse of alveoli during expiration.

Type II Cells (Pneumocytes)

Alveoli lined by type I and II cells

Electron micrograph of lung alveoli. This electron micrograph shows two alveolar spaces separated by an

alveolar septum containing capillaries, some of which contain red blood cells. Note the areas of thin and thick portions

of the alveolar septum. These are shown at a higher magnification in Figure 19.19. 5,800. Inset. Photomicrograph of

an alveolus for comparison with the alveolar wall as seen in an electron micrograph. Arrows indicate alveolar

capillaries containing red blood cells. X480.

Electron micrograph of a type II alveolar cell. The type II alveolar cell has a dome-shaped apical surface

with a number of short microvilli at its periphery and a relatively smooth-contoured apical center. The lateral cell

margins are overlain to a variable degree by the type I alveolar cells that are joined to the type II cell by occluding

junctions. Both cell types rest on the basal lamina (BL). The secretory vesicles (G ) in this specimen are largely

dissolved, but their lamellar character is shown to advantage in Figure 19.17b. X24,000.

Diagram of a type II alveolar cell and electron micrograph of lamellar bodies. a. Surfactant is an oily

mixture of proteins, phospholipids, and neutral lipids that are synthesized in the rER from precursors in the blood.

These precursors are glucose, fatty acids, choline, and amino acids. The protein constituents of surfactant are

produced in the rER and stored in the cytoplasm within lamellar bodies, which are discharged into the lumen of the

alveolus. With the aid of surfactant protein, surfactant is distributed on the surface of epithelial cells lining the

alveolus as a thin film that reduces the surface tension. b. Higher magnification electron micrograph showing the

typical lamellar pattern of the secretory vesicles of type II alveolar cells. These vesicles contain the pulmonary

surfactant precursor proteins. X38,000. ( Courtesy of Dr. A. Mercuri.)

* Surfactant decreases the alveolar surface tension and actively

participates in the clearance of foreign materials.

- The most specific phospholipid called dipalmitoylphosphatidylcholine

(DPPC), which accounts for almost all surface tension.

- Surfactant synthesis in the fetus occurs after the 35th week of gestation

and is modulated by a variety of hormones, including;

. Cortisol, insulin, prolactin, and thyroxine.

* Surfactant proteins help organize the surfactant layer and

modulate alveolar immune responses.- The hydrophobic proteins are necessary for the structure and function of

surfactant.

- These proteins are listed here:

• Surfactant protein A (SP-A), the most abundant surfactant protein.

- It modulates immune responses to viruses, bacteria, and fungi.

• Surfactant protein B (SP-B), an important protein for the transformation

of the lamellar body into the thin surface film of surfactant.

• Surfactant protein C (SP-C), maintenance of the thin film layer within the

alveoli.

• Surfactant protein D (SP-D), a primary protein involved in host defense.

- It binds to various microorganisms (e.g., Gram-negative bacteria) and to

lymphocytes.

- SP-D participates in a local inflammatory response to acute lung injury and

with SP-A modulates an allergic response to various inhaled antigens.

* The alveolar septum is the site of the air–blood barrier.- The air–blood barrier refers to the cells and cell products across which

gases must diffuse between the alveolar and capillary compartments.

- The thinnest air–blood barrier consists of

. a thin layer of surfactant,

. a type I epithelial cell and its basal lamina, and

. a capillary endothelial cell and its basal lamina.

- Often, these two basal laminae are fused.

- Connective tissue cells and fibers that may be present between the two basal

laminae.

- These two arrangements produce a thin portion and a thick portion of the

barrier.

- Most gas exchange occurs across the thin portion of the barrier.

. The thick portion is thought to be a site in which tissue fluid can

accumulate and even cross into the alveolus.

- Lymphatic vessels in the connective tissue of the terminal bronchioles drain

fluid that accumulates in the thick portion of the septum.

Diagram of the interalveolar septum. This diagram shows the thick and thin portions of the interalveolar

septum. The thin portion forms the air–blood barrier and is responsible for most of the gas exchange that occurs in the

lung. Arrows indicate the direction of CO2 and O2 exchange between the alveolar air space and the blood. The thick

portion of the interalveolar septum plays an important role in fluid distribution and its dynamics. It contains

connective tissue cells. Note the macrophage in the thick portion that extends its processes into the lumen of the

alveolus.

Electron micrograph of the alveolarseptum. This high-magnification micrographshows the thin portion of the air– blood barrierwhere it consists of type I alveolar cells, capillaryendothelium, and the fused basal lamina sharedby both cells. In the thick portion, the type Ialveolar cell (arrows) rests on a basal lamina, andon the opposite side is connective tissue in whichcollagen fibrils and elastic fibers are evident.X33.000

* Found on surface of alveoli, within alveoli and in interstitial

connective tissue.

* Remove debris and particles that escape mucus and cilia in

conducting portion of respiratory tract

Alveolar Macrophages (Dust Cells)

Macrophages (dust cells)

Pleura

* The outer surface of the lung and the inner surface of the thoracic

cavity are covered by the pleura, which is a serous membrane

(serosa).

* Parietal pleura lines the thoracic cavity;

* visceral pleura covers the lungs.

* Serous membranes consist of simple squamous epithelial cells

called mesothelium plus a thin layer of connective tissue.

* The pleural cavity contains serous fluid made by the pleura.

Parietal and Visceral Pleura