The Respiratory System

Pulmonary Anatomy and Physiology
The Respiratory System
Functions to supply the body with O2 and remove CO2
There are actually 4 distinct processes:
Ventilation – Movement of air into & out of the lungs
External Respiration – Gas exchange between blood and air-
filled chambers of the lungs
Transport of Gases – Accomplished by Cardiovascular system
Internal Respiration – Gas exchange between systemic blood
and the tissue cells
Organs include: nose,
nasal cavity, pharynx,
trachea, bronchi,
bronchioles, and the
Divided into Respiratory
and Conducting Zones.
Gas exchange with the
blood occurs in the
respiratory zones. It
does NOT occur in the
conducting zones.
The conducting zones
transport, cleanse,
warm and humidify the
incoming air.
The Upper Airway
 Oral Cavity
 Pharynx
 Larynx
The Nose
Only externally visible part of the respiratory
 Functions include:
– Providing an airway for respiration- Conduct
– Moistening and warming air- Warm and Humidify
– Filtering inspired air- Protect
– Serving as a resonating center for speech
– Housing the olfactory receptors.
Skeletal Framework of
External Nose
Fashioned by the:
– Nasal and frontal
bones superiorly
– Maxillary bones
– Plates of hyaline
cartilage (lateral,
septal, and alar
Nasal Cavity
Lies in and posterior to the
external nose
Divided by a midline nasal
septum – formed anteriorly
by septal cartilage and
posteriorly by the vomer
bone and perpendicular
plate of the ethmoid bone.
Continuous with the
nasopharynx via the
internal nares.
Roof is formed by the
sphenoid & ethmoid
Nasal Cavity
Floor is formed
by the palate.
– Hard palate
contains portions
of the maxillary
and palatine
– Soft palate lacks
bone, a flexible
mass of collagen
Nasal Cavity
Lined by 2 types of
– Slit-like superior region is
lined by olfactory
 What does it do?
– The rest is line by
respiratory epithelium
(pseudostratified ciliated
columnar with goblet
 It rests on a connective
tissue layer richly
supplied with mucous
and serous glands.
Nasal Cavity
Produces 1 quart of mucus per day.
Mucus vs Sputum
Sputum is matter that is coughed up from
the respiratory tract, such as mucus or
phlegm, mixed with saliva and then
expectorated from the mouth.
 Mucus is a slippery secretion of the lining
of the mucous membranes in the body..
Mucus is produced by goblet cells in the
mucous membranes that cover the surfaces
of the membranes.
Mucus is produced by goblet cells in the
mucous membranes that cover the
surfaces of the membranes. It is made up
of mucins and inorganic salts suspended
in water. Contains Lysosomes.
 Phlegm is a type of mucus that is
restricted to the respiratory tract, while
the term mucus refers to secretions of the
nasal passages as well.
High H2O content of mucus humidifies
inward air
 Ciliary current moves mucus to pharynx
for swallowing.
– Cold temps disable these cilia; runny nose
Rich plexuses of capillaries and veins
underlie the nasal epithelium and warm
incoming air
Protruding medially from
each lateral wall of the
nasal cavity are 3 scrolllike, mucosa-covered
projections: the superior,
middle, and inferior
conchae or turbinates
They increase the mucosal
surface area exposed to air
The groove inferior to each
concha is a meatus.
Nasal cavity is surrounded
by a ring of paranasal
sinuses located in the
frontal, sphenoid, ethmoid
and maxillary bones.
Nasal Cavity
Anatomy and Physiology Revealed
How Does this System function..
Air first enters the nares into a slightly
dilated area called the vestibule. The
vestibule is lined with hairs called
vibrissae, a protective mechanism
against foreign particles. PROTECT
 The anterior 1/3rd of the nasal cavity is
lined with stratified squamous epithelium,
posterior 2/3rd lined with pseudostratified
ciliated columnar epithelium
How Does this System function..
Air then travels through the turbinates
(conchae) where the function is to
separate inspired air into separate
 This allows for an increase of surface area.
 This increased surface area increases the
temperature of the air and adds moisture.
Oral Cavity
Accessory Respiratory Passage
– Lined with stratified squamous epithelium
– Air enters the vestibule (small outer portion
between gums and lips) and large opening
that extends to the back of the oropharynx
– Roof of the oral cavity
 Hard Palate
 Soft Palate
 Uvula
Oral Cavity
Soft palate rises, shutting off the passage
between the nasal and oral cavity
– Levator veli palatinum muscle draws up and back
– Palatopharyngeal muscle draws down and forward
Palantine Arches
– Palatopharyngeal Arch
– Palatoglossal Arch
Contains the tonsils, adenoids and lymph
tissue; Front line protection
 Connects the nasal
cavity and mouth
superiorly to the
larynx and
esophagus inferiorly
 3 regions. From
superior to inferior:
– Nasopharynx
– Oropharynx
– Laryngopharynx
Posterior portion of the nasal
cavity, superior portion of the
soft palate, contains
 Only an air passage. During
swallowing, the soft palate
and its uvula move superiorly
and close it off.
 Lined by pseudostratified
ciliated columnar epithelium.
 High on its posterior wall is
the pharyngeal tonsil
(adenoids) which traps
entering pathogens.
 The eustachian tubes open
into its lateral walls. They
connect the middle ear to
the nasal cavity; pressure
release function
Lies posterior to the oral
Extends from the soft
palate to the base of the
tongue – hyoid bone
Lined by stratified
squamous epithelium
Paired palatine tonsils lie in
the lateral walls while the
lingual tonsil covers the
base of the tongue
Also called “hypopharynx”
Extends from the base of
the tongue to the entrance
of the esophagus.
Common passage for both
food and air
Lined by stratified
squamous epithelium
Superiorly attached to the hyoid bone and
opens into the laryngopharynx.
 Inferiorly, it’s continuous with the trachea
 Main tasks are:
– Provision of a patent airway for air and food.
– Routing of air and food to proper pathways.
– Voice production.
Consists of an intricate arrangement of 9
cartilages connected by membranes and
 3 single cartilages
– Epiglottis, Cricoid, and Thyroid
3 paired cartilages
– Cuneiform, corniculate and arytenoid
The large, shield-shaped thyroid
cartilage is formed by the fusion of
2 cartilage plates.
– The fusion point is the laryngeal
prominence (adam’s apple).
The ridge is called the thyroid
Inferior to the thyroid cartilage is
the cricoid cartilage. Signet ring
shape with increased size to the
posterior. Cricoid membrane is the
site for emergent airways.
– The first “C” shaped tracheal
ring lies below the cricoid
3 pairs of small cartilages, the
arytenoid, cuneiform, & corniculate
cartilages form part of the lateral &
posterior walls of the larynx
The important arytenoids anchor
the vocal cords
The 9th cartilage (epiglottis)
is spoon-shaped &
composed of elastic
Epiglottis is covered almost
entirely by a taste-bud
containing mucosa.
During swallowing, the
larynx is pulled superiorly
and the epiglottis tips to
cover the laryngeal inlet.
If anything other than air
enters the larynx – a
cough/gag reflex is initiated
by the sensory nerve;
glossopharyngeal and the
motor nerve; vagus.
Epiglottis and Vallecula
The space between the base of the
tongue and the epiglottis is called the
 This is an important landmark in the
– While intubating, if using a macintosh blade
the tip of the blade slides into the vallecula
causing the epiglottis to lift. If using a miller
blade, the epiglottis is directly lifted up to
allow access to the airway.
Lying under the laryngeal mucosa on
each side are the vocal ligaments
These ligaments (made mostly of
elastic fibers) form the core of
mucosal folds called the vocal folds or
true vocal cords
 Vocal cords vibrate, producing sounds
as air rushes up from the lungs.
 Superior to the true vocal cords is a
similar pair of mucosal folds called the
vestibular folds or false vocal cords.
 The superior portion of the larynx is
lined by stratified squamous
epithelium, while below the vocal
cords, it’s a pseudostratifed ciliated
columnar epithelium.
The medial opening between
them thru which the air passes
is the rima glottidis or
 In an adult, the glottis is the
narrowest point of the adult
 In an infant and small child,
the cricoid cartilage is the
narrowest point.
 Subglottic swelling in an infant
or small child, due to infection
or trauma can cause stridor
during inspiration
Epiglottitis vs Croup (LTB)
Croup – Steeple Sign
Epiglottitis – Thumb Sign
The larynx is closed by the epiglottis during
In addition to opening and closing the glottis for
speech, the vocal folds can act as a sphincter during
conditions such as coughing, sneezing or straining
The Arytenoid cartilage
is shaped like a pyramid
which rests on the
posterior portion of the
cricoid cartilage
At the base of the
arytenoid cartilage a
projection called the
vocal process, the vocal
ligaments attach vocal
process and the thyroid
The cuneiform and
corniculate are
accessory cartilages at
the superior portion of
the arytenoids
Animated Airways..
Laryngeal Musculature
– Infrahyoid (below the hyoid)
– Pulls the larynx and hyoid down the neck
 Sternohyoid, sternothyroid, throhyoid and
– Suprahyoid (above the hyoid)
– Pulls the hyoid bone forwards, upwards and
 Stylohyoid, mylohyoid, digastric, geniohyoid and
Laryngeal Musculature
– They all deal with the arytenoid cartilage and
vocal cord movement.
– Posterior cricoarytenoid, lateral cricoarytenoid,
transverse, thyroarytenoid and cricothyroid.
Ventilatory Function of the
Ensures a free flow of air to and from the lungs
 During inspiration, vocal cords move apart; abduct,
and widens glottis for improved airflow
 Forced expiration against a closed glottis, “Valsalva’s
maneuver” causes massive adduction preventing air
from escaping during cough, vomiting, urination,
defecation and parturition.
 Forced inspiration against a closed glottis, “Mueller
maneuver” --- missed sputum bowl question!
The Lower Airways
After passing through the larynx, inspired
air enters the Tracheobronchial Tree
 The traceobronchial tree consists of a
series of branching airways called “orders”
or “generations”
 It is believed that there are 28 generations
or orders of the tracheobroncial tree
Dichotomous Branching
Tracheobronchial Tree
The tracheobronchial tree is divided into two
general zones
– Conducting Zone or Cartilaginous Airways
 No Gas Exchange occurs in this zone
– Respiratory Zone or Non Cartilaginous
 The site of Gas Exchange
– There a transition zone where no cartilage
surrounds the airway, yet no gas exchange
Histology of the
Tracheobroncial Tree
Three layers
– Epithelial Lining
– Lamina propria
– Cartilaginous Layer
Epithelial Lining
Psuedostratified ciliated columnar epithelium
 Numerous Mucous glands interspersed
 Anchored to a basement membrane that
contains basal cells (reserve cells and
replenish mucus glands and ciliated cells)
 200 cilia per cell
 Cells move from columnar to cuboidal and
cilia disappear as you move down the tree
Epithelial Lining
A mucus layer, or “mucous blanket” covers the
epithelial lining of the tracheobronchial tree.
Produced by goblet cells and submucosal
/bronchial glands
Goblet cells are located between the epithelial
Submucosal glands extend into the lamina
propria and are innervated by the
parasympathetic nervous system.
Composed of 95% water and the remainder is
carbohydrates, glyocproteins, lipids, DNA,
cellular debris and foreign particles.
Mucous Blanket
The body produces about 100ml of
secretions per day.
 The viscosity of the secretions increase as
you move from the lining to the lumen.
 Two distinct layers
– SOL Layer, adjacent to the epithelial lining
 Less viscous
– GEL Layer, adjacent to the inner lumen
 More viscous
Mucous Blanket
Cilia move in a wavelike fashion, beating
1500 times per minute through the less
viscous sol layer and strike the inner layer of
the more viscous gel layer.
 This action propels the mucus layer, along
with any foreign particles attached to the
“sticky” gel layer towards the larynx at 2cm
per minute
 The cough mechanism moves the secretions
above the larynx and into the oropharynx
Mucous Blanket
This cleansing process is called Mucociliary
Transport or the Mucociliary Escalator
 What slows the rate down:
Cigarette smoke,
Positive Pressure Ventilation
Endotracheal Suctioning
High FiO2
Atmospheric pollutants
General anesthetics
Lamina Propria
This is a submucosal layer
Loose fibrous tissue containing blood vessels,
lymphatic vessels, vagus nerve innervation
Two sets of smooth muscle that wrap in spirals
both clockwise and counterclockwise
The smooth muscle fibers extend to the alveolar
The lamina propria is surrounded by a thin
connective tissue layer called the peribronchial
Lamina Propria
Mast Cells are located in the lamina propria
 Their cytoplasm is loaded with granules
containing mediators of inflammation.
– Histamine, heparin, SRS-A (slow reacting substance of
anaphylaxis), PAF (platelet activating factor), ECF-A
(eosinophilic chemotaxic factor of anaphylaxis)
Their surface is coated with a variety of
receptors which, when engaged by the
appropriate antigen trigger exocytosis of the
 Destablization of mast cells in the lungs can
be extremely dangerous, and is what we see in
patients with an allergic asthmatic episode
Cartilaginous Layer
Outermost layer of the tracheobronchial
 This layer progressively diminishes in size
as the airway extend into the lungs.
 Cartilage is absent in bronchioles less than
1 mm in diameter
Lower Airway
Cartilaginous Airways
 Main stem Bronchi
 Lobar Bronchi
 Segmental Bronchi
 Subsegmental Bronchi
In the adult, it is 11 to 13
cm long, 1.5 – 2.5 cm in
 Descends from the
cricoid to the 2nd costal
It bifurcates, divides into
the right and left main
stem bronchi, this division
is called the carina
There are 15-20 Cshaped rings of
cartilage that
support the trachea,
keeping the airway
patent and prevent
its collapse.
The open posterior
parts of the rings
are adjacent to the
the esophagus and
are connected by
fibers of the
trachealis muscle,
which is involved in
Main Stem and Lobar
The Main Stem Bronchi are the 1st generation
of the tracheobronchial tree
The right main stem bronchus branches off the
trachea at a 25 degree angle, the left bronchus
forms a 40 – 60 degree angle
The right bronchus is wider, more vertical, and
5 cm shorter than the left.
Main Stem and Lobar Bronchi
Main Stem bronchi are supported by ‘C’
shaped cartilage
Each bronchus runs obliquely into the
mediastinum before plunging into the medial
depression (hilus) of the lung on its own side.
Inside the lungs, the main stem or primary
bronchi divide into lobar or secondary bronchi,
3 on the right and 2 on the left, each of which
supplies one lung lobe.
3 lobes on the right, 2 lobes on the left (room
for heart)
Bronchi and Subdivisions
The lobar bronchi become segmental; third
– 10 in the right lung and 8 in the left lung
Subsegmental bronchi range in diameter from
1 – 4 mm. Peribronchial sheaths containing
nerves, vessels and lymphatic tissue surround
the subsegmental bronchi down to the 1 mm
– These are the 4th – 9th generation.
Non Cartilaginous Airways
– When the diameter decreases to less than
 they are no longer surrounded by a connective
 cartilage is absent, rigidity is absent-airway
patency can be compromised
 A muscle sheath surrounding the bronchioles
 Columnar epithelial becomes cuboidal
– Generations 10 - 15
Terminal Bronchioles
Diameter is about 0.5 mm
Cilia and mucous glands progressively disappear
Epithelium is cuboidal and thin
Channels called “Canals of Lambert” appear
 Connect the surface of terminal bronchioles to adjacent alveoli
 Thought to aid in collateral ventilation in individuals with
respiratory disorders such as COPD
– Presence of Clara Cells
 Function unknown, they have a thick protoplasmic extensions
that bulge into the bronchial lumen - perhaps they secrete an
enzyme that detoxifies inhaled substances
16th – 19th Generation;
 TERMINAL- END: Structures beyond this point are
not part of the Tracheobronchial tree.
 Structures distal to this point are the sites of gas
exchange; referred to as the Respiratory Zone
As conducting tubes become
The cartilage support changes. It goes from
rings in the trachea to irregular plates in the
bronchi to none in the bronchioles.
– Why?
The epithelium changes. It goes from respiratory
to simple columnar to simple cuboidal.
– Why?
The number of cilia and goblet cells present
– Why?
The amount of smooth muscle increases.
– Why?
Gas Flow
Once in the Respiratory Zone, the cross
sectional area of the lung increases
 Forward motion of gas flow stops-no bulk
flow, no laminar flow
 The movement of gas becomes molecular
Bronchial Blood Supply
Tracheobronchial tree requires blood flow
 Arteries follow the tracheobronchial tree as far
as the terminal bronchioles
 Beyond the terminal bronchioles, this vascular
system merges with the pulmonary vascular
– RA – RV– PA - Lungs – LA – LV –Aorta - Systemic
Bronchial Blood Supply
Approximately 1% of Cardiac Output serves
the tracheobronchial tree.
 Of that 1%, 1/3rd of it returns to the Right
Atrium as unoxygenated venous blood.
 The vessels responsible for this return
– Azygos
– Hemiazygos
– Intercostal veins
– RA – RV – PA - Lungs – LA – LV –Aorta - Systemic
The remaining 2/3rd of the bronchial venous
blood drains into the pulmonary circulation, into
the pulmonary arteries and capillaries
via bronchopulmonary anastomoses
 This results in a mixture of low oxygenated and
high carbon dioxide blood from the
tracheobronchial tree with highly oxygenated
low carbon dioxide blood returning to the left
atrium for systemic circulation
– RA – RV – PA - Lungs – LA – LV –Aorta – Systemic
2/3rd of blood returning from bronchical circulation
This is an example of an anatomical shunt, and called venous
The Respiratory Zone
Distal to the terminal bronchioles are the
functional units of gas exchange
 Consists of:
– 3 generations of Respiratory Bronchioles
– 3 generations of Alveolar Ducts
– Ending in 15-20 grapelike clusters called Alveolar Sacs
These units, Respiratory Bronchioles, Ducts and
Alveoli are called a primary lobule or acinus, or
terminal respiratory unit or lung parenchyma or
functional units
Composed of smooth muscle fiber
 Approximately 300 million alveoli which
are 90% covered with capillaries.
 The surface area of the alveoli is 70
square meter, the surface of a tennis court
 Each primary lobule, 130,000 each stem
from a single terminal bronchiole and
contains about 2000 alveoli
Alveolar Epithelium
Alveoli are consist of 3 cell types:
Type I cells; Squamous Epithelium
– Cover 95% of the alveolar surface
– 0.1 – 0.5 micrometers thick
– Major site of gas exchange
Type II cells; Granular pneumocytes
Have microvilli
Cuboidal in shape
Produce pulmonary surfactant
Form the remaining 5% of the alveolar surface
Type III cells; Alveolar macrophages
– Migrate through the blood stream and are embedded in
the extracellular lining of the alveoli
Pores of Kohn
Pores of Kohn
– Small holes in the alveolar wall or
interalveolar septa
– Allow gas to move between alveoli
– Formed by
 Shedding of cells – desquamation
 Normal degeneration due to age
 Movement/detachment of macrophages
among the Type
I’s are Type II
cells which
Pores of Kohn
adjacent alveoli.
(dust cells)
crawl along the
internal alveolar
Surrounds, supports and shapes the alveolar
capillary clusters
 Gel like substance of hyaluronic acid molecules
bound together by a network of collagen fibers
 Two compartments
– Tight Space; the area between the alveolar epithelium
and the endothelium of the pulmonary capillaries-Site of
gas exchange
– Loose Space: the area the surround the bronchioles,
respiratory bronchioles, alveolar ducts and sacs.
Lymphatic vessels and neural fibers are in this area
The Pulmonary Vascular System
Function is to deliver blood to and from
the lungs for gas exchange
 It also supplies nutrition to portions of the
lung distal to the terminal airways
Pulmonary Vascular System
 Arterioles
 Capillaries
 Venules
 Veins
Pulmonary Arteries
Three Layer
– Tunica Intima; inner layer
 Endothelium and a thin layer of connective tissue
– Tunica Media; middle layer
 Thickest layer of the vessel
 Elastic connective tissue in large arteries, smooth muscles in
smaller and medium arteries
– Tunica Adventitia: outer layer
 Connective tissue
 Contains small vessels that nourish all layers
Stiff vessels, capable of carrying blood under
high pressures.
Endothelial layer
 Elastic layer
 Smooth muscle fibers
Called “resistance vessels” due to the
ability of the smooth muscles to regulate
the blood flow
Surround 90% of the alveoli
 Composed of endothelial layer; single
layer of squamous epithelial cells
 Essentially an extension of the inner lining
of the larger vessels
 This is where gas exchange occurs
 Some prostaglandins are produced here,
some biological substances are destroyed
Veins and Venules
3 Layers, same as arteries, 2 layers in the
smaller veins-no tunica adventitia
 Middle layer is poorly developed and contain
less smooth muscle and elastic tissues
 Due to less elastic and smooth muscle tissue,
veins can hold a greater volume of blood with
little pressure changes. Because of this, veins
are called “capacitance vessels”
 Veins return to the heart in a more direct route
out of the lungs
The Lymphatic System
Function is to remove excess fluid and protein that
leak out of the capillaries
Located in a dense connective tissue sheath
around the bronchioles, also in the loose space of
the interstitium
More lymphatic channels located on the left side,
increased incidence of right sided pleural effusions
due to less drainage
Like veins, they have one way valves/flaps
Bronchopulmonary lymph nodes, end of the line,
are located outside the lung parenchyma
No lymph vessels in the alveoli, they are located
immediately adjacent to the alveoli called
juxta-alveolar lymphatics
Neural Control of the Lungs
Controlled by the autonomic nervous
– Regulates involuntary vital functions
 Cardiac muscles
 Smooth muscles
 Glands
– Two divisions
 Sympathetic
 Parasympathetic
Neural Control of the Lungs
– Accelerates heart rate
– Constricts blood vessels
– Relaxes bronchial smooth muscles
– Raises blood pressure
– Slows heart rate
– Constricts bronchial smooth muscles
– Increases intestinal peristalsis and gland activity
Neural Control of the Lungs
Sympathetic neural transmitters
– Epinephrine
– Norepinehrine
These agents stimulate the
– Beta 2 receptors in the bronchial smooth
muscles; causing airway muscle relaxation
– Alpha receptors in the arteriole smooth
muscles, causing the pulmonary vascular
system to contrict
Neural Control of the Lungs
Parasympathetic neural transmitters
– Acetylcholine
 Causes constriction of the bronchial smooth muscles
Inactivity of either portion allows the other one to
dominate the bronchial smooth muscles
 There must be careful attention to the role of
pharmacological agents
– Beta blockers- causes parasympathetic to dominate
– Atropine-a parasympathetic blocker allows sympathetic
to dominate
Lung Gross Anatomy
Occupies all of the thoracic cavity except the
Each lung is within its own pleural cavity
Anterior, lateral, and posterior surfaces are costal,
adjacent to ribs
Rises above the clavicle to the level of the 1st rib
The concave bases sit on the diaphragm.
The mediastinal border is concave to heart and
the other mediastinal structure
The hilum is at the center of the mediastinal
border, and is where the main stem bronchi, blood
and lymph vessels, and nerves enter and exit the
Lung Gross Anatomy
The right lung is larger
and heavier than the left.
The Right lung has 3
lobes; upper, middle, and
 The lobes are divided by
– oblique fissure which
divides the upper and
middle lobe from the
lower lobe
– Horizontal fissure,
divides the upper from
the lower lobe.
Lung Gross Anatomy
The left lung is
smaller than the right,
contains 2 lobes;
upper and a lower
lobe and has an
indentation (cardiac
notch) where the
heart sits.
– The lobes are
divided by the
oblique fissure
Lung Segments
All Lobes are
further divided into
 10 on the right
 8 on the left
 Careful with the
A cavity that contains the organs and tissues
in the center of the thoracic cage between
the right and left lung
 Bordered anteriorly by the sternum, and
posteriorly by the vertabrae
 Contains the trachea, heart, the great vessels
the major vessels that enter and exit the
heart, the esophagus, thymus gland, lymph
nodes, and nerves
The Pleural Membranes
Two moist, slick surfaced membranes
Parietal pleura covers the thoracic wall, superior
diaphragm and lateral portion of the mediastinum
Visceral pleura firmly attached and covers the external
lung surface, extends into the interlobar fissures
The potential space between visceral and parietal
pleurae is called the pleural cavity
The two membranes are held together by a thin film
of serous fluid. The fluid allows the membranes to
glide over each other during inspiration and exhalation
The pleural membranes hold the lung tissue to the
inner surface of the thorax and diaphragm, allowing
for lung expansion during inspiration
Because the lungs
have a natural
tendency to collapse
and the thorax has a
natural tendency to
expand, a negative
pressure normally
exists between these
two layers. Should
air enter this space,
the pleural
membranes would
separate causing a
condition known as a
The Diaphragm
The diaphragm is the major muscle of
Dome-shaped musculofibrous partition located
between the thoracic cavity and the abdominal
Two separate muscles; the right and left
hemidiaphragm, joined at midline by the central
Pierced by the esophagus, aorta, nerves and the
inferior vena cava
Innervated mainly by the phrenic nerve, the lower
thoracic nerves contribute to some motor
When the diaphragm is
stimulated to contract, it
moves downward and the
lower ribs move upward
and outward
 This increases the thoracic
 Which causes the lung
volume to increase
 The increased lung volume
causes lung pressure;
intrapleural and intra
alveolar to decrease
 As a result, gas from the
atmosphere flows into the
During expiration, the
diaphragm relaxes and
moves upward into the
thoracic cavity
 This increases the
intrapleural and intraalveolar pressures and
causes gas to flow out of
the lungs
 Quiet expiration is a passive
process that is due to the
elasticity of the lungs.
 Forced expiration is an
active process due to
contraction of oblique and
transverse abdominus
muscles, internal
intercostals, and the
latissimus dorsi.