The Anatomy & Physiology of the Respiratory System
REVIEW OF EPITHELIAL TISSUE TYPES
THE AIRWAYS
The passageway between the external environment and the gas exchange areas are
called the conducting airways. The conducting airways are divided into the upper and
lower airways.
THE UPPER AIRWAY
The upper airway consists of the nose, the oral cavity, the pharynx, and the larynx.

Primary Functions
o Conductor of air
o Humidify and warm the inspired air
o Prevent foreign materials from entering the tracheobronchial tree
o Important area for speech and smell
The Nose
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Primary Functions
o Filter
o Warm
o Humidify
Structure
o The nose consists of a combination of bones and cartilages.
 Nasal Septum
 Divides the nasal cavity into two equal chambers
 Formed by the perpendicular plate of the ethmoid bone, the
vomer, and the septal cartilage
Nasal Cavity
o Epithelial Tissue
 Anterior 1/3 lined with stratified squamous epithelium
 Posterior 2/3 lined with pseudostratified ciliated columnar
epithelium

Cilia propel nasal mucous towards the nasopharynx
The Oral Cavity
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Location
o The oral cavity extends from the end of the nasal passage to the pharynx.
Structures
o Teeth
o Hard Palate
o Soft Palate
 A flexible mass of densely packed collagen fibers that ends with the
soft fleshy process called the uvula
 During swallowing, the soft palate closes off the opening between
the nasal and oral pharynx by moving upward and backward
o Tongue
 Anterior ⅔ of the tongue
o Palatine Tonsils
 Have a role in immunologic defense
Epithelial Tissue
o Stratified squamous epithelium
The oral cavity is essential for speech and taste functions.
The Pharynx
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Location
o The pharynx lies between the oral cavity and the larynx, or voice box
The pharynx consists of the nasopharynx, oropharynx, and laryngopharynx.
Nasopharynx
o Location
 Posterior to the nasal cavity and superior to the soft palate at the
back of the oral cavity
o Epithelial tissue
 Pseudostratified ciliated columnar epithelium
o Structures
 Posterior ⅓ of the tongue
 Pharyngeal tonsils
 Opening for eustachian tubes
 Connect the nasopharynx to the middle ear
o Equalizes pressure in the middle ear
Oropharynx
o Location
 Lies between the soft palate superiorly and the base of the tongue
inferiorly
o Epithelial tissue
o Stratified squamous epithelium
o Structures
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 Lingual tonsil
Laryngopharynx
o Location
 Lies between the base of the tongue and the entrance to the
esophagus
o Epithelial tissue
 Stratified squamous epithelium
o Function
 Laryngopharyngeal musculature
 Stimulation of these muscles and nerves produce the “gag”
or “swallowing” reflex
o Helps to prevent the aspiration of foods and solids
o Helps to prevent the base of the tongue from
obstructing the airway
The Larynx
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Location
o The larynx is located between the base of the tongue and the upper
end of the trachea.
Function
o Allows air to pass between the pharynx and the trachea
o Protects against the aspiration of solids and liquids
o Generates sounds for speech
Structure
o Cartilages
 3 single cartilages
 Thyroid – largest
 Cricoid
 Epiglottis
o Covers the larynx during swallowing to prevent
aspiration of foods and solids
 3 paired cartilages
 Arytenoids
 Corniculate
 Cuneiform
 The cartilages are held in place by ligaments, membranes, and
intrinsic and extrinsic muscles
o Mucous Membrane
 Forms two pairs of folds:
 False vocal folds
 True vocal folds
o Produce speech.
o Rima glottides (Glottis)
 The space between the true vocal folds
o Vallecula
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The small space between the epiglottis and the base of the
tongue
 Important anatomical landmark for endotracheal intubation
o Laryngeal Musculature
 The larynx also contains a complex system of muscle groups
that control the movement of the vocal cords.
Epithelial tissue
o Above the vocal cords
 Stratified squamous epithelium
o Below the vocal cords
 Pseudostratified ciliated columnar epithelium
Ventilatory Function
o Free flow of air to and from the lungs
 During quiet inspiration, the vocal folds move apart (abduct) and
widen the glottis
 During exhalation, the vocal folds move toward midline slightly
(adduct), but always maintain an open airway
o Valsalva’s Maneuver
 Prevents air from escaping during physical exertion
 Lifting, pushing, coughing, vomiting, urination, defecation,
parturition
THE LOWER AIRWAYS
The Tracheobronchial Tree
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Description
o A series of branching airways referred to as generations or orders
o These airways progressively narrow, shorten, and become more
numerous as they progress throughout the lungs
o Two major forms
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Cartilaginous airways
 Serve only to conduct air from the external environment to
the sites of gas exchange
 The cartilaginous airways consist of the:
o Trachea
o Main stem bronchi
o Lobar bronchi
o Segmental bronchi
o Subsegmental bronchi
Noncartilagenous
 Serve both as conductors of air and as sites of gas
exchange
 The noncartilaginous airways consist of the:
o Bronchioles
o Terminal bronchioles
Histology
o Epithelial lining
 Pseudostratified ciliated columnar epithelium
 Extends from the trachea down to and including the terminal
bronchioles
 Cilia
o 200 cilia per ciliated cell
o 5 to 7 microns in length
 The cilia progressively disappear in the terminal bronchioles
and are completely absent in the respiratory bronchioles
 Numerous mucous glands
 Separated from the lamina propria by a basement membrane
 Basal cells along the basement membrane serve to replenish the
ciliated cells and mucous cells as needed
o Mucous blanket
 Covers the epithelial lining of the T-B tree
 Composition
 95% water
 5% glycoproteins, carbohydrates, lipids, DNA, cellular
debris, and foreign particles
 Mucus is produced by:
 Goblet cells
o Located intermittently between the epithelial cells
reaching down to and including the terminal
bronchioles
 Submucosal glands
o Produce most of the mucous blanket
o Extend deep into the lamina propria
o Produce about 100 mL of bronchial secretions per
day
o Numerous in the medium sized bronchi and disappear
in the distal terminal bronchioles
o Innervated by vagal parasympathetic nerve fibers
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Structure
 Sol layer
o Adjacent to the epithelial lining
o Thin, watery layer
 Gel layer
o Viscous layer adjacent to the inner luminal surface
Mucociliary Transport Mechanism (Mucociliary Escalator)
 Process
o The cilia move in a wave-like fashion through the less
viscous sol layer and strike the innermost portion of
the gel layer
o This action propels the mucus layer, along with any
foreign particles stuck to the gel layer, toward the
larynx at a rate of 2 cm/min.
o At the larynx, the cough mechanism moves the
secretions beyond the larynx, into the oropharynx
where they are expectorated or swallowed
Factors that Slow the Rate of Mucociliary Transport
Cigarette smoke
Dehydration
Positive pressure ventilation
Endotracheal suctioning
High FiO2
Hypoxia
Atmospheric pollutants
General anesthesia
Parasympatholytics (Atropine)
o Lamina Propria
 Contents
 Loose fibrous tissue that contains:
o Tiny blood vessels
o Lymphatic vessels
o Branches of the vagus nerve
o Mast cells
o Submucosal glands
 Smooth muscle
o Two sets of smooth muscle fibers wrap around the TB tree in fairly close spirals, one clockwise, one
counter clockwise
o Smooth muscle extends down to and including the
alveolar ducts
 Peribronchial Sheath
o The outer portion is surrounded by a thin connective
tissue layer called the peribronchial sheath
 Mast Cells
o Function
 Play an important role in the immunologic
mechanism
o Location
 Airways, skin, intestines
o Process
 When activated, numerous substances are
released that significantly alter the diameter of
the bronchial airways
 Histamine
 Heparin
 Slow-reacting substance of anaphylaxis
(SRSA)
 Platelet-activating factor (PAF)
 Eosinophilic chemotactic factor of
anaphylaxis (ECFA)
o Consequences of mast cell degranulation
 Increased vascular permeability
 Smooth muscle contraction
 Increased mucus secretion
 Vasodilation
 Edema
o Cartilaginous Layer
 Description
 The outermost layer of the T-B tree
 Present in the trachea and all levels of the bronchi
The Cartilaginous Airways
Structure
Trachea
Generation
0
Main Stem Bronchi
1
Lobar Bronchi
2
Segmental Bronchi
3
Subsegmental Bronchi
4-9
The Noncartilaginous Airways
Description
Length:11-13 cm long
Diameter: 1.5-2.5 cm
15-20 C-shaped cartilages
Left Main Stem Bronchus:
Branches at 40-60˚
Right Main Stem Bronchus:
Branches at 25˚
Shorter and wider than the
left main stem bronchus
Both have C-shaped
cartilages
Right Lung: upper, middle,
and lower lobar bronchi
Left Lung: upper and lower
lobar bronchi
Supported by cartilaginous
plates
Right Lung: 10 bronchi
Left Lung: 8 bronchi
Cartilaginous plates
Diameter: 1-4 mm
Cartilaginous plates
Structure
Bronchioles
Generation
10 -15
Terminal Bronchioles
16 - 19
Description
Surrounded by spiral muscle
fibers
Diameter: < 1 mm
Diameter: 0.5 mm
Canals of Lambert: pathway
to alveoli
Clara Cells: secretory and
enzymatic function
Bronchial Blood Supply
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Bronchial arteries provide blood supply to the tracheobronchial tree to support
the lung’s own metabolic needs.
Arise from the aorta and follow the T-B tree as far as the terminal bronchioles
where they merge with the pulmonary arteries and capillaries.
Some bronchial venous blood (about ⅓) returns to the right atrium of the heart
through the azygos, hemiazygos, and intercostal veins.
The remainder (about ⅔) drains into the pulmonary circulation via
bronchopulmonary anastomoses and then to the left atrium via the pulmonary
veins.
SITES OF GAS EXCHANGE
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Gas exchange occurs in structures distal to the terminal bronchioles.
Structure
Respiratory Bronchioles
Generation
20 – 23
Alveolar ducts
24 – 27
Alveolar Sacs
28
Description
Have alveoli budding from
their walls
Alveoli separated by septal
walls that contain smooth
muscle fibers
Amount:300 million alveoli in
the adult
Diameter: 75 - 300 microns
Collectively, these three structures are known as a primary lobule. Each lobule
contains approximately 2,000 alveoli.
Alveolar Epithelium
o Type I Cells
 Simple squamous pneumocytes
 95% of the alveolar surface
 Major sites of gas exchange
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o Type II Cells
 Cuboidal and have microvilli
 Source of pulmonary surfactant
 decreases the surface tension of the fluid that lines the
alveoli
o prevents alveolar collapse and facilitates gas
exchange
o Pores of Kohn
 small holes in the walls of the septa between alveoli
 permit gas to move between adjacent alveoli
 number and size of pores increase with age
 formation is accelerated with lung disease
o Type III Cells (Alveolar Macrophages)
 help to remove bacteria and other foreign matter from the primary
lobule
Interstitium
o Description
 A gel-like substance that supports and shapes the alveolar-capillary
clusters
 Consists of a network of collagen fibers and hyaluronic acid
molecules
o Types
 Tight-space
 Located between the alveoli and the capillary walls
 Loose-space
 Surrounds the bronchioles, respiratory bronchioles, alveolar
ducts and sacs
o Function
 Limits alveolar distension that may cause:
 Pulmonary capillary occlusion
 Damage to the alveolar walls
THE PULMONARY VASCULAR SYSTEM
The pulmonary vascular system delivers blood to and from lungs for gas exchange, and
provides nutrition to the structures distal to the terminal bronchioles.
This blood supply system is made up of the following structures.

Arteries
o Pathway
 The right ventricle of the heart pumps deoxygenated blood into the
pulmonary trunk
 The pulmonary trunk divides into the right and left pulmonary
arteries
 These arteries penetrate their respective lung through the hilum
 The pulmonary arteries follow the T-B tree
o Structure
 There are 3 layers of tissue in their walls:
 Tunica intima
o The inner layer composed of endothelium and a thin
layer of connective and elastic tissue
 Tunica media
o The middle layer composed of elastic connective
tissue in large arteries and smooth muscle in mediumsized to small arteries
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o Thickest layer
 Tunica adventitia
o The outermost layer composed of connective tissue
o Contains small blood vessels that nourish all three
layers
o Arteries are relatively stiff vessels well suited for carrying blood under high
pressure
Arterioles
o Structure
 Endothelial layer
 Elastic layer
 Smooth muscle layer
o Function
 supply nutrients to the respiratory bronchioles, alveolar ducts and
alveoli
 have an important role in distributing and regulating blood flow
 arterioles are called the resistance vessels
Capillaries
o Structure
 A complex network of small vessels that surround the alveoli
 Composed of an endothelial layer which consists of a single layer
of squamous epithelial cells
o Function
 This is where gas exchange occurs
 Has a selective permeability to substances such as water,
electrolytes, and sugars
 Play an important role in the production and destruction of a broad
range of biologically active substances
Venules and Veins
o Structure
 The venules are tiny veins continuous with the pulmonary
capillaries
 Veins have three layers of tissue in their walls similar to the arteries
 Smaller veins lack the tunica adventitia
 Veins have thinner walls and contain less smooth muscle and
elastic tissue
 The tunica media is poorly developed
o Pathway
 The venules empty into the veins which carry blood back to the
heart
 The veins take a more direct route out of the lungs by moving away
from the bronchi
 The veins merge into two large veins and exit through the hilum
 The four pulmonary veins carrying oxygenated blood then empty
into the left atrium of the heart
o Function
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Veins are capable of collecting a large amount of blood with very
little pressure change
Veins are called the capacitance vessels
THE LYMPHATIC SYSTEM
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Location
o Lymph vessels
 Found around the lungs just beneath the visceral pleura and in the
connective tissue wrapping of the bronchioles, bronchi, pulmonary
arteries and pulmonary
 There are more vessels on the surface of the lower lung lobes than
the upper or middle lobes
 The lymphatic channels on the left lower lung lobe are more
numerous and larger in diameter than that of the right lower lung
lobe
 More fluid will collect in the right lower lobe as compared to
the left lower lobe
o This is called a pleural effusion
 The vessels end in pulmonary and bronchopulmonary lymph nodes
located just inside and outside the lung parenchyma
o Lymph nodes
 Collections of lymphatic tissue that are interspersed along the
lymphatic stream
Function
o Lymph Vessels
 The primary function of the lymphatic vessels is to remove excess
fluid and protein molecules that leak out of the pulmonary
capillaries
 The smooth muscle activity of the vessels and the cyclic pressure
changes generated in the thoracic cavity move lymphatic fluid
towards the hilum
o Lymph Nodes
 Produce lymphocytes and monocytes that attack bacteria or other
foreign matter
 Act as filters to keep particulate matter and bacteria from entering
the bloodstream.
NEURAL CONTROL OF THE LUNGS
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The muscle tone of the bronchi and arteries of the lungs is controlled by the
autonomic nervous system, which regulates involuntary vital functions in the
body
The autonomic nervous system consists of the:
o Sympathetic nervous system
 increases heart rate
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 constricts blood vessels
 raises blood pressure
 relaxes bronchial smooth muscle
o Parasympathetic nervous system
 decreases heart rate
 constricts bronchial smooth muscle
 increases intestinal peristalsis and gland activity
Changes in respiratory function are triggered by neural transmitters
o Epinephrine, and norepinephrine are released by the sympathetic nervous
system causing stimulation of:
 Beta 2 receptors which produces bronchial smooth muscle
relaxation, thus opening the bronchial passages.
 Alpha1 receptors which constricts the smooth muscle of the
arterioles
o Acetylcholine is released by the parasympathetic nervous system causing
 the bronchial muscles to constrict, thus narrowing the bronchial
passages
Inactivity of either the sympathetic or parasympathetic nervous system allows the
action of the other to dominate the bronchial smooth muscle response
THE LUNGS
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Anatomical Description
o Apex
 rise to the level of the 1st rib
o Base
 extends anteriorly to about the level of the 6th rib (xiphoid process)
o The mediastinal border
 concave to fit the heart and other mediastinal structures
o Hilum
 The area where the mainstem bronchi, vessels and nerves enter
and exit the lungs
o Right lung
 Larger and heavier than the left lung
 Divided into upper, middle, and lower lobes
 Oblique fissure separates the middle and lower lobes
 Horizontal fissure separates the upper and middle lobes
 Lobes are further divided into bronchopulmonary segments
o Left Lung
 Divided into upper and lower lobes
 Oblique fissure separates the lobes
 Lobes are further divided into bronchopulmonary segments
Bronchopulmonary segments
Right Lung
Segment Left Lung
Upper Lobe
Upper Lobe
Segment
Apical
Posterior
Anterior
Middle Lobe
Lateral
Medial
Lower Lobe
Superior
Medial Basal
Anterior Basal
Lateral Basal
Posterior Basal
1
2
3
4
5
6
7
8
9
10
Upper Division
Apical/Posterior
Anterior
Lower Division
Superior lingula
Inferior lingula
Lower Lobe
Superior
Anterior medial
Lateral basal
Posterior basal
1&2
3
4
5
6
7&8
9
10
THE MEDIASTINUM
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The cavity that contains the organs and tissues in the center of the thoracic cage
between the right and left lungs
Contains the trachea, the heart, the major blood vessels that enter and exit from
the heart, various nerves, portions of the esophagus, the thymus gland, and
lymph nodes
If the mediastinum is compressed or distorted, it can severely compromise the
cardiopulmonary system
THE PLEURAL MEMBRANES
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Visceral pleura
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o Attached to the outside surface of each lung and extends into each
interlobar fissure
Parietal pleura
o Lines the inside of the thoracic walls, the thoracic surface of the
diaphragm, and the lateral surface of the mediastinum
Pleural cavity
o The potential space between the visceral and parietal pleurae
The visceral and parietal pleurae are held together by a thin film of serous fluid
o Allows the two membranes to glide over each other during inspiration and
expiration
During inspiration the pleural membranes hold the lung tissue to the inner
surface of the thorax and diaphragm, causing the lungs to expand
Because the lungs have a natural tendency to collapse and the chest wall has a
natural tendency to expand, a negative or subatmospheric pressure (negative
intrapleural pressure) normally exists between the visceral and parietal pleurae
Pneumothorax
o Should air or gas enter the pleural cavity (chest puncture wound), the
intrapleural pressure will rise to atmospheric pressure and cause the
pleural membranes to separate
o This is a life-threatening condition that may severely compromise the
cardiopulmonary system
THE THORAX
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Houses and protects the organs of the cardiopulmonary system
Thoracic vertebrae
o Twelve thoracic vertebrae form the posterior midline border of the thoracic
cage
Sternum
o The sternum forms the anterior border of the thoracic cage
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Manubrium sterni
Body (gladiolus)
Xiphoid process
Ribs
o Twelve pairs of ribs form the lateral border of the thorax
 The ribs attach directly to the vertebral column posteriorly
 The ribs attach indirectly by way of the costal cartilages anteriorly
 True ribs
 First 7 rib pairs
o Attach to the sternum
 False ribs
 Rib pairs 8, 9, and 10
o Attach to the cartilage of the ribs above
 Floating ribs
 Rib pairs 11 and 12
o Float freely anteriorly
Intercostal spaces
o The spaces between the ribs
o Contain blood vessels, intercostal nerves, and the external and internal
intercostal muscles
 Blood vessels and nerves run just below the ribs
THE DIAPHRAGM

Description
o The major muscle of ventilation
o Dome-shaped muscle that divides the thoracic and abdominal cavities
o Composed of two separate muscles
 Left and right hemidiaphragms

o The diaphragm is pierced by the esophagus, the aorta, several nerves,
and the inferior vena cava
o The phrenic nerves supply the primary motor innervation to the diaphragm
 The lower thoracic nerves also contribute
Role of the Diaphragm in Respiration
o When stimulated to contract, the diaphragm moves downward and the
lower ribs move upward and outward
o This increases the volume in the thoracic cavity, which, in turn lowers the
intra-alveolar pressure
o As a result, gas from the atmosphere enters the lungs (inspiration)
o At end-inspiration, the diaphragm relaxes and domes upward into the
thoracic cavity, increasing the intra-alveolar pressure and causing gas to
move out of the lungs (expiration)
THE ACCESSORY MUSCLES OF VENTILATION
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During normal ventilation, the diaphragm alone can manage the task of moving
gas in and out of the lungs
During vigorous exercise, and in persons with diseased lungs, the accessory
muscles of inspiration and expiration are activated to assist the diaphragm
Accessory muscles of inspiration help to increase the volume in the thorax
o Scalene muscles
o Sternocleidomastoid muscles
o Pectoralis major muscles
o Trapezius muscles
o External intercostal muscles
Accessory muscles of expiration assist in exhalation when airway resistance
becomes significantly elevated by increasing the intrapleural pressure
(decreasing the volume in the thorax)
o Rectus abdominus
o External abdominus obliquus
o Internal abdominus obliquus
o Transverses abdominus
o Internal intercostal muscles