Chapter 8
The Respiratory System
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Learning Objectives




State the major developmental events of the
respiratory system.
Describe how genes control lung
development .
Describe the key elements of normal fetal
circulation.
State what happens to the respiratory system
at birth.
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Learning Objectives (cont.)




Describe the developmental events in the
respiratory system that continue after birth.
Identify the main structures in the thorax and
describe their functions.
Identify and describe the primary and
accessory muscles of breathing.
Describe how the pulmonary and bronchial
circulations are organized and their functions.
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Learning Objectives (cont.)



Describe how somatic and autonomic
nervous systems connect to and control the
lungs and respiratory muscles.
Identify the major structures of the upper
respiratory tract and how they function.
Describe how the lungs are organized into
lobes and segments and the airways that
supply them with ventilation.
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Learning Objectives (cont.)



Describe how and why airways produce and
move mucus.
Describe how the structures in the respiratory
bronchioles and alveoli are organized.
Describe the blood-gas barrier.
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Introduction



Primary function: absorption of O2 & excretion of
CO2 called “external respiration”
“Internal respiration” gas exchange between
tissue cells & systemic capillary blood
During lifetime, about 250 million liters partake in
external respiration.


Performed with minimal work
Secondary function: filters both inhaled
contaminants and small clots or chemicals from
blood
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What is meant by “external respiration”?
A. The continuous absorption of O2 & excretion of
CO2
B. any gas exchange that occurs inside the body
C. consumption of oxygen in the mitochondria
D. exchange of gases between the systemic
capillary blood & tissue cells
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Development of the Respiratory
System


Extends from almost conception into childhood.
Developmental stages between fertilization & birth divided into:



1. Embryonic period,
2. Fetal Period
Embryonic period


Occurs during the first 8 weeks
Where all major organs begin development
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Development of the Respiratory
System (cont.)

The fetal period

Occurs during remaining 32 weeks of gestation
 Organized into 23 stages (Carnegie stages)
 Organs continue to develop, refine their structure
& function

17 days following fertilization (Embryonic
Period)

Formation of mass of cells composed of 3 distinct
germinal tissue layers
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Development of the Respiratory
System (cont.)

3 germinal tissue layers will form all tissues
and organs:



Endoderm
Mesoderm
Ectoderm
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At what point in fetal development do all major
organs begin their development?
A.
B.
C.
D.
fetal period
Childhood
embryonic period
alveolar period
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Development of the Respiratory
System (cont.)

Endoderm



Forms epithelium lining layer for entire respiratory
system
Forms mucous & gas exchange membranes
Mesoderm


Surrounds the lung bud
Forms supporting structures of tracheobronchial
tree
• Muscle
• Connective tissues
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Development of the Respiratory
System (cont.)

Ectoderm


Forms nervous system of respiratory tract
Respiratory System Development:

The beginning:
• On or about day 22 after fertilization
• First a mass of cells forms a pouch-like bud
• Many stages of development into branching airways &
blood vessels
• Highly regulated by activation of various genes
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Development of the Respiratory
System (cont.)
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Genetics Mutations


40 out of 22,000 human genes required for
normal respiratory system development
Failure or mutation of NKX2-1 (aka TTF-1)


Failure of lung bud formation &
tracheoesophageal malformations
Cystic fibrosisdefect on chromosome 7
results in pulmonary, gastrointestinal, &
endocrine dysfunction
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Genetics Mutations (cont.)


Emphysema can result from an 1-antitrypsin
deficiency due to mutation on chromosome
14
Asthma may be associated with multiple gene
alterations.

Affects about 10% of population
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Fetal Lung Development


The canalicular stage is from week 16-23; life is
possible
End of canalicular stage:


2-4 generations of respiratory bronchioles form
• Primitive acini form, covered with type I & II pneumocytes
• Life viable if airway, MV, surfactant provided
Terminal saccular stage - functional acinus
forms


Thinning of type I pneumocyte cells
Type II pneumocyte cells mature & produce surfactant
at about 24 to 25 weeks
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Fetal Lung Development (cont.)

Alveolar period: 32 weeks until years after
birth


Development of mature alveoli & capillaries in
alveolar walls
Alveolarization occurs
• Crests form along immature airway wall, develop into
septa, then into terminal saccule lumen
• Mature alveoli/capillary membranes appear

At birth, full-term newborn has about 50
million alveoli


Will increase in number for about 2-3 years after
birth
By age 8about 300 million
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Fetal Lung Development (cont.)

Genes: responsible for development of
branching of lungs’ airways & blood vessels
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Development of the Respiratory
System (cont.)
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Development of the Respiratory
System (cont.)
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The Fetal Lung

Lung maturation: determined by pulmonary
surfactant

Regulated by genes & hormones
• Including glucocorticoids, prolactin, insulin, thyroid hormones,
oestrogens, androgens, catecholamines



Begins production about 24-25th week by type II
pneumocytes
Promotes lung inflation & protects alveolar surface
Composed primarily of phospholipids & small amount
of protein, trace carbohydrates
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The Fetal Lung (cont.)

Phospholipid components:



Phosphatidylcholine levels predictive of lung
maturity
Lecithin/sphingomyelin ratio (L/S ratio)
• L/S >=2 indicates low risk for respiratory distress
• L/S <=1.5 indicates high risk for respiratory distress
Phosphatidylglycerol (PG) concentration
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The Fetal Lung (cont.)

Glucocorticosteroid production increases at the of
gestation



Stimulates receptors in type II pneumocytes to
increase surfactant production
Improves L/S ratio
Various key genes are responsible for normal
surfactant production

Gene mutation: linked w/ development of respiratory
distress syndrome (RDS) & other pulmonary disorders
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The Fetal Lung (cont.)

Fetal lung fluid is constantly produced

Slight positive pressure keeps lungs inflated
• Promotes normal lung development
• At birth, lungs hold about 40 ml of fluid
• If deficient, can result in hypoplastic (poorly developed) lung
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The Fetal Lung (cont.)

Gender differences in lung development

Male/female: similar growth in developmental
period
 Male lungs larger, on average, at birth
• Greater number of respiratory bronchioles
 Females have better developed lung function
• Breathing efforts & surfactant production at 26-36 weeks
gestation

Females: slightly less susceptible to get RDS
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The Fetal Lung (cont.)
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All of the following make up mature surfactant,
except?
A.
B.
C.
D.
trace carbohydrates
Phospholipids
Proteins
glucocorticosteroid
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Uterine Life

In utero, life depends on placental structure,
providing, among many things:




Fetal circulation incorporates placenta by
umbilicus & use of three special shunts:


Gas exchange
Nutrients & waste removal
Defense against disease
Ductus venosus, ductus arteriosus, and foramen
ovale
Growth similar in male & female fetuses
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Uterine Life (cont.)
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Fetal to Adult Circulatory Patterns
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Fetal Circulation




Placenta large volume, low resistance system,
fetal SVR is low
Umbilical vein returns oxygenated blood from
placenta to fetus via the ductus venosus
Flows into IVC & on to RA
Oxygenated blood is preferentially shunted
through foramen ovale from right to left atrium

Provides oxygenated blood to systemic circulation
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Fetal Circulation (cont.)

In utero fetal lungs have high PVR due to low
PAO2.

Ductus arteriosus shunts blood from highresistance pulmonary artery to low-resistance
aorta
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Cardiopulmonary Events at Birth

Fetal lung fluid




Prior to birth, production stops & absorption starts
1/3 of fluid is expelled by vaginal squeeze
Pulmonary lymphatics absorb remaining fluid
Tactile & thermal stimuli initiate first breath


Initial breath requires transpulmonary pressures >40
cm H2O.
Subsequent breaths require progressively less
pressure as lung volume increases
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Cardiopulmonary Events at Birth
(cont.)

Air in lung increases PO2 and pH, while PCO2
decreases, which results in:

Pulmonary vasodilation & decreased PVR
 Ductus arteriosus constriction/closure
 Increased pulmonary blood flow

At the same time, placenta removal results in:


Sudden increase in SVR
Net results:


LAP > RAP, so foramen ovale closes
Transition to extrauterine circulation complete
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All of the following cardiopulmonary events happen
at birth, except?
A.
B.
C.
D.
Ductus ateriosus dilation
Pulmonary vasodilation
Ductus arteriosus constriction/closure
Increased pulmonary blood
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Postnatal Upper Airway


Head flexion can cause airway obstruction
Contributing factors:

Tongue is relatively larger compared with adults
 Nasal passages are relatively smaller
• Most infants nose breathe exclusively
• At 4 to 5 months, most infants can breathe orally
 Infections or Intubations can cause obstruction at the
cricoid cartilage (narrowest point) or the epiglottis
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Postnatal Upper Airway (cont.)

Epiglottis is relatively longer & less flexible than
adult’s
 Mechanical & chemical irritant laryngeal reflexes
develop at birth
• Can initiate protective laryngeal closure
• Can trigger prolonged apnea in some
• May be linked to cause Sudden Infant Death Syndrome
(SIDS)
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Postnatal Upper Airway
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Postnatal Lower Airway & Alveoli

Alveoli continue to develop for years until stable
stage is reached



Total of approximately 480 million alveoli by 10 years old
Most develop in first 1½ postnatal years
By adulthood :

Alveolar-capillary (AC) membrane has gas exchange
surface area of approximately 140 m2
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Postnatal Lower Airway & Alveoli
(cont.)

Prior to current research data:

Prior belief: alveolar development ended several years
after birth, however, numerous studies prove:
• Compensatory lung growth can rapidly occur when part or all of
other lung is removed
• Due to stem cell activation
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Vascular Development



Basic structure is in place at birth
Subsequent vascular growth involves increased
smooth muscle growth & increased density of
arterioles & capillaries in distal regions
Lungs are unique as blood from RV & LV
provide flow to alveoli microcirculation



Pulmonary circulation from RV
Bronchial circulation from LV
Provides greater stability & resistance against impact
of disease processes
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Lymphatic & Nervous Development

Lymph nodes & vessels are located in connective
tissues beside pulmonary structures


Provide fluid control & defense
• Absorbed fluid travels to hilar lymph nodes
Nervous tissue development

Brainstem centers for automatic control
 Phrenic & intercostal nerves form to carry motor signals
to diaphragm & intercostal muscles
 Autonomic fibers form for smooth muscle control
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Chest Wall Development, Diaphragm
& Lung Volume




Infant thorax is more compliant than that of adult
FRC is established by equal & opposing forces
of chest wall to expand against lungs tendency
to collapse
Infant’s more compliant thorax results in reduced
lung volume
Predisposes infant to early airway closure,
atelectasis, V/Q mismatch, & resultant
hypoxemia
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Chest Wall Development, Diaphragm
& Lung Volume
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Chest Wall Development, Diaphragm
& Lung Volume (cont.)

Infants, especially those in distress can
actively increase lung volume by ending
expiration early



Traps gas
Improves V/Q mismatch & gas exchange
Mechanically accomplished on exhalation by:


Actively using diaphragm to slow exhalation
Adducting vocal cords to narrow the glottis
• Patient will make a grunting sound, “laryngeal braking”
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Adult Respiratory System

Thoracic surface features

Imaginary lines establish reference points and
thoracic landmarks
• See Figures 8-13, 8-14, and 8-15 (slides 49-50)


Thoracic size & volume continue to increase
throughout childhood & especially during puberty
Both adolescent & adult males tend to have larger
lungs compared to females
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Adult Respiratory System (cont.)

Chest wall



Cone-shaped cavity contains vital organs
Functions to protect those organs
Ability to change shape facilitates breathing
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Adult Respiratory System (cont.)
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Adult Respiratory System (cont.)
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Adult Respiratory System (cont.)
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Thoracic Wall Cross Section
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Components of Thoracic Wall




Skin, fat, skeletal muscles, & bony structures
form outer portion of wall
Inner layer lined with serous membrane
parietal pleura
Serous membrane that covers the lungs
visceral pleura
Pleura separated by thin fluid layer

Pleural space
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Components of Thoracic Wall (cont.)

Sternum composed of: manubrium, body, &
xiphoid process (see Figure 8-18, A)

Sternal angle at joining of body & manubrium
• External landmark for tracheal division into mainstem
bronchi

12 pairs of ribs, pairs 1 to 7 (true ribs)
connect directly to the sternum

Immediately below each rib run artery, vein, &
nerves for particular portion of chest wall
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Components of Thoracic Wall (cont.)
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Components of Thoracic Wall (cont.)
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Rib Movement: Facilitates Breathing


Pair 1: raise slightly, pulling sternum up,
which increases AP diameter
Rib pairs 2 to 7 move in two directions (see
Figure 8-20)



Increase AP diameter, “pump action”
Increase lateral space, “bucket handle”
Rib pairs 8 to 10 move similar to 2 to 7


However, slight reduction of AP diameter
While lateral space increases
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Rib Movement: Facilitates Breathing
(cont.)
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Respiratory Muscles

Diaphragm & intercostals: primary muscles of
respiration

Active during resting breathing
 75% of work performed by diaphragm
 Muscle relaxation results in passive exhalation

Accessory muscles of inspiration



Active only during increased demand
Primarily scalene & sternocleidomastoids
See Table 8-4, p. 167
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Accessory Muscles of Expiration


During resting, breathing exhalation is
passive
During times of increased demand, expiratory
muscle contraction increases speed of
exhalation



Compression of abdomen by an array of
abdominal muscles
Ribs pulled down & together by internal intercostal
muscles
See Table 8-5, p. 168
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Diaphragm

Normal diaphragmatic excursion 1 to 2 cm


With maximal inspiration may be 10 cm
Hyperinflation (increased lung volumes) flattens
domes





Contraction may decrease AP diameter
Decreased efficiency with increased work of breathing
Seen in severe asthma and COPD
To compensate for this, individuals must recruit other
accessory muscles to enlarge the thorax
Result is less efficient breathing & excessive muscle
work
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Diaphragm (cont.)

Other nonpulmonary diseases can also affect
diaphragm function:
• Abdominal wall muscle tensioning (splinting) due to pain
• Abdominal distention with fluid (ascities)
• Any other causes of abdominal wall rigidity can interfere
with descent
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Diaphragm (cont.)
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Respiratory Muscles
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Respiratory Muscles
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Respiratory Muscles
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Diaphragm (cont.)


Innervated by phrenic nerves that arise from
C3, C4, & C5
Prolonged diaphragmatic contraction
concurrent with abdominal muscle contraction
aids in compression of abdomen for:

Vomiting, coughing, defecation, parturition
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Diaphragm (cont.)

Diaphragmatic Paralysis



Spinal cord injuries at or above level of 3rd
cervical vertebrae
Loss of all nervous control of respiratory muscles
Unable to breathe
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Pleural Membranes,Space,
& Fluid

Visceral & parietal pleuraactually two sides of
one membraneform sac or space, “pleural
space”





Clear pleural fluid fills pleural space
Pleural fluid is secreted & reabsorbed by both pleural
membranes
Pleura produce ~0.26 ml/kg or about 18 ml in a 70 kg
adult
Parietal pleura produces little more than ½ of fluid
Both pleura together produce about 150-250 ml per
day
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Pleural Membranes, Space,
& Fluid (cont.)

Pleural fluid acts as:



Lubricant, decreasing lung friction as lungs slide
across inner chest wall
Airtight seal adhering 2 pleural membranes together
Pleural fluid has:

pH of 7.60-7.65
 Small amount of protein (about 1 g/dL)
 Glucose
 Electrolytes in concentrations similar to plasma

Pleural pressure is negative due to opposing
tendency of lung to collapse & thorax to expand
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Pleural Membranes, Space,
& Fluid (cont.)


Most pleural fluid is absorbed by visceral
pleura capillaries
The rest is cleared by:



Lymphatic drainage located in parietal pleura
By solute-coupled liquid absorption
Through some transcytosis
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Pleural Membranes, Space,
& Fluid (cont.)

Fluid & solutes or cells cleared by lymphatic
drainage are:



Carried by pulmonary lymphatic's to hilar region
Enters major lymphatic vessels
Drainage back to subclavian veins & right heart
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Pleural Membranes, Space,
& Fluid (cont.)

Costophrenic angle





Where costal parietal pleura joins diaphragmatic parietal
pleura
Located in right & left lateral & inferior regions of thoracic
cavities
On normal chest radiograph, angle is clearly visible & is
important landmark
Normal - sharp angle of about 30 to 45 degrees
Excess fluids between visceral & parietal pleura tend to pool
here in an upright person
• Causes angle to appear blunted or flattened to 90 degrees on
chest radiograph
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How is most of the pleural fluid absorbed?
A.
B.
C.
D.
through the visceral pleura capillaries
through the parietal pleura capillaries
through the right main stem bronchus
it’s excreted through urine
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Lungs



Cone-shaped, sponge-like organs
Apices extend 1 to 2 cm above clavicles
Each lung has two (left) or three (right) lobes separated by fissures (see Figure 8-28)



Left upper & lower lobes divided by oblique fissure
Right lower lobe is also delineated by oblique fissure,
while transverse fissure separates upper & middle
lobes
Lungs elasticity results from alveolar surface
tension, elastic properties of tissue, & connective
tissue
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Lungs (cont.)
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Lungs (cont.)
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Lungs (cont.)

3 different fiber supporting systems

Scaffold structure to support lungs as they inflate
• Axial system:


Composition: collagen & reticulin fibers
Begins in hilum & extends outward in all airway walls
• Septal fiber system:


Composition: collagen, reticulin & elastic
Supports alveoar walls
• Peripheral fiber system


Composition: primarily collagen
Effectively divides up lung tissue into interlobular regions
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Pulmonary Circulation



Supplied with blood from right heart at flow rate
equal to entire blood volume each minute at rest
Capillaries cover about 90% of alveolar surface
Functions of lungs



Gas exchange at the alveolar-capillary (A/C)
membrane (primary function)
• Pick up oxygen & drop off CO2
A/C membrane controls fluid exchange in lung
Production, processing, & clearance of variety of
chemicals & blood clots
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Pulmonary Circulation (cont.)
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Pulmonary vs. Systemic Circulation

Hemodynamic values are very different between systems


Pulmonary: low pressure, low resistance
Systemic: high pressure, high resistance
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Bronchial Circulation




Separate arterial supply; systemic circuit branch
Supplied with blood from aorta via minor thoracic
branches
Supplies blood to larger lung structures (1%-2% CO)
Lung metabolic demands are fairly low


Most lung parenchyma gets oxygen directly from inspired
gas
Bronchial veins drain via various routes


Some drain to pulmonary veins, contributing to anatomic
shunt
When pulmonary circulation is compromised, bronchial flow
increases, & vice versa
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Bronchial Circulation (cont.)
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Bronchial Circulation (cont.)
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Nervous Control of Lungs


Somatic nerves innervate chest wall &
respiratory muscles
Autonomic (sympathetic & parasympathetic)
nerves innervate:



Airway smooth muscles & glands
Pulmonary arteriole smooth muscle
Result in balanced control of:
• Bronchodilation/bronchoconstriction
• Vasodilation/vasoconstriction
• Glandular secretion
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Nervous Control of Lungs
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Nervous Control of Lungs
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Lung Reflexes

Inflation (Hering-Breuer) reflex



Stretch receptors function to limit further stretch
Probably inactive during resting breathing
Irritant receptors are found in posterior of
trachea & bifurcations of larger bronchi

When stimulated, can result in cough, sneeze,
bronchospasm, hyperpnea, & vagal response
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Upper Respiratory Tract (URT)


Defined as airways starting at the nose,
extend to trachea
URT is composed of




Nasal cavities & sinuses
Oral cavity
Pharynx
Larynx
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Upper Respiratory Tract (URT)
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Nasal Cavity



External nares give entrance into cavities
Vestibules contain gross hairs working as filter
Concha or turbinates3 shelf-like bones
projecting from lateral walls

Function: Increase surface area for filtering, warming,
& humidifying of inhaled gases
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Nasal Cavity (cont.)


Contain olfactory cells providing sense of
smell
Surface fluid is provided by goblet cells &
submucosal glands in cavity & sinuses
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Sinuses


Hollow spaces in the facial bones
Four sets of sinuses


Frontal, ethmoid, sphenoid, maxillary
Function of sinuses



Reduce weight of head
Strengthen skull
Modify voice during phonation
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Sinuses (cont.)
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Oral Cavity



Forms common passage for air, food, & fluids
Tip of soft palate, uvula, marks posterior
aspect of cavity
Posterior portion of tongue has nerve endings
triggering gag reflex to protect airway
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Oral Cavity (cont.)
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Pharynx

Oral & nasal cavities open into the pharynx



Nasopharynx (from nasal cavity to uvula)
• Adenoids lie right where many particles impact
• Eustachian tubes link to middle ear
Oropharynx (from uvula to tip of epiglottis)
• Palatine tonsils (removed in tonsillectomy)
Laryngopharynx (tip epiglottis to larynx)
• Anatomic location where respiratory & digestive tracts divide
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Larynx

Contains nine cartilages (see Figure 8-39)



Thyroid (Adam’s apple)
Cricoid falls just below thyroid cartilage
Epiglottis attaches to thyroid cartilage
• Closes laryngeal opening during swallowing, to create tight seal to
prevent liquids & food from entering respiratory tract
• Swallowing :

Muscular contractions resulting in early vocal cord closure & downward
epiglottis movement
• Folds connecting epiglottis & tongue form space called “vallecula”


Key landmark for oral intubation
3 paired cartilages involved in phonation: Arytenoid, corniculate,
cuneiform
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Larynx (cont.)
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Larynx (cont.)
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Patent Upper Airway

Relative positions of oral cavity, pharynx, &
larynx are major determinant of patency,
particularly in unconscious patient


Head tilts forward, partial or total occlusion can
occur
Extend head into “sniff position” to open airway &
facilitate artificial airway insertion
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Patent Upper Airway (cont.)
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Lower Respiratory Tract



Everything distal to larynx
Made up of conducting & respiratory airways
Conducting airwaysfirst 15 generations


Purpose: convey gas from URT to area of gas
exchange (lung parenchyma)
Respiratory airways


Microscopic airways distal to conducting zone
Participate in gas exchange with blood
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Trachea & Bronchi



Trachea: extends below cricoid cartilage to
sternal angle
Anterior & sides supported by 16 to 20 C-shaped
cartilage
Trachealis muscle connects tips of C-shaped
cartilage & forms posterior wall
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Trachea & Bronchi (cont.)
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Trachea & Bronchi (cont.)


Right & left mainstem bronchi bifurcate at
carina
Right bronchus branches at 20- 30-degree
angle


Due to angle, most foreign aspirate goes to right
lower lobe
Left bronchus branches at 45- 55-degree
angle
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Trachea & Bronchi (cont.)
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Trachea & Bronchi (cont.)
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Lobar & Segmental
Pulmonary Anatomy



Each lung is divided into lobes and segments
Right lung has 3 lobes and 10 segments
Left lung has 2 lobes and 8 or 10 segments

See Table 8-8 (p. 191)
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Lobar & Segmental
Pulmonary Anatomy (cont.)
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Lobar & Segmental
Pulmonary Anatomy (cont.)


Each segment is supplied by segmental
bronchus
These further divide numerous times until
conducting airways end in terminal bronchioles

All airways up to this point constitute anatomic
deadspace.
• ~2 ml/kg of lean body weight, typically 150 ml
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Lobar & Segmental
Pulmonary Anatomy (cont.)
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Histology of Airway Wall


Conducting airways (trachea to bronchioles)
Walls constructed of 3 layers

MucosaInner layer forms mucous membrane
• Composed of epithelia
• Pseudostratified, ciliated, columnar epitheliamost
numerous cell type


Submucosacomposed of connective tissue,
bronchial glands & smooth fibers wrap around
airway
Adventitiaouter covering of connective tissue
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Histology of Airway Wall (cont.)

Basal cells



Contribute to appearance of pseudostratified
cellular layer
Mature into pseudostratified cells
Thought to play role in repair of mucous
membrane following diseases & injury
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Histology of Airway Wall (cont.)

Neuroendocrine cells





Connected to vagus nerve
Are thought to function during lung development
Hypoxia & stress-strain sensors
Secrete bioactive chemicals (e.g., serotonin,
calcitonin, & gastrin releasing peptide)
Mixed with lymphocytes & thought to be migratory
in nature
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Histology of Airway Wall
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Histology of Airway Wall (cont.)
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What is a function of neuroendocrine cells?
A. contribute to the appearance of a
pseudostratified cellular layer
B. hypoxia & stress strain sensors
C. work as an anti inflammatory response
D. repair the mucous membrane following
diseases & injury
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Respiratory Zone Airways

Respiratory bronchioles arise from terminal
bronchioles & have 2 functions
1.
2.

Conduct gas deeper into respiratory zone
Participate in gas exchange
• Bronchiole walls sprout alveoli
All structures distal to one terminal bronchiole
form primary lobule or acinus, each
composed of:


Respiratory bronchioles, alveolar ducts, alveolar
sacs, & about 10,000 alveoli
See Figures 8-51 and 8-52
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Respiratory Zone Airways (cont.)
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Respiratory Zone Airways (cont.)
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Respiratory Zone Airways (cont.)
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Alveoli

Saclike growths that sprout on walls of
respiratory bronchioles, alveolar ducts, &
alveolar sacs


Primary function is gas exchange
Type I pneumocytes: very flat & cover about
93% of alveolar surface


Very thin facilitating gas exchange
Form very tight joints, which limits movement of
materials into alveolar space
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Alveoli (cont.)
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Alveoli (cont.)

Type II pneumocytes are cuboidal



Twice as many as type I cells
Manufacture & storage of surfactant
• Reduces surface tension & alveolar tendency to collapse
• Increases compliance & decreases work of breathing
Have “Stem” cell like action
• Can differentiate into type I cells, so as to repopulate &
repair damaged alveolar surface


Alveolar macrophages provide defense
Clara cells also store & manufacture
surfactant
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Blood-Gas Barrier

A/C membrane provides area for gas exchange
(typically about 140 m2 and 1 µm thick)

O2 & CO2 diffuse from alveoli through
• Surfactant layer
• Type I cell
• Basement membrane
• Interstitial space containing basement membrane, elastin &
collagen fibers, & capillaries
• Capillary endothelial cells
• Plasma
• Finally, into erythrocytes (RBCs)
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Blood–Gas Barrier (cont.)
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