Biology 322 – Human Anatomy
Respiratory system
Respiratory system function and anatomy
•In biology, RESPIRATION can have several
meanings:
1. Movement of air in/out of the lungs
(really known as ventilation)
2. Exchange of O2 for CO2 in the
bloodstream
3. Use of O2 during aerobic respiration
(ATP generation)
•Main function of respiration is to replenish O2 and expel CO2
•Need O2 for aerobic respiration
•Need to eliminate CO2 waste
•CO2 + H2O → H2CO3 → HCO3- + H+→ acid build up in blood and tissues
•Additional, important functions:
•Need to move air for vocalization (speech, laugh, cry, etc…), sense of smell
•Respiratory pump aids blood flow back to heart
•Breath holding – allows us to survive underwater briefly, aids in urination, defacation,
childbirth
Respiratory system anatomy
1. Nose
2. Pharynx
• nasopharynx, oropharynx, laryngopharynx
3. Larynx
4. Trachea
5. Bronchi – airways within the lung
6. Lungs
• Most distal portions end in sacs called
ALVEOLI
 How we get O2 from atmosphere to blood
 Essentially a “dead end” circuit – air enters
resp. system, travels to distal parts of lung,
then goes back the way it came in
•Two main functional divisions exist:
1. Conducting division –
 Organs which allow air to get
to lungs, but do not exchange
O2 and CO2
 Nose, pharynx, larynx,
trachea, most bronchi
2. Respiratory division –
 Portion of resp. system that
carries out gas exchange
 Some smaller distal bronchi
and alveoli
Conducting Division
•Main function is to move air from outside → inside → back outside
•Several organs also play other roles and contribute to normal function of respiratory system
•Nasal cavity –
Conditions the air as it enters the body
Nasal conchae cause air to swirl as it enters
Nasal cavity lined by MUCOSAL MEMBRANE
•GOBLET CELLS that secrete mucous, pseudostratified ciliated cells that “sweep”
material back so that it can be swallowed
Air is warmed and humidified
•ERECTILE TISSUE in inferior conchae fills with warm blood (common site of nosebleed)
Dust, allergens, other particles get trapped in nasal hairs VIBRISSAE and in mucous that
lines the nasal cavity
OLFACTORY MUCOSA – patch of cells located mostly in
superior portion of nasal cavity
•Contains nerve fibers (C.N. I) that pass through
cribiform plate
•Pharynx – divided anatomically into 3 regions
1. Nasopharynx – posterior to the nasal cavity, slightly superior to oral cavity
 Air enters through nose and is directed inferiorly
 Lined by mucus membrane
 Lymphoid tissues (tonsils) are well positioned to combat this
 Eustachian tube (auditory tube) opens here
2. Oropharynx – posterior to the oral cavity, inferior to the soft palate (roof of mouth)
 Also well supplied with lymphoid tissues
3. Laryngopharynx – inferior to oropharynx
 Lined by pseudostratified columnar epithelia (like much of the conducting
airway)
Laryngopharynx and oropharynx transmit BOTH
food/liquid and air
•Lined by non-keratinized, stratified
squamous epithelia
Nasopharynx ONLY transmits air
EPIGLOTTIS covers opening to
laryngopharynx – prevents choking!
Larynx – a.k.a. the “voicebox”
•Obviously hugely important for its social and psychological function
•Also very important for keeping food, fluids out of the airways
Epiglottis – larynx is PULLED upwards when we swallow and closes opening of larynx
Vestibular folds – also close shut when we swallow
•Very well protected by a number of thick cartilage structures
Thyroid cartilage – forms “Adam’s Apple”
Cricoid cartilage – connects larynx to trachea below
Many smaller cartilages that are important for vocal portion of larynx
•Several INTRINSIC muscles and ligaments allow opening/closing of glottis (opening of larynx)
as well as movement of vocal cords
•Vestibular fold controls opening of glottis – prevents choking
•Vocal fold - fold of tissue overlying the vocal ligaments
Inferior to vestibular fold
Vibrate as air moves over them
Varying tension on vocal cords allow us to create various “rough” sounds
Trachea – rigid tube found ANTERIOR to the esophagus, continuous with inferior part of larynx
•C-shaped rings of hyaline cartilage offer structural support
Rings get smaller, thinner as you go deeper into the lung tissue (more distal)
•Interior lined by pseudostratified columnar epithelium and many GOBLET CELLS (mucus cells)
Particles get trapped in mucus and cilia on columnar cells sweep upwards
Dirty mucus gets moved towards esophagus where it can be swallowed
Referred to as the MUCOCILIARY ELEVATOR
Bronchial Tree
•Trachea branches at a point known as the
CARINA to form a right and left PRIMARY
BRONCHUS
•After 2-3 cm the primary bronchi enter the
lung tissue and branch into successively
smaller SECONDARY and TERTIARY bronchi
•Distal to the bronchi are the BRONCHIOLES
•Each bronchiole branches into TERMINAL
BRONCHIOLES – the end of the conducting
system
This is where gas
exchange actually occurs
•Terminal bronchioles branch into
RESPIRATORY BRONCHIOLES that have small
ALVEOLAR DUCTS branching off
•The ends of the alveolar ducts have small, thin
sacs known as ALVEOLI budding from them
Air Flow
Mostly pseudostratified
columnar, goblet cells
Often cuboidal, with or
without cilia
Simple squamous cells
1.
2.
3.
4.
5.
6.
7.
8.
9.
Trachea
Primary bronchi
Secondary bronchi
Tertiary bronchi
Bronchioles
Terminal bronchioles
Respiratory bronchioles
Alveolar ducts
Alveoli
Supported
by cartilage
No cartilage
Lung structure
•Inferior is BASE, superior is APEX
•Surfaces :
Costal surface – anterior, lateral, posterior
(faces the rib cage)
Mediastinal surface – medial surface
Diaphragmatic surface – portion of the base
that rests on the diaphragm
•There is a large impression on the left lung where
the heart projects laterally – CARDIAC IMPRESSION
•Right lung has 3 lobes – superior, middle, inferior
•Left lung has 2 lobes – superior and inferior
Lobes are separated by fissures:
•Oblique fissure in left lung
•One oblique and one horizontal
fissure in right lung
Pleura
•Like the heart, the lungs reside in the
thoracic cavity and are surrounded by two
layers of serous membranes
VISCERAL PLEURA – lies directly on
top of the lung
PARIETAL PLEURA – lines part of the
thoracic cavity, mediastinum, and
diaphragm
•The PLEURAL CAVITY is space between the
pleurae filled with PLEURAL FLUID that has
three functions
1. Reduces friction during ventilation
2. Creates pressure gradient
3. Compartmentalization of organs
Pulmonary Blood Supply
•Pulmonary arteries and veins enter
the heart near where the primary
bronchi enter – HILUM
•Pulmonary vessels follow roughly the
same path and branching pattern as
the bronchial tree
•The more distal parts of the lung
(respiratory bronchioles, alveolar
ducts, alveoli) receive O2 and
nutrients directly from the pulmonary
circulation
•The more proximal parts (primary bronchi, secondary bronchi, pleura, etc…) need their
own supply
BRONCHIAL ARTERIES branch off of the descending aorta
BRONCHIAL VEINS drain blood that eventually reaches the superior vena cava
However, some venous blood from the bronchial veins ends up in the pulmonary
veins….
Alveolar Structure
•Most gas exchange occurs near the
terminal ends of the bronchial tree in
closed sacs called ALVEOLI
•Average human lung has about 150
MILLION alveoli with a huge surface
area of well over 200 ft2
•Most cells are simple squamous
epithelial cells
•Each alveolus is surrounded by a
capillary bed
Simple squamous cell of alveolus +
endothelial cell/basement membrane of capillary =
RESPIRATORY MEMBRANE
Creates a gas exchange barrier of only 0.5 microns!!
Gases are easily exchanged
Type 1 alveolar cell
Alveolar Structure, cont…
•Alveoli are composed of two unique cell types:
Type 2
alveolar cell
Alveolar
macrophage
1. Type 1 alveolar epithelial cells – simple squamous cells
• Most numerous
• Well suited for gas exchange
2. Type 2 alveolar epithelial cells – more cuboid in shape
• Much less prevalent
• Main job is to produce PULMONARY SURFACTANT
Alveoli also contain many MILLIONS of ALVEOLAR MACROPHAGES
•Monitor the alveolus for bacteria, debris that didn’t get filtered out
•After phagocytosis, macrophages make their way up the mucociliary elevator
Alveolar Structure
•Interior surface of alveolus is lined by a thin
layer of fluid called the HYPOPHASE
•Polar nature of H2O causes surfaces of alveoli
to cling together
•Type 2 cells produce a thin layer of
PULMONARY SURFACTANT that reduces surface
tension
Prevents alveoli from collapsing when
we exhale!!
Pulmonary surfactant –
•Complex mix of phospholipids and 4 proteins secreted by TYPE 2 ALVEOLAR CELLS
•Plays two roles :
1. Reduces alveolar surface tension
2. Assists alveolar macrophages in digestion of bacteria/viruses
Alveolar gas exchange
•In order to exchange O2 and CO2, the gasses
must cross the respiratory membrane and the
hypophase
•This occurs via diffusion
•O2 and CO2 that are being exchanged are first
dissolved in the water part of the hypophase
AIR (alveolus)
WATER (hypophase)
CO2
O2
RESPIRATORY MEMBRANE
BLOOD
Pulmonary Ventilation
Movement of air into and out of the lungs
•RESPIRATORY CYCLE is the repetitive cycle of inhalation (air in) and exhalation (air out)
•Movement of air between two spaces requires a pressure gradient
Air moves from an area of high pressure → low pressure
Important to remember that an increase in volume → decrease in pressure
As pressure within the alveoli decreases air rushes in from the atmosphere
Respiratory muscles
•Main muscle responsible for ventilation is the
DIAPHRAGM
Contraction increases volume of cavity (and
decreases pressure)
•Intercostal muscles of ribs are also important
External intercostals assist inhalation
Internal intercostals assist in FORCED
exhalation
Visceral
Pleura
Parietal
Pleura
•Organization of the pleurae is critical to
ventilation
•Visceral pleura – attached to lung
•Parietal pleura – attached to thoracic wall
•Pleural cavity – fluid filled space between
Normally a slight vacuum exists in this
space (~4mm Hg)
•Vacuum within pleural cavity and fluid on
surfaces of membranes cause the two layer to
“stick” together
•As the thoracic cavity enlarges the parietal pleura
moves and draw the visceral pleura with it.
•Lung puncture (PNEUMOTHORAX) breaks this
vacuum and prevents that lung from expanding
Pleural
Cavity
Mechanics of Inhalation
•Air movement is driven by pressure gradients
•At rest, there is no pressure gradient between atmosphere and alveoli – called
TRANSPULMONARY PRESSURE
•INTRAPLEURAL PRESSURE –
Pressure within the two pleural membranes, normally a -4 mm Hg vacuum
This vacuum causes pleural membranes to stick together and move at the same time
Creates a mechanical coupling of thoracic cavity volume to lung volume
•INTRAPULMONARY PRESSURE –
Pressure within the alveoli themselves
As volume of alveoli increases, pressure
drops below that of atmosphere→ air flows
INTO the lung
•Eventually, intrapulmonary pressure EQUALS
atmospheric pressure and flow stops
•Since inhalation requires muscle contraction, it is
considered to be an active process
“Balloon and Pipe Model of Respiration”
AIR

•In VERY simple terms the bronchial tree and alveoli can be
thought of as a pipe (conducting airways) and a balloon (alveoli)
Expansion of thoracic cavity
Decrease in intrapleural pressure
P
P
Decrease in intrapulmonary pressure
Air rushes into lung
Mechanics of Exhalation
•As opposed to inhalation, exhalation is normally a passive process
However, we can force exhalation (blowing up a balloon or blowing out candles)
•Exhalation is primarily caused by three factors:
1. Recoil of thoracic cavity
Decreases volume of thoracic cavity/lung
2. Relaxation of diaphragm
3. Recoil of elastic fibers that surround the alveoli
•
Decrease in volume of thoracic cavity/alveoli → increase in intrapulmonary pressure
 As intrapulmonary pressure exceeds atmospheric pressure, air is forced out of the lung
•
Rate of exhalation is slowed by CONTROLLED relaxation of diaphragm
 Allows respiratory cycle to occur smoothly
 Phrenic nerve continues to stimulate diaphragm slightly
• Forced exhalation results from contraction of abdominal muscles (pull downward on rib
cage) and internal intercostals (contract ribs)
Forced exhalation DOES have its limits……..
Exhalation
Contraction of thoracic cavity
(Could be passive or forced)
AIR

Increase in intrapleural pressure
Increase in intrapulmonary pressure
P
Air rushes out of lung
P
Inhalation/Exhalation
Rate of expiration
Limits to expiratory rate
“Balloon and squishy pipe model”
Force
Increased AIRWAY RESISTANCE
limits rate of forced expiration
1. Cartilage decreases moving from 1° bronchi, 2° bronchi, etc…
2. Increases in thoracic pressure can deform some airways (point
of diminishing return)
3. Bernoulli’s principle – the faster air flows, the less pressure it
exerts
• Airways partially collapse when air flows rapidly out of
the lung
Airflow resistance
•Air flowing through lungs encounters resistance like blood flowing through arteries and veins
•Resistance decreases airflow in/out of the lung
•Resistance is influenced by factors:
1. Bronchiole diameter –
• ↓ airway diameter leads to ↑resistance to airflow, and vice versa
• Resistance decreases airflow in/out of lung
• Increase/decrease in airway diameter is known as BRONCHODILATION or
BRONCHOCONSTRICTION
• Mainly occurs in distal portions of lung – trachea, larger bronchi don’t really
change diameter that much (lots of supporting cartilage)
•Diameter is affected by contraction/relaxation of smooth
muscle surrounding airway as well as the lining of the airway
(inflammation of membranes)
•Sympathetic stimulation (epinephrine, norepinephrine)
cause bronchodilation
•Parasympathetic stimulation causes bronchoconstriction
Resistance to airflow…..
2. Pulmonary compliance – ability of the lungs to expand and contract
• Alveoli are surrounded by elastic fibers
• Damage to fibers can limit lung compliance
 Alveoli can expand but don’t want to contract
• Accumulation of scar tissue can cause lungs to become stiff (no expansion)
Emphysema is a disease of decreased lung compliance
•Exposure to toxic chemicals, smoke trigger
formation of scar tissue and degradation of elastic
fibers
Resistance to airflow…..
3. Alveolar surface tension –
• Presence of hypophase within alveoli and distal bronchioles increases surface
tension - walls want to stick together
• Pulmonary surfactant reduces surface tension – prevents airway and alveolar
collapse
• Lack of surfactant in preterm infants causes respiratory distress syndrome
•Water molecules on opposing surfaces want to
form hydrogen bonds with themselves
•Surfactant prevents formation of these bonds
Alveolar Ventilation
Describes the air that actually enters/exits the alveoli
•During normal breathing we inhale about 500 mL of
air (TIDAL VOLUME)
•Some of this air only gets as far as the conducting
airways (trachea, bronchi) and not involved in gas
exchange
•ANATOMICAL DEADSPACE – portion of lung where air
is present but not involved in gas exchange
•PHYSIOLOGICAL DEADSPACE – anatomical deadspace
plus non-functional alveoli
Increases with diseases like emphysema
Alveolar ventilation, cont….
•Normal tidal volume is 500 mL, but about 150 mL is
deadspace ventilation
About 350 mL of air reaches the alveoli (ALVEOLAR
VENTILATION)
•Alveolar ventilation rate = alveolar ventilation
(mL/breath) X breaths per minute
•Ex.: 350 mL/ breath X 12 breaths/min. = 4,200 mL/min
•Not all air in lungs is expelled each breath, so goal of A.V.
is to “refresh” the air in the lungs at any given time rather
than completely replace it
Spirometry - Assessment of pulmonary/respiratory function
Spirometry - Assessment of pulmonary/respiratory function
Some common respiratory capacities: usually the sum of two or more respiratory volumes