RESPIRATORY
PHYSIOLOGY
5 Functions of the
Respiratory System
1. Provides extensive gas exchange
surface area between air and circulating
blood
2. Moves air to and from exchange
surfaces of lungs
5 Functions of the
Respiratory System
3. Protects respiratory surfaces from outside
environment
4. Produces sounds
5. Participates in olfactory sense
Components of the
Respiratory System
Figure 23–1
Organization of the
Respiratory System
• The respiratory system is divided into the
upper respiratory system, above the
larynx, and the lower respiratory system,
from the larynx down
The Respiratory Tract
• Consists of a conducting portion:
– from nasal cavity to terminal bronchioles
• Consists of a respiratory portion:
– the respiratory bronchioles and alveoli
What is the difference
between external respiration
and internal respiration?
Respiration
• Refers to 2 integrated processes:
– external respiration
– internal respiration
External Respiration
• Includes all processes involved in
exchanging O2 and CO2 with the
environment
Internal Respiration
• Also called cellular respiration
• Involves the uptake of O2 and production
of CO2 within individual cells
What are the major
steps involved in
external respiration?
3 Processes of
External Respiration
1. Pulmonary ventilation (breathing)
2. Gas diffusion:
– across membranes between alveolar air
spaces and alveoalar capillaries, and across
capillary walls between blood and other
tissues
3 Processes of
External Respiration
3. Transport of O2 and CO2:
– between alveolar capillaries and capillary
beds in other tissues
What physical principles
govern the movement
of air into the lungs?
Pulmonary Ventilation
• Is the physical movement of air in and out
of respiratory tract
• Provides alveolar ventilation (movement of
air into and out of the alveoli)
• Alveolar ventilation: prevents the buildup
of carbon dioxide in the alveoli and
ensures a continuous supply of oxygen
Atmospheric Pressure
• The weight of air:
– has several important physiological effects
Gas Pressure and Volume
• At normal atmospheric pressures, gas
molecules are much farther apart than the
molecules in a liquid, so the density of the
air is relatively low.
• The forces acting between gas molecules
are minimal so an applied pressure can
push them closer together.
Gas Pressure and Volume
Figure 23–13
Boyle’s Law
• Defines the relationship between gas
pressure and volume:
P = 1/V
• In a contained gas:
– external pressure forces molecules closer
together
– movement of gas molecules exerts pressure
on container
Mechanisms of Pulmonary
Ventilation
Pressure Difference
• Air flows from area of higher pressure to
area of lower pressure
A Respiratory Cycle
• Consists of:
– an inspiration (inhalation)
– an expiration (exhalation)
Respiration
• Causes volume changes that create
changes in pressure
• Volume of thoracic cavity changes:
– with expansion or contraction of diaphragm or
rib cage
Compliance of the Lung
• An indicator of expandability
• Low compliance requires greater force
• High compliance requires less force
Factors That Affect Compliance
1. Connective-tissue structure of the lungs
2. Level of surfactant production
3. Mobility of the thoracic cage
Compliance of the lung
• Loss of supporting connective structure of the
lung increases compliance (emphysema): easier
to expand the lungs, although respiratory
exchange surfaces are damaged
• Inadequate surfactant decreases compliance
(respiratory distress syndrome, skeletal muscle
disorders): more pressure of force required to fill
the lungs
Gas Pressure
• Can be measured inside or outside the
lungs
• Normal atmospheric pressure:
– 1 atm or Patm at sea level: 760 mm Hg
Pressure and Volume Changes
with Inhalation and Exhalation
Figure 23–15
Intrapulmonary Pressure
• Also called intra-alveolar pressure
• Pressure inside the respiratory tract at the
alveoli
• Is relative to Patm
• In relaxed breathing, the difference
between Patm and intrapulmonary pressure
is small:
– about —1 mm Hg on inhalation or +1 mm Hg
on expiration
Maximum
Intrapulmonary Pressure
• Maximum straining, a dangerous activity,
can increase range:
– from —30 mm Hg to +100 mm Hg
Intrapleural Pressure
• Pressure in space between parietal and visceral
pleura
• Averages —4 mm Hg
• Maximum of —18 mm Hg
• Remains below Patm throughout respiratory cycle
due to the relationship between the lungs and
the body wall. The elastic fibers continuously
oppose the fluid bond and pull the lungs away
from the chest wall and diaphragm, lowering the
atmospheric pressure.
The Respiratory Pump
• Cyclical changes in intrapleural pressure
operate the respiratory pump:
– which aids in venous return to heart
* On inspiration, diaphragm lowers, raising
pressure in abdomen and lowers pressure in
thorax, which creates partial vacuum in thorax
that favors venous return to right atrium.
Tidal Volume
• Amount of air moved in and out of lungs in
a single respiratory cycle
• Average value: 500 cc
What are the origins and
actions of the respiratory
muscles responsible for
respiratory movements?
The Respiratory Muscles
Figure 23–16a, b
The Respiratory Muscles
Figure 23–16c, d
The Respiratory Muscles
• Most important are:
– the diaphragm
– external intracostal muscles of the ribs
– accessory respiratory muscles :
activated when respiration increases
significantly
The Mechanics of Breathing
• Inhalation:
– always active
• Exhalation:
– active or passive
3 Muscle Groups of Inhalation
1. Diaphragm:
– contraction draws air into lungs
– 75% of normal air movement
3 Muscle Groups of Inhalation
2. External intracostal muscles:
– assist inhalation
– 25% of normal air movement
3 Muscle Groups of Inhalation
3. Accessory muscles assist in elevating
ribs:
–
–
–
–
–
–
–
sternocleidomastoid
serratus anterior
pectoralis minor
scalene muscles
internal intercostals
external and internal oblique
rectus abdominis
Muscles of Active Exhalation
1. Internal intercostal and transversus
thoracis muscles:
– depress the ribs
2. Abdominal muscles:
– compress the abdomen
– force diaphragm upward
Modes of Breathing
• Respiratory movements are classified:
– by pattern of muscle activity
– into quiet breathing and forced breathing
Quiet Breathing (Eupnea)
• Involves active inhalation and passive
exhalation
• Diaphragmatic breathing or deep
breathing: is dominated by diaphragm
• Costal breathing or shallow breathing:
is dominated by ribcage movements
Elastic Rebound
• When inhalation muscles relax:
– elastic components of muscles and lungs
recoil
– returning lungs and alveoli to original position
Forced Breathing
•
•
•
•
Also called hyperpnea
Involves active inhalation and exhalation
Assisted by accessory muscles
Maximum levels occur in exhaustion
Respiratory Rates and Volumes
• Respiratory system adapts to changing
oxygen demands by varying:
– the number of breaths per minute (respiratory
rate); adult: 12-18/min; children: 18-20/min
– the volume of air moved per breath (tidal
volume)
Respiratory Minute Volume
(VE)
• Amount of air moved per minute
• Is calculated by:
respiratory rate (f)  tidal volume (VT)
(normal: 6 L/min)
• Measures pulmonary ventilation (how
much air is moving into and out of the
respiratory tract)
Anatomic Dead Space
(VD)
• Only a part of respiratory minute volume
reaches alveolar exchange surfaces
• Volume of air remaining in conducting
passages is anatomic dead space
Alveolar Ventilation
(VA)
• Amount of air reaching alveoli each minute
• Calculated as:
(tidal volume — anatomic dead space) 
respiratory rate (f)
Alveolar Ventilation
• A typical inhalation pulls about 500 cc of air into
the respiratory system (VT)
• The first 350 cc enters the alveolar spaces
• The last 150 cc does not reach the alveolar
spaces and thus not participate in gas
exchange. This represents the anatomical dead
space.
• The alveolar ventilation rate is more important
than the respiratory minute volume because it
determines the rate of oxygen delivery to the
alveoli, not just the total air moved per minute.
Alveolar Gas Content
• Air in the alveoli contain less O2, more
CO2 than atmospheric air:
– because air mixes with exhaled air
Alveolar Ventilation Rate
• Determined by respiratory rate and tidal
volume:
*for a given respiratory rate:
• increasing tidal volume increases alveolar
ventilation rate
*for a given tidal volume:
• increasing respiratory rate increases alveolar
ventilation
Respiratory Volumes
and Capacities
Figure 23–17
Lung Volume
• Total lung volume is divided into a series
of volumes and capacities useful in
diagnosis
4 Pulmonary Volumes
1. Resting tidal volume:
– in a normal respiratory cycle
2. Expiratory reserve volume (ERV):
– The amount of air you can force out after a
normal quiet respiratory cyle
4 Pulmonary Volumes
3. Residual volume:
– The amount of air remains in the lungs after
maximal exhalation
– minimal volume (in a collapsed lung): the
amount of air that would remain in a
collapsed lung (30-120 ml)
4. Inspiratory reserve volume (IRV):
– Amount of air you take in over and above
the tidal volume
4 Calculated
Respiratory Capacities
1. Inspiratory capacity: the amount air that you
can draw into your lungs after you have
completed a quiet respiratory cycle
tidal volume + inspiratory reserve volume
2. Functional residual capacity (FRC): the
amount of air remaining in your lungs after you
have completed a quiet respiratory cycle
expiratory reserve volume + residual volume
4 Calculated
Respiratory Capacities
3. Vital capacity: the maximum amount of
air that you can move into or out of your
lungs in a single respiratory cycle
expiratory reserve volume + tidal volume +
inspiratory reserve volume
4. Total lung capacity:
vital capacity + residual volume
Pulmonary Function Tests
• Measure rates and volumes of air
movements