Overview of Respiration and
Respiratory Mechanics
Dr Shihab Khogali
Ninewells Hospital & Medical School, University of Dundee
 This lecture is the
first of four-linked
lectures …in this
lecture:
 Understand what is
meant by the terms
“internal respiration”
and “external
respiration”
What is
This
Lecture
About?
 Know the four steps
of external
respiration
 Understand
Ventilation - the first
step of external
respiration
See blackboard for detailed learning objectives
 Understand ventilation (Step 1 of external respiration).
 Know that gases move from higher to lower pressure, with the Boyle’s
Law.
 Understand the respiratory mechanics and the relationship between
atmospheric, intra-alveolar, and intrapleural pressures.
 understand the significance of transmural pressure gradient. Know
that peumothorax abolishes the transmural pressure gradient.
 Understand that inspiration is an active process and that normal
resting expiration is a passive process.
 Know the inspiratory muscles and the accessory muscles of
respiration (link with anatomy).
 Describe the role and importance of pulmonary surfactant, with the
Law of Laplace and alveolar stability.
 Know the lung volumes and capacities. Understand the changes in
dynamic lung volumes in obstructive and restrictive lung disease.
 Know the factors which influence airway resistance.
 Define the compliance of lungs and thorax.
 Understand what is meant by the term work of breathing.
 Our body systems
are made of cells
 These cells need a
constant supply of
oxygen (O2) to
produce energy and
function
 The carbon dioxide
(CO2) produced by
the cellular reactions
must continuously be
removed from our
bodies
 The internal
respiration refers to
the intracellular
mechanisms which
consumes O2 and
produces CO2
Internal
Respiration
‘food’ +
‘energy’ +
O2
CO2
Atmosphere
External Respiration
 The term external
respiration refers to the
sequence of events that
lead to the exchange of
O2 and CO2 between the
external environment
and the cells of the
body
 External respiration is
the topic for our fourlinked physiology
lectures
 External respiration
involves four steps
O2
Alveoli of lungs
CO2
CO2
O2
Pulmonary
circulation
Systemic
circulation
CO2
O2
Food + O2
CO2 + HO2 + HTP
Tissue cell
Atmosphere
O2
Alveoli of lungs
Steps of external respiration
1
Ventilation or gas exchange between
the atmosphere and air sacs (alveoli)
in the lungs
2
Exchange of O2 and CO2 between air
in the alveoli and the blood
3
Transport of O2 and CO2 between the
lungs and the tissues
4
Exchange of O2 and CO2 between the
blood and the tissues
CO2
CO2
O2
Pulmonary
circulation
Systemic
circulation
CO2
O2
Food + O2
CO2 + HO2 + ATP
Tissue cell
Internal respiration
Fig. 13-1, p. 452
The Four Steps of External Respiration
Ventilation
The mechanical process of moving gas in and out of the lungs
Gas exchange between alveoli and blood
The exchange of O2 and CO2 between the air in the alveoli and
the blood in the pulmonary capillaries
Gas transport in the blood
The binding and transport of of O2 and CO2 in the circulating
blood
Gas exchange at the tissue level
The exchange of O2 and CO2 between the blood in the systemic
capillaries and the body cells
Three body systems are involved
in external respiration
 The Respiratory System
Atmosphere
O2
Alveoli of lungs
CO2
CO2
O2
 The Cardiovascular System
Pulmonary
circulation
Systemic
circulation
 The Haematology System
CO2
O2
Food + O2
CO2 + HO2 + HTP
Tissue cell
Ventilation
The mechanical process of
moving air between the
atmosphere and alveolar sacs
 Air flow down pressure
gradient from a region of
high pressure to a region
of low pressure
 The intra-alveolar
pressure must become
Boyle’s Law
less than atmospheric
pressure for air to flow
At any constant temperature the
into the lungs during
inspiration. How is this
pressure exerted by a gas varies
achieved?
inversely with the volume of the gas
 Before inspiration the
intra-alveolar pressure is
equivalent to atmospheric
pressure
 During inspiration the
thorax and lungs expand
as a result of contraction
as the volume of a gas
of inspiratory muscles
increases the pressure
 But: How the movement
of the chest wall expand
exerted by the gas
the lungs as there is no
decreases
physical connection
between the lungs and
chest wall?
Ventilation
Linkage of Lungs to Thorax
Two forces hold the thoracic wall and the lungs in
close opposition:
(1) The intrapleural fluid cohesiveness: The water
molecules in the intrapleural fluid are attracted to each
other and resist being pulled apart. Hence the pleural
membranes tend to stick together.
(2) The negative intrapleural pressure: the subatmospheric intrapleural pressure create a transmural
pressure gradient across the lung wall and across the
chest wall. So the lungs are forced to expand outwards
while the chest is forced to squeeze inwards.
Three Pressures are Important in Ventilation
Inspiration is an active process
depending on muscle contraction
 The volume of the thorax is increased vertically by
contraction of the diaphragm (major inspiratory muscle),
flattening out its dome shape.
Phrenic nerve from cervical 3,4 and 5
The external intercostal muscle contraction lifts the ribs
and moves out the sternum.
The “bucket handle” mechanism.
 Inspiration is an active
process brought about by
contraction of inspiratory
muscles
Inspiration
760
 The chest wall and lungs
stretched
Size of thorax on
contraction of
inspiratory muscles
 The Increase in the size of
the lungs make the intraalveolar pressure to fall
 This is because air
molecules become
contained in a larger
volume (Boyle’s Law)
 The air then enters the
lungs down its pressure
gradient until the intraalveolar pressure become
equal to atmospheric
pressure
759
754
Size of lungs as they
are stretched to fill
the expanded thorax
 Normal expiration is a
passive process brought
about by relaxation of
inspiratory muscles
 The chest wall and
stretched lungs recoil to
their preinspiratory size
because of their elastic
properties
Expiration
760
Size of thorax on
relaxation of
inspiratory
muscles
 The recoil of the lungs
make the intra-alveolar
pressure to rise
 This is because air
molecules become
contained in a smaller
volume (Boyle’s Law)
 The air then leaves the
lungs down its pressure
gradient until the intraalveolar pressure become
equal to atmospheric
pressure
761
756
Size of lungs as
they recoil
Changes in intra-alveolar and intra-pleural pressures
during the respiratory cycle
Inspiration
Expiration
Intra-alveolar
pressure
Atmospheric
pressure
Transmural pressure
gradient across the
lung wall
Intrapleural
pressure
Pneumothorax (air in the pleural space) abolishes the
transmural pressure gradient
What causes the lungs to recoil during expiration?
(i.e. what gives the lungs their elastic behaviour)
Elastic connective tissue in the lungs
The whole structure bounces back into shape
But even more important is the alveolar
surface tension
What is alveolar surface tension?
 Attraction between water molecules at liquid air interface
 In the alveoli this produces a force which resists the
stretching of the lungs
 If the alveoli were lined with water alone the surface
tension would be too strong so the alveoli would collapse
 According to the law of
LaPlace: the smaller alveoli
(with smaller radius - r) have
a higher tendency to
collapse
Surfactant Reduces the
Alveolar Surface Tension
 Pulmonary surfactant is a
complex mixture of lipids
and proteins secreted by
type II alveoli
 It lowers alveolar surface
tension by interspersing
between the water molecules Surfactant prevent this happening
lining the alveoli
 Surfactant lowers the
surface tension of smaller
alveoli more than that of
large alveoli
 This prevent the smaller
alveoli from collapsing and
emptying their air contents
into the larger alveoli
If we regard the alveoli as spherical
2T (LaPlace’s Law)
bubles, then:
P
r
P = inward directed collapsing pressure
T = Surface Tension
r = radius of the buble
Respiratory Distress Syndrome of the New Born
 Developing fetal lungs are unable to synthesize surfactant
until late in pregnancy
 Premature babies may not have enough pulmonary
surfactant
 This causes respiratory distress syndrome of the new born
 The baby makes very strenuous inspiratory efforts in an
attempt to overcome the high surface tension and inflate the
lungs.
Another factor which helps keep the alveoli open is:
The Alveolar Interdependence
If an alveolus start to collapse the surrounding alveoli are
stretched and then recoil exerting expanding forces in the
collapsing alveolus to open it
Fig. 13-11, p. 459
Lung Volumes and Capacities
See Practical Class and Online Tutorial
Predicted normal values vary with age, height, gender,..
Lung Volumes and Capacities
Description
Average
Value
Tidal volume
(TV)
Volume of air entering or leaving lungs
during a single breath
500 ml
Inspiratory
reserve volume
(IRV)
Extra volume of air that can be
3000 ml
maximally inspired over and above the
typical resting tidal volume
Inspiratory
capacity (IC)
Maximum volume of air that can be
inspired at the end of a normal quiet
expiration
(IC =IRV + TV)
Expiratory
reserve volume
(ERV)
Extra volume of air that can be actively 1000 ml
expired by maximal contraction
beyond the normal volume of air after
a resting tidal volume
Residual volume Minimum volume of air remaining in
(RV)
the lungs even after a maximal
expiration
3500 ml
1200 ml
Lung Volumes and Capacities
Description
Average
Value
Functional residual
capacity (FRC)
Volume of air in lungs at end of 2200 ml
normal passive expiration
(FRC = ERV + RV)
Vital capacity (VC)
Maximum volume of air that
can be moved out during a
single breath following a
maximal inspiration (VC =
IRV + TV + ERV)
Total lung capacity
(TLC)
Maximum volume of air that the 5700 ml
lungs can hold (TLC = VC +
RV)
Forced expiratory
volume in one
second (FEV1):
Dynamic volume
FEV1% =
Volume of air that can be
expired during the first second FEV1/FVC ratio
of expiration in an FVC (Forced Normal >75%
Vital Capacity) determination
4500 ml
Spirometry for Dynamic Lung Volumes
Volume time curve - allow you to determine:
FVC = Forced Vital Capacity (maximum volume that can be forcibly
Expelled from the lungs following a maximum inspiration)
FEV1 = Forced Expiratory volume in one second
FEV1% = FEV1/FVC ratio
Normal
Obstructive Lung Disease
Airway Resistance
F  P
R
F: Flow P: Pressure R: Resistance
 Resistance to flow in the airway normally is very low and
therefore air moves with a small pressure gradient
 Primary determinant of airway resistance is the radius of
the conducting airway
 Parasympathetic stimulation causes bronchoconstriction
 Sympathetic stimulation causes bronchodilatation
 Disease states (e.g. COPD or asthma) can cause
significant resistance to airflow
 Expiration is more difficult than inspiration
Dynamic Airway Compression
During inspiration the airways are pulled open by the expanding
thorax. Therefore in cases of increased airway resistance
expiration tends to be more difficult.
Transairway Pressure = Airway Pressure – Pleural pressure
The transairway pressure tends to compress airways during active expiration pleural pressure rises during expiration (increases airway resistance)
If no obstruction: the increased airway resistance causes an increase in
airway pressure upstream. This helps open the airways (i.e. reduce the
compressive transairway pressure)
If there is an obstruction (e.g. COPD), the driving pressure between the
alveolus and airway is lost over the obstructed segment. This causes a
fall in airway pressure along the airways resulting in airway
compression by the transairway pressure during active expiration.
 Gives an estimate of peak
flow rate
 The peak flow rate assess
airway function
 The test is useful in
patients with obstructive
lung disease (e.g. asthma
and COPD)
 It is measured by the
patient giving a short
sharp below into the peak
flow meter
 The average of three
attempts is usually taken
 The peak flow rate in
normal adults vary with
age and height
 You will practice taking
the peak flow rate in the
Clinical Skills Centre
Peak Flow Meter
Compliance
During inspiration the lungs are stretched
– Compliance is measure of effort that has to go
into stretching or distending the lungs
– Volume change per unit of pressure change
across the lungs
– The less compliant the lungs are, the more work
is required to produce a given degree of inflation
– Decreased by factors such as pulmonary fibrosis
Work of Breathing
Normally requires 3% of total energy
expenditure for quiet breathing
Lungs normally operate at about “half full”
Work of breathing is increased in the
following situations
–
–
–
–
When pulmonary compliance is decreased
When airway resistance is increased
When elastic recoil is decreased
When there is a need for increased ventilation