36 - Ventilatio. n.ppt

25-Mar-24 Ventilation 1
Ventilation

Pulmonary Ventilation
Tidal volume 500 ml
Anatomical dead
space 150 ml
Alveolar gas
3000 ml
Pulmonary capillary
blood 70 ml
Total ventilation
7500 ml/min
Frequency = 15
per min
Alveolar
ventilation
5250 ml/min
Pulmonary
blood flow
5000 ml/min

 Minute ventilation (VE)
 Volume of air inspired or expired per
minute
 Depends on the frequency (f)
 Depth of breathing (tidal volume, VT)
 VE = ( VT * f)

 At rest
 VT = 500 ml , f = 12 to 15 breath per
minute
 VE = (500 * 12) = 6000 ml/min
 VE = (500 * 15) = 7500 ml/min

The Anatomical Dead Space
 The first 16
generation plus
trachea and upper
respiratory tract form
 Conducting zone
of the airways
 Transport gas from
& to exterior
From Textbook of Work Physiology by
Astrand, Rodahl, Dahl & Stromme

The Anatomical Dead Space
 Made up of
 Upper respiratory
tract
 Trachea
 Bronchi,
bronchioles,
terminal bronchioles
 Constitute the
anatomical dead space

Dead Space Ventilation (VD)
 This is a portion of
the minute
ventilation
 That fails to reach
areas of lungs
involved in gas
exchange
Portion of tidal
volume air that
remain in dead
space (150 ml)
Portion of tidal
air that gets into
alveoli (350 ml)
Alveolar
air

Dead Space Ventilation (VD)
 Anatomical dead
space (VD)
 Volume of gas
occupying the
conducting zone of
airways
 Is equal to 150 ml
 Dead space ventilation
 Is equal to VD * f
 150 * 15 = 2.25 l/min
Portion of tidal
volume air that
remain in dead
space (150 ml)
Portion of tidal
air that gets into
alveoli (350 ml)
Alveolar
air

Function of Anatomical Dead
Space
 Conditioning of inspired air
 Warming the air to body temp
 Adding moisture
 Saturate with water vapour
 Addition of water vapour dilutes oxygen and
nitrogen concentration of inspired air

Space
 Removal of foreign material
 Foreign particles
 Filtered by nose
 Impacted in lower airways
 Dissolved on moist surface of airways
 Small particles (soot, pollen)
 Impact on the surface of the airways

Space
 Impaction
 Stick to mucus lining
 Carried in the mucus towards the mouth
 Expectorated
 Swallowed
 Mucus is propelled upwards towards the
mouth
 Cilia of the respiratory epithelium

Space
 Foreign materials in inspired gas
(cigarette smoke, smog)
 Stimulate irritant receptors in the
airways
 Cause coughing
 Increase secretion of mucus
 Hypertrophy of mucus glands

Space
 Prolonged breathing air containing
foreign material
 Cause chronic bronchitis
 Increase airway resistance, difficult in
breathing

Alveolar Dead Space
 In health
individuals
 Anatomical dead
space represent the
entire dead space
volume
 In people with lung
diseases
 Some alveoli do not
get blood supply

The Alveolar Dead Space
 Such alveoli do not
participate in gas
exchange
 They constitute
alveolar dead space
 Total (physiologic)
dead space include
 Anatomical dead
space
 Alveolar dead space

Alveolar Ventilation
 Volume of fresh gas
that reaches the
alveoli per minute
 Participate in
exchange of O2 & CO2
 It is equal to
 Amount of new air
reaching the alveoli
times the breathing
frequency
Portion of tidal
volume air that
remain in dead
space (150 ml)
Portion of tidal
air that gets into
alveoli (350 ml)
Alveolar
air

 Alveolar ventilation
(VA)
 VA = (VT – VD) * f
 VA = (500 – 150) * 12
 VA = 4200 ml/min
Portion of tidal
volume air that
remain in dead
space (150 ml)
Portion of tidal
air that gets into
alveoli (350 ml)
Alveolar
air

 Major factor in
determining the conc
of O2 and CO2 in the
alveoli
 Alveolar CO2 tension
(PACO2)
 Regulated at value of
40 mm Hg
 Determined by the
 Rate of production
Portion of tidal
volume air that
remain in dead
space (150 ml)
Portion of tidal
air that gets into
alveoli (350 ml)
Alveolar
air

 Alveolar O2 tension (PA O2)
 O2 is continually removed
from the alveoli by
diffusion
 Inspiration brings
 Fresh air into the alveoli
 Maintain the alveolar O2
tension (PA o2)at about 100
mm Hg
Portion of tidal
volume air that
remain in dead
space (150 ml)
Portion of tidal
air that gets into
alveoli (350 ml)
Alveolar
air

Alveolar – Capillary Gas
Exchange
Pulmonary capillary
blood 70 ml
Pulmonary
blood flow
5000 ml/min
alveoli

Exchange
 Composition of
alveolar gas
mixture
 Contain respiratory
gases
 Oxygen, carbon
dioxide
 Together with
 Nitrogen, water
vapour
CO2
CO2
CO2
O2
O2
O2
Alveolar
space

Exchange
 The volume of
alveolar space
 Functional residual
capacity (FRC)
 2.4 to 3 liters
 To this vol fresh air
is added
 O2 is removed
 CO2 is added
CO2
CO2
CO2
O2
O2
O2
Alveolar
space

Exchange
 The conc of O2 in
the alveoli (FAO2)
depends on
 Rate of diffusion of
oxygen in blood
(VO2)
 Oxygen uptake
 Rate of entry of O2
into the lung
 (FIo2) * (VA)
CO2
CO2
CO2
O2
O2
O2
Alveolar
space

Exchange
 Where
 (FIO2) is the conc of
O2 in inspired air
 (VA) is alveolar
ventilation
CO2
CO2
CO2
O2
O2
O2
Alveolar
space

Exchange
 The alveolar CO2
conc (FACO2)
depends on
 Rate of excretion of
CO2 from blood
into alveolar
 Rate of CO2
removal from the
alveoli
 (FACO2) * (VA)
CO2
CO2
CO2
O2
O2
O2
Alveolar
space

Exchange
 Where
 (FACO2) is the
alveolarCO2 conc
 (VA) is alveolar
ventilation
CO2
CO2
CO2
O2
O2
O2
Alveolar
space

Alveolar Partial Pressures
 In a mixture of gases
 Each gas exerts its own partial pressure
(tension)
 According to Dalton’s law
 Partial pressure equal
 Fraction of gas present (concentration) times the total
pressure
 Partial pressure of gas in a mixture
 A measure of the concentration of the gas in the
mixture

Partial Pressure
 % Composition of dry air at sea level
contain
 O2 = 20.93%
 Co2 = 0.03%
 N2 = 79.04%
 Partial pressure
 Total pressure * % conc
 For O2
 Po2 = 760 * 0.2093 = 159 mm hg

Partial Pressure
 For CO2
 PCO2 = 760 * 0.0003 = 0.2 mm Hg
 For N2
 PN2 = 760 * 0.7904 = 600 mm Hg

Partial pressures & conc of
O2, CO2 in alveoli
 Oxygen
 Conc of O2 in
alveoli (FAO2) &
 PAO2
 Depend on
 Rate of diffusion
into blood (VO2)
 Rate of entry of
O2 in lungs
 (FIO2) * (VA)
CO2 O2
Alveoli
Pulmonary capillary
PACO2
PAO2
CO2 O2
FAO2

O2, CO2 in alveoli
 Hence
 If you increase O2
consumption (VO2)
 You need to
increase alveolar
ventilation (VA)
 To maintain PAO2 at
100 mmHg
CO2 O2
Alveoli
Pulmonary capillary
PACO2
PAO2
CO2 O2
FAO2

Partial Pressures & conc of
O2, CO2 in Alveoli
 When the oxygen
uptake (VO2) is 250
ml/min
 You require
alveolar vent of
about 5 liters /min
to maintain PAO2 =
100mm Hg
150
100
40
5 10 15 20 30
Alveolar ventilation (L/min)
VO2 = 250 ml/min
VO2 = 1000 ml/min
PAO2 = 100
mm Hg
PACO2 = 40
mm Hg

O2, CO2 in Alveoli
 When the oxygen
uptake (VO2) is
1000 ml/min
 You require
alveolar vent of
about 20 liters
/min to maintain
PAO2 = 100mm Hg
150
100
40
5 10 15 20 30
Alveolar ventilation (L/min)
VO2 = 250 ml/min
VO2 = 1000 ml/min
PACO2 = 40
mm Hg
PAO2 = 100
mm Hg

O2, CO2 in alveoli
 For CO2
 The alveolar CO2
conc (FACO2) and
the PACO2 depend
on rate of
 Excretion of CO2
from blood into the
alveoli
 CO2 removal from
alveoli
 (VA * FACO2)
CO2 O2
Alveoli
Pulmonary capillary
PACO2
PAO2
CO2 O2
FAO2

Partial Pressure of Respiratory
Gases (mm Hg)
Gas Atmospheric air Alveolar gas Expired air
O2 159.0 (20.84%) 104.0 (13.6%) 120.0 (15.7%)
CO2 0.3 (0.04%) 40.0 (5.3%) 26.0 (3.6%)
N2 597.0 (78.62%) 569.0 (74.9%) 566.0 (74.5)
H2O 3.7 (0.5%) 47.0 (6.2%) 47.0 (6.2%)
Total 760 (100%) 760 (100%) 760 (100%)
From Guyton

Diffusion

Diffusion of Gases Through the
Respiratory Membrane
 Fick’s law
 The rate of transfer of
gas through a sheet of
tissue is proportional
to
 Tissue area
 Diffusing gas partial
pressures
 Is inversely
proportional to
 Tissue thickness
Vgas  (A/T)D(P1-P2)
P1 P2
T
A
D  Sol/ √ MW
Vgas = gas transferred
A =area
T = thickness
D = diffusion const

 With respect to the
lungs
 The area of blood
gas barrier is large
 Thickness is very
small
 The dimensions
are ideal for
diffusion
P1 P2
T
A
D  Sol/ √ MW
A =area
T = thickness
D = diffusion const

 The rate of transfer is
proportional to a
diffusion constant
which depends on
 Properties of the tissue
 Particular gas
 The diffusion constant
is
 Proportional to
solubility of the gas
 Inversely proportional
to MW of the gas
P1 P2
T
A
D  Sol/ √ MW
A =area
T = thickness
D = diffusion const

 Hence CO2 diffuses
about 20 times
more fast than O2
because
 Has much higher
solubility
 But not very
different MW
P1 P2
T
A
D  Sol/ √MW
A =area
T = thickness
D = diffusion const

O2, CO2 in Alveoli
 The partial
pressure of the
respiratory gases
in the alveoli
 PAO2 = 100 mmHg
 PACO2 = 40 mm hg
 In the capillary at
arterial end
 Pvo2 = 40 mmHg
 Pvco2 = 46 mm hg
CO2 O2
Alveoli
PACO2 = 40 PAO2 = 100
CO2 O2
PvCO2 = 46 mm Hg
PvO2 = 40 mm Hg
PaCO2 = 40 mm Hg
PaO2 =100 mm Hg

O2, CO2 in Alveoli
 Thus there is
 Partial pressure
difference which
form the driving
force for diffusion
of O2 and CO2
 In the capillary at
venous end
 PaO2 = 100 mmHg
 PaCO2 = 40 mm Hg
CO2 O2
Alveoli
PACO2 = 40 PAO2 = 100
CO2 O2
PvCO2 = 46 mm Hg
PvO2 = 40 mm Hg
PaCO2 = 40 mm Hg
PaO2 = 40 mm Hg

Diffusion Path in the Lungs
 Alveolar capillary
membrane
 Made up of
 Capillary endothelium
 Single layer endothelial
cells
 Basement membrane
 Elastic collageneous
tissue
 Alveolar epithelium
 Single layer epithelial
cells
From: www.pdh-odp.co.uk/diffusion.htm

Diffusion Path in the Lungs
 Also to be
included
 RBC membrane
From: www.pdh-odp.co.uk/diffusion.htm

Diffusion Capacity of the
Lung
 Ability of
respiratory
membrane (RM )
 To exchange gas
between alveoli &
pulmonary blood
 Diffusion capacity
 Volume of gas that
will diffuse through
the RM/min/mm
Hg
CO2 O2
Alveoli
Pulmonary capillary

Lung
 Factors affecting diffusing capacity of
the lung include
 Membrane component
 Blood component
 Membrane component
 Pulmonary diseases may affect diffusion
process by
  The SA (destruction of alveoli)
  Diffusion distance (oedema)

Lung
 Reducing the partial pressure
gradient for the diffusion of gases
 Ventilation/perfusion abnormalities

Lung
 Blood component
 Chemical combination of gases with Hb
require finite time
  In Hb conc enhances the transfer of gases
 Anaemic individuals would have impaired
diffusion capacity
 Increase in cardiac output (C.O) enhance
diffusion capacity

Diffusion Capacity for O2
 The extent to
which diffusion can
occur in the whole
human lung
 Can be obtained
from Fick’s law of
diffusion
 Vgas  (A/T)D(P1 –
P2)
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60
PO2 = 0

 Vgas = K(A/T)P
 VO2 = K(A/T)PO2
 The amount that
diffuses must be
identical to the
oxygen uptake
(VO2)
 K, A, & T can not
be measured in the
human lung
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60
PO2 = 0

 K(A/T) = DL
 DL new constant
 Equals the diffusion
capacity of the lung
 Oxygen uptake
 VO2 = DLO2 * (meanPO2)
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60
PO2 = 0

 DLO2 is the diffusion
capacity of the lung
for O2
 MeanPO2
 is the mean oxygen
partial pressure
difference between the
alveolar space and the
blood in the lung
 It is about 10 mm Hg
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60
PO2 = 0

 In the human lung
 VO2 = 250 ml/min
 MeanPO2 = 10 mm
Hg
 Thus
 DLO2 = (VO2)/ MeanPO2
= 250/10 = 25 ml
of O2 / min/ mm
Hg
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60
PO2 = 0

 Changes in O2
diffusion capacity
 During exercise there
is increase
 Pulmonary blood flow
 Alveolar ventilation
 Diffusion capacity for
O2 increase
 Maximum of about 3
times resting value
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60
PO2 = 0

 The increase is due
to
 Opening up of
dormant capillaries
 Extra dilatation of
already open
capillaries
 All these lead to
 Increase in blood
flow
 Increase in SA
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60
PO2 = 0

 There is also better
matching between
 Ventilation of
alveoli
 Perfusion of
capillaries
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60
PO2 = 0

Diffusion Capacity for CO2
 Diffusion capacity
of the lung for CO2
 Has been estimated
to be equal to
 400 to 450 ml of
CO2 /min/mm Hg
PCO2 = 40
Alveoli
Pulmonary capillary
PCO2 = 40
PCO2 = 46
PCO2 = 40
PO2 = 60
PO2 = 0

Equilibration for O2
 Diffusion of O2 occurs
from alveolar gas to
pulmonary capillary
blood
 Normal Alveolar O2
tension (PAO2) = 100
mm Hg
 Oxygen tension of
blood entering the
capillary (PvO2) = 40
mm Hg
PaO2 = 100
Alveoli
Pulmonary capillary
PAO2 = 100
PvO2 = 40
PAO2 = 100
PO2 = 60
PO2 = 0

 Diffusion of O2 occurs
from alveolar gas to
pulmonary capillary
blood
 Normal Alveolar O2
tension (PAO2) = 100
mm Hg
 Oxygen tension of
blood entering the
capillary (PvO2) = 40
mm Hg
PaO2 = 100
Alveoli
Pulmonary capillary
PAO2 = 100
PvO2 = 40
PAO2 = 100
PO2 = 60
PO2 = 0
O2
O2
HbO2
O2
Hb

 After crossing the
alveolar/capillary
membrane
 O2 diffuse in
plasma
 Raising plasma O2
tension
 Cause O2 to diffuse
into RBC
PaO2 = 100
Alveoli
Pulmonary capillary
PAO2 = 100
PvO2 = 40
PAO2 = 100
PO2 = 60
PO2 = 0
O2
O2
HbO2
O2
Hb

 Equilibration time
 Enough O2 diffuse
across the alveolar/
capillary membrane
 Blood O2 tension
and alveolar O2
tension
 Equalize in about
0.25 seconds
PaO2 = 100
Alveoli
Pulmonary capillary
PAO2 = 100
PvO2 = 40
PAO2 = 100
PO2 = 60
PO2 = 0
O2
O2
Hb O2
HbO2

Equilibration for CO2
 Diffusion of CO2
occurs from
pulmonary capillary
blood to alveolar gas
 Normal Alveolar CO2
tension (PACO2) = 40
mm Hg
 CO2 tension of blood
entering the capillary
(PvCO2) = 46 mm Hg
PaCO2 = 40
Alveoli
Pulmonary capillary
PACO2 = 40
PvCO2 = 46
PACO2 = 40
PCO2 = 6
PCO2 = 0

Equilibration for CO2
 CO2 diffuse
 From capillary blood
into alveoli
 It is estimated that the
time required for
 The blood CO2 tension
and the alveolar CO2
tension to equalize
 Is approximately
0.25 sec
PaCO2 = 40
Alveoli
Pulmonary capillary
PACO2 = 40
PvCO2 = 46
PACO2 = 40
PCO2 = 6
PCO2 = 0
Hb
Hb Hb
CO2
CO2
CO2

Equilibration
 Blood transit time
during its passage
through the
capillaries
 At rest transit time is
0.75 sec
 By 0.25 sec blood
and alveolar air have
equalized for O2 and
CO2 tensions
 During exercise
blood transit time
 Reduced to 0.34
sec
100 mm Hg
40
46
Oxygen
Carbon dioxide
0 0.25 0.50 0.75 seconds
Transit time
RBC CO2, O2
Alveolus

Factors Affecting Gas
Exchange
 Amount of gas
exchanged across
the respiratory
membrane may be
dependent on
 Perfusion or
 Diffusion
properties
Alveoli
Pulmonary capillary

Perfusion Limited Gas
Exchange
 As soon as the O2
equilibrates
 Net transfer of O2
ceases
 No additional
uptake of O2
occurs until
 Capillary blood is
replaced by new
blood
100 mm Hg
40
46
Oxygen
Carbon dioxide
0 0.25 0.50 0.75 seconds
Transit time
RBC CO2, O2
Alveolus

Exchange
 Increase in gas
exchange can only
 Be achieved by
increase in blood
flow
 Average RBC
 Spends 0.75 sec in
pulmonary capillary
 O2 equilibration
occurs in 0.25 sec
100 mm Hg
40
46
Oxygen
Carbon dioxide
0 0.25 0.50 0.75 seconds
Transit time
RBC CO2, O2
Alveolus

Exchange
 There is
normally no
increase in the
O2 content for
the last 0.5 sec
 This provides for
a safety factor
100 mm Hg
40
46
Oxygen
Carbon dioxide
0 0.25 0.50 0.75 seconds
Transit time
RBC CO2, O2
Alveolus

Diffusion Limited Gas
Exchange
 Occurs whenever
 Equilibration does not
occur
 Many pulmonary
diseases
 Reduce the rate of O2
transfer
 By altering with RM
 Reduce alveolar O2
tension
 Reduces diffusion
rate
100 mm Hg
40
46
Oxygen
Carbon dioxide
0 0.25 0.50 0.75 seconds
Transit time
RBC CO2, O2
Alveolus

Diffusion Limited Gas
Exchange
 The diffusion
rate can be
increased by
 Raising the
alveolar O2
tension (PAO2)
100 mm Hg
40
46
Oxygen
Carbon dioxide
0 0.25 0.50 0.75 seconds
Transit time
RBC CO2, O2
Alveolus  PAO2

Blood Flow
Q

Pulmonary Blood Flow
 The entire blood
flow from the right
ventricle
 Distributed to the
pulmonary vessels
 Pulmonary blood
flow is essentially
equal to cardiac
output (5 l/min)
Alveoli
Pulmonary capillary
Q

Pressure in Pulmonary
System
 Pressure in the pulmonary system
 Pressure in the RV = 25/0 mm hg
 In the PA = 25/8 mm hg
 Mean pressure of 15 mm hg
 Capillary = 7 mm hg
 LA & PV = 2 mm hg
 Varies between 1 – 5 mm hg

Blood Volume
 Blood volume of the lungs
 Is about 450 ml
 9% of total blood volume
 About 70 ml of this is in the capillaries
 The remaining is divided equally
between arteries and veins

Distribution of Blood Flow
 Effect of gravity
 Gravity has marked
effect on pulmonary
circulation
 In upright position
 Upper portion of the
lung are well above the
level of the heart
 The bases are well
below the level of the
heart
Level of
RA
Zone 1
PA >Pa >Pv
Zone 2
Pa >PA >Pv
Zone 3
Pa >Pv >PA

 There are marked
pressure gradients
 In the pulmonary
arteries from top to
bottom of the lung
Level of
RA
Zone 1
PA >Pa >Pv
Zone 2
Pa >PA >Pv
Zone 3
Pa >Pv >PA

 Pressure in capillaries
at apex (zone 1)
 Close to atmospheric in
the alveoli
 Pulmonary arterial
pressure is normally
sufficient to maintain
perfusion
Level of
RA
Zone 1
PA >Pa >Pv
Zone 2
Pa >PA >Pv
Zone 3
Pa >Pv >PA

 If it is reduced or if
alveolar pressure
increases
 Some capillaries
collapse
 Thus there will be
 No gas exchange
 Cause alveolar dead
space
Level of
RA
Zone 1
PA >Pa >Pv
Zone 2
Pa >PA >Pv
Zone 3
Pa >Pv >PA

 In the middle of the
lung (zone 2)
 Pulmonary arterial
pressure exceed
alveolar pressure
 Venous pressure is still
low
 Blood flow is
determined by
difference between
arterial & alveolar
pressure
Level of
RA
Zone 1
PA >Pa >Pv
Zone 2
Pa >PA >Pv
Zone 3
Pa >Pv >PA

 In the lower portion of
the lung (zone 3)
 The alveolar pressure is
 Lower than
pressures in all
parts of the
pulmonary
circulation
 Blood flow is
determined by
 Arterial – venous
pressure difference
Level of
RA
Zone 1
PA >Pa >Pv
Zone 2
Pa >PA >Pv
Zone 3
Pa >Pv >PA

Control of Distribution of
Blood Flow
 When conc of O2 in
the alveolus
decrease
 Less than 70%
normal ; or <73
mm Hg
 Adjacent blood
vessel constrict
within 3 to 10 sec
 This increases
resistance
under ventilated alveolus
PAO2, PACO2
vasoconstriction
Well ventilated
alveolus PAO2 =
104, PACO2 = 40

Blood Flow
 This restrict blood
flow through the
affected alveoli
 Diverts blood to
well oxygenated
alveoli
 An important
mechanism for
 Balancing blood
flow and ventilation
PAO2, PACO2
vasoconstriction
Well ventilated
alveolus PAO2 =
104, PACO2 = 40

Blood Flow
 Generalized hypoxia
as in
 Exposure to high
altitude (>5000 – 7000
feet)
 Hypoventilation
 Hypoxic
vasocosntriction can
cause
 Increase in total
pulmonary resistance
 Pulmonary
hypertension
PAO2, PACO2
vasoconstriction
Well ventilated
alveolus PAO2 =
104, PACO2 = 40

Ventilation – Perfusion Ratio
 The alveolar O2 (PAO2)
tension and CO2(PACO2)
tension
 Determined by the rate
of
(VA) and
 Transfer of O2 &
CO2 through the
respiratory
membrane
Alveoli
Pulmonary capillary
Q
VA

 In the lung with
normal ventilation &
blood flow some areas
are well
 Ventilated but poorly
perfused
 Perfused but poorly
ventilated
 In either of these
situation
 Gas exchange at the
respiratory membrane
would be impaired
Alveoli
Pulmonary capillary
Q
VA

 Ventilation –
perfusion ration
 Expressed as VA/Q
 Where
 VA = alveolar
ventilation for a
given alveolus
 Q = capillary blood
flow for the same
alveolus
Alveoli
Pulmonary capillary
Q
VA

 For the entire lung
 VA = 4.2 liters /
min
 Q = 5 liters/ min
 Thus the VA/Q =
4.2/5 = 0.84
 This is the normal
ratio
Alveoli
Pulmonary capillary
Q (5)
VA (4.2)

Effect of Ventilation-
perfusion Ratios
 If an alveolus is well
ventilated & well
perfused
 The VA/Q = 0.84
 In this case there will
be normal gas
exchange
 The alveolar gas
equilibrates with the
capillary blood partial
pressures of O2 & CO2
VA
Q
PaCO2 = 40
PVCO2 = 46
PAO2 = 104
PACO2 = 40
VA / Q = 0.84
PaO2 = 98
PVO2 = 40

perfusion Ratios
 If an alveolus is not
ventilated but is well
perfused
 The VA/Q = 0
be no gas exchange
 Pulmonary capillary
blood not oxygenated
 Shunt
VA
Q
PVO2 = 40
PVCO2 = 46
PAO2 = 40
PACO2 = 46
VA / Q = 0
PaO2 = 40
PaO2 = 46

perfusion Ratios
equilibrates with
the venous blood
partial pressures of
O2 & CO2
 If an alveolus is
well ventilated but
not perfused
 The VA/Q = ∞
VA
Q
PVO2 = 40
PVCO2 = 46
PAO2 = 149
PACO2 = 0
VA / Q = ∞

perfusion Ratios
be no gas exchange
 Pulmonary capillary
blood not oxygenated
equilibrates with the
atmospheric air partial
pressures of O2 & CO2
 Dead space
VA
Q
PVO2 = 40
PVCO2 = 46
PAO2 = 149
PACO2 = 0
VA / Q = ∞

Physiologic Shunt
 In a poorly
ventilated alveolus
 VA is low while Q is
normal
 The VA/Q < 0.8
VA
Q
PVO2 = 40
PVCO2 = 46
VA / Q < 0.8

Physiologic Shunt
 Certain portion of
venous blood does
not become
oxygenated
 Poorly aerated
blood leaves
pulmonary capillary
(shunted blood)
VA
Q
PVO2 = 40
PVCO2 = 46
VA / Q < 0.8

Physiologic Shunt
 Physiologic shunt
 There is a fall in
PaO2
 Only slight
elevation of PaCO2
 CO2 is eliminated
in ventilated alveoli
VA
Q
PVO2 = 40
PVCO2 = 46
VA / Q < 0.8

Physiologic Dead Space
 When VA is normal but
Blood flow (Q) is
decreased
 The VA/Q > 0.8
 Some of the alveolar
ventilation (VA) is wasted
 No blood flow to carry out
gas exchange
 This is physiologic dead
space
VA
Q
PVO2 = 40
PVCO2 = 46
VA / Q > 0.8

Ventilation – Perfusion Ratios
in Lung
 In the lung of
upright individual
 Upper part is less
well ventilated than
the lower part, but
 It is also poorly
perfused
 VA/Q > 0.8
 This amounts to
Dead space
Level of
RA
Zone 1
VA/Q > 0.8
Zone 2
VA/Q = 0.8
Zone 3
VA/Q < 0.8

Ventilation – Perfusion Ratios
in Lung
 In the lung of
upright individual
 Lower part is well
ventilated, but
 It is also very well
perfused
 VA/Q < 0.8
 This amounts to
physiologic shunt
Level of
RA
Zone 1
VA/Q > 0.8
Zone 2
VA/Q = 0.8
Zone 3
VA/Q < 0.8

36 - Ventilatio. n.ppt

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36 - Ventilatio. n.ppt