May 3, 2023 Ventilation 1
Ventilation
May 3, 2023 Ventilation 2
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
May 3, 2023 Ventilation 3
Pulmonary Ventilation 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)
May 3, 2023 Ventilation 4
Pulmonary Ventilation At rest
VT = 500 ml , f = 12 to 15 breath per minute VE = (500 * 12) = 6000 ml/min VE = (500 * 15) = 7500 ml/min
May 3, 2023 Ventilation 5
Anatomical Dead Space The first 16
generation plus trachea and upper respiratory tract form Conducting zone
of the airways Transport gas
from & to exterior
17
1819
20
21
22
23
1
0
34
2
Bro
nchi
R
esp i
rato
ry
bron
chi o
leA
lve o
lar
duct
Conducting zone
Respiratory zone
Trachea
May 3, 2023 Ventilation 6
Anatomical Dead Space Made up of
Upper respiratory tract
Trachea Bronchi,
bronchioles, terminal bronchioles
Constitute the anatomical dead space
17
1819
20
21
22
23
1
0
34
2
Bro
nchi
R
esp i
rato
ry
bron
chi o
leA
lve o
lar
duct
Conducting zone
Respiratory zone
Trachea
May 3, 2023 Ventilation 7
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)
Tid
al v
ol =
50
0 m
l
Alveolar air
May 3, 2023 Ventilation 8
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)
Tid
al v
ol =
50
0 m
l
Alveolar air
May 3, 2023 Ventilation 9
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
May 3, 2023 Ventilation 10
Function of Anatomical Dead 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
May 3, 2023 Ventilation 11
Function of Anatomical Dead 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
May 3, 2023 Ventilation 12
Function of Anatomical Dead 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
May 3, 2023 Ventilation 13
Function of Anatomical Dead Space Prolonged breathing air
containing foreign material Cause chronic bronchitis
Increase airway resistance, difficult in breathing
May 3, 2023 Ventilation 14
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 Such alveoli do not
participate in gas exchange
They constitute alveolar dead space
17
1819
20
21
22
23
1
0
34
2
Bro
nchi
R
esp i
rato
ry
bron
chi o
leA
lve o
lar
duct
Conducting zone
Respiratory zone
Trachea
May 3, 2023 Ventilation 15
Total Dead Space Total
(physiologic) dead space include Anatomical dead
space Alveolar dead
space
17
1819
20
21
22
23
1
0
34
2
Bro
nchi
R
esp i
rato
ry
bron
chi o
leA
lve o
lar
duct
Conducting zone
Respiratory zone
Trachea
May 3, 2023 Ventilation 16
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)
Tid
al v
ol =
50
0 m
l
Alveolar air
May 3, 2023 Ventilation 17
Alveolar Ventilation 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)
Tid
al v
ol =
50
0 m
l
Alveolar air
May 3, 2023 Ventilation 18
Alveolar Ventilation Alveolar ventilation
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 Alveolar
ventilation
Portion of tidal volume air that remain in dead space (150 ml)
Portion of tidal air that gets into alveoli (350 ml)
Tid
al v
ol =
50
0 m
l
Alveolar air
May 3, 2023 Ventilation 19
Alveolar Ventilation 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)
Tid
al v
ol =
50
0 m
l
Alveolar air
May 3, 2023 Ventilation 20
Alveolar – Capillary Gas Exchange
Pulmonary capillary blood 70 ml
Pulmonary blood flow 5000 ml/min
alveoli
May 3, 2023 Ventilation 21
Alveolar/capillary 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
May 3, 2023 Ventilation 22
Alveolar/capillary 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
May 3, 2023 Ventilation 23
Alveolar/capillary 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
May 3, 2023 Ventilation 24
Alveolar/capillary Exchange
Where (FIO2) is the
conc of O2 in inspired air
(VA) is alveolar ventilation
CO2
CO2
CO2
O2
O2
O2
Alveolar space
May 3, 2023 Ventilation 25
Alveolar/capillary 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
May 3, 2023 Ventilation 26
Alveolar/capillary Exchange
Where (FACO2) is the
alveolarCO2 conc
(VA) is alveolar ventilation
CO2
CO2
CO2
O2
O2
O2
Alveolar space
May 3, 2023 Ventilation 27
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 is a measure of the concentration of the gas in the mixture
May 3, 2023 Ventilation 28
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
May 3, 2023 Ventilation 29
Partial Pressure For CO2
PCO2 = 760 * 0.0003 = 0.2 mm Hg
For N2 PN2 = 760 * 0.7904 = 600 mm
Hg
May 3, 2023 Ventilation 30
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
May 3, 2023 Ventilation 31
Partial pressures & conc of 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
May 3, 2023 Ventilation 32
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)
PA
O2 &
PA
CO
2 mm
Hg
VO2 = 250 ml/min
VO2 = 1000 ml/min
PAO2 = 100 mm Hg
PACO2 = 40 mm Hg
May 3, 2023 Ventilation 33
Partial Pressures & conc of 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)
PA
O2 &
PA
CO
2 mm
Hg
VO2 = 250 ml/min
VO2 = 1000 ml/min
PACO2 = 40 mm Hg
PAO2 = 100 mm Hg
May 3, 2023 Ventilation 34
Partial pressures & conc of 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
May 3, 2023 Ventilation 35
Partial Pressure of Respiratory Gases (mm Hg)Gas Atmospheric
airAlveolar 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
May 3, 2023 Ventilation 36
Diffusion
May 3, 2023 Ventilation 37
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
May 3, 2023 Ventilation 38
Diffusion of Gases Through the Respiratory Membrane
With respect to the lungs The area of
blood gas barrier is large
Thickness is very small
The dimensions are ideal for diffusion
Vgas (A/T)D(P1-P2)
P1 P2
T
A
D Sol/ MW
Vgas = gas transferred
A =area
T = thickness
D = diffusion const
May 3, 2023 Ventilation 39
Diffusion of Gases Through the Respiratory Membrane
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
Vgas (A/T)D(P1-P2)
P1 P2
T
A
D Sol/ MW
Vgas = gas transferred
A =area
T = thickness
D = diffusion const
May 3, 2023 Ventilation 40
Diffusion of Gases Through the Respiratory Membrane
Hence CO2 diffuses about 20 times more fast than O2 because Has much
higher solubility But not very
different MW
Vgas (A/T)D(P1-P2)
P1 P2
T
A
D Sol/ MW
Vgas = gas transferred
A =area
T = thickness
D = diffusion const
May 3, 2023 Ventilation 41
Partial Pressures & conc of 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 HgPaCO2 = 40 mm Hg
PaO2 = 100 mm Hg
May 3, 2023 Ventilation 42
Partial Pressures & conc of O2, CO2 in Alveoli
In the capillary at venous end PaO2 = 100 mmHg PaCO2 = 40 mm Hg
Thus there is Partial pressure
difference which form the driving force for diffusion of O2 and CO2
CO2 O2
Alveoli PACO2 = 40 PAO2 = 100
CO2 O2
PvCO2 = 46 mm Hg
PvO2 = 40 mm HgPaCO2 = 40 mm Hg
PaO2 = 100 mm Hg
May 3, 2023 Ventilation 43
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
Capillary endothelium
Basement membrane
Alveolar epithelium
Surface lining
Alveolar capillary membrane (0.2 m)
Alveolar diameter = 300 m
Diameter of RBC = 7.5 m
May 3, 2023 Ventilation 44
Alveolar Capillary Membrane
Also to be included RBC membrane
Capillary endothelium
Basement membrane
Alveolar epithelium
Surface lining
Alveolar capillary membrane (0.2 m)
Alveolar diameter = 300 m
Diameter of RBC = 7.5 m
May 3, 2023 Ventilation 45
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
May 3, 2023 Ventilation 46
Diffusion Capacity of the 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)
May 3, 2023 Ventilation 47
Diffusion Capacity of the Lung Reducing the partial pressure
gradient for the diffusion of gases Ventilation/perfusion
abnormalities
May 3, 2023 Ventilation 48
Diffusion Capacity of the 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
May 3, 2023 Ventilation 49
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 = 60PO2 = 0
May 3, 2023 Ventilation 50
Diffusion Capacity for O2
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 = 60PO2 = 0
May 3, 2023 Ventilation 51
Diffusion Capacity for O2
K(A/T) = DL DL new
constant Equals the
diffusion capacity of the lung
Oxygen uptake VO2 = DLO2 *
(meanPO2)
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60PO2 = 0
May 3, 2023 Ventilation 52
Diffusion Capacity for O2
DLO2 is the diffusion capacity of the lung for O2
MeanPO2 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 = 60PO2 = 0
May 3, 2023 Ventilation 53
Diffusion Capacity for O2
In the human lung VO2 = 250 ml/min MeanPO2 = 10
mm Hg Thus
DLO2 = (VO2)/
MeanPO2 = 250/10 = 25 ml of O2 / min/ mm Hg
PO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60PO2 = 0
May 3, 2023 Ventilation 54
Diffusion Capacity for O2
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 = 60PO2 = 0
May 3, 2023 Ventilation 55
Diffusion Capacity for O2
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 = 60PO2 = 0
May 3, 2023 Ventilation 56
Diffusion Capacity for O2
There is also better matching between Ventilation of
alveoli Perfusion of
capillariesPO2 = 100
Alveoli
Pulmonary capillary
PO2 = 100
PO2 = 40
PO2 = 100
PO2 = 60PO2 = 0
May 3, 2023 Ventilation 57
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 = 60PO2 = 0
May 3, 2023 Ventilation 58
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 = 60PO2 = 0
May 3, 2023 Ventilation 59
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 = 60PO2 = 0
HbHb Hb
O2
O2
O2
May 3, 2023 Ventilation 60
Equilibration for O2 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 = 60PO2 = 0
HbHb Hb
O2
O2
O2
May 3, 2023 Ventilation 61
Equilibration for O2 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 = 60PO2 = 0
HbHb Hb
O2
O2
O2
May 3, 2023 Ventilation 62
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 = 6PCO2 = 0
May 3, 2023 Ventilation 63
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 = 6PCO2 = 0
HbHb Hb
CO2
CO2
CO2
May 3, 2023 Ventilation 64
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
May 3, 2023 Ventilation 65
Factors Affecting Gas Exchange
Amount of gas exchanged across the respiratory membrane may be dependent on Perfusion or Diffusion
properties
Alveoli
Pulmonary capillary
May 3, 2023 Ventilation 66
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
May 3, 2023 Ventilation 67
Perfusion Limited Gas 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
May 3, 2023 Ventilation 68
Perfusion Limited Gas 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
May 3, 2023 Ventilation 69
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
May 3, 2023 Ventilation 70
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
May 3, 2023 Ventilation 71
Blood Flow
Q
May 3, 2023 Ventilation 72
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
May 3, 2023 Ventilation 73
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
May 3, 2023 Ventilation 74
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
May 3, 2023 Ventilation 75
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
May 3, 2023 Ventilation 76
Distribution of Blood Flow
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
May 3, 2023 Ventilation 77
Distribution of Blood Flow
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
May 3, 2023 Ventilation 78
Distribution of Blood Flow
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
May 3, 2023 Ventilation 79
Distribution of Blood Flow
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
May 3, 2023 Ventilation 80
Distribution of Blood Flow
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
May 3, 2023 Ventilation 81
Control of Distribution of Blood Flow
When conc of O2 in the alveolus decease 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
May 3, 2023 Ventilation 82
Control of Distribution of 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
under ventilated alveolus PAO2, PACO2
vasoconstriction
Well ventilated alveolus PAO2 = 104, PACO2 = 40
May 3, 2023 Ventilation 83
Control of Distribution of 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
under ventilated alveolus PAO2, PACO2
vasoconstriction
Well ventilated alveolus PAO2 = 104, PACO2 = 40
May 3, 2023 Ventilation 84
Ventilation – Perfusion Ratio
The alveolar O2 (PAO2) tension and CO2(PACO2) tension Determined by the
rate of Alveolar
ventilation (VA) and
Transfer of O2 & CO2 through the respiratory membrane
Alveoli
Pulmonary capillary
Q
VA
May 3, 2023 Ventilation 85
Ventilation – Perfusion Ratio
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
May 3, 2023 Ventilation 86
Ventilation – Perfusion Ratio
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
May 3, 2023 Ventilation 87
Ventilation – Perfusion Ratio
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)
May 3, 2023 Ventilation 88
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 = 40PVCO2 = 46
PAO2 = 104
PACO2 = 40
VA / Q = 0.84
PaO2 = 98
PVO2 = 40
May 3, 2023 Ventilation 89
Effect of Ventilation-perfusion Ratios
If an alveolus is not ventilated but is well perfused The VA/Q = 0
In this case there will be no gas exchange Pulmonary
capillary blood not oxygenated Shunt
VA QPVO2 = 40
PVCO2 = 46
PAO2 = 40
PACO2 = 46
VA / Q = 0
PaO2 = 40PaO2 = 46
May 3, 2023 Ventilation 90
Effect of Ventilation-perfusion Ratios
The alveolar gas 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 = ∞
May 3, 2023 Ventilation 91
Effect of Ventilation-perfusion Ratios
In this case there will be no gas exchange Pulmonary capillary
blood not oxygenated
The alveolar gas equilibrates with the atmospheric air partial pressures of O2 & CO2
Dead space
VA Q
PVO2 = 40
PVCO2 = 46
PAO2 = 149
PACO2 = 0
VA / Q = ∞
May 3, 2023 Ventilation 92
Physiologic Shunt In a poorly
ventilated alveolus VA is low while
Q is normal The VA/Q < 0.8
VA QPVO2 = 40
PVCO2 = 46
VA / Q < 0.8
May 3, 2023 Ventilation 93
Physiologic Shunt Certain portion
of venous blood does not become oxygenated Poorly aerated
blood leaves pulmonary capillary (shunted blood)
VA QPVO2 = 40
PVCO2 = 46
VA / Q < 0.8
May 3, 2023 Ventilation 94
Physiologic Shunt Physiologic
shunt There is a fall
in PaO2
Only slight elevation of PaCO2 CO2 is
eliminated in ventilated alveoli
VA QPVO2 = 40
PVCO2 = 46
VA / Q < 0.8
May 3, 2023 Ventilation 95
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 = 40PVCO2 = 46
VA / Q > 0.8
May 3, 2023 Ventilation 96
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
May 3, 2023 Ventilation 97
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