ventilation 27-apr-17 ventilation

May 3, 2023 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


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)


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


Anatomical Dead Space The first 16

generation plus trachea and upper respiratory tract form Conducting zone

of the airways Transport gas

from & to exterior

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Conducting zone

Respiratory zone

Trachea


Anatomical Dead Space Made up of

Upper respiratory tract

Trachea Bronchi,

bronchioles, terminal bronchioles

Constitute the anatomical dead space

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Conducting zone

Respiratory zone

Trachea


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


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



Tid

al v

ol =

50

0 m

l

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


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


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


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


Function of Anatomical Dead 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 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


Total Dead Space Total

(physiologic) dead space include Anatomical dead

space Alveolar dead

space

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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


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



Tid

al v

ol =

50

0 m

l

Alveolar air


Alveolar Ventilation Alveolar

ventilation (VA) VA = (VT – VD) *

f VA = (500 – 150) *

12 VA = 4200

ml/min



Tid

al v

ol =

50

0 m

l

Alveolar air


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



Tid

al v

ol =

50

0 m

l

Alveolar air


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



Tid

al v

ol =

50

0 m

l

Alveolar air


Alveolar – Capillary Gas Exchange

Pulmonary capillary blood 70 ml

Pulmonary blood flow 5000 ml/min

alveoli


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



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



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



Where (FIO2) is the

conc of O2 in inspired air

(VA) is alveolar ventilation

CO2

CO2

CO2

O2

O2

O2

Alveolar space



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



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 is 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



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)

PA

O2 &

PA

CO

2 mm

Hg

VO2 = 250 ml/min

VO2 = 1000 ml/min

PAO2 = 100 mm Hg

PACO2 = 40 mm Hg



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



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

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


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

Vgas (A/T)D(P1-P2)

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

Vgas (A/T)D(P1-P2)

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

Vgas (A/T)D(P1-P2)

P1 P2

T

A

D Sol/ MW


A =area

T = thickness

D = diffusion const



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



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


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


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


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


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)


Diffusion Capacity of the Lung Reducing the partial pressure

gradient for the diffusion of gases Ventilation/perfusion

abnormalities


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


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



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



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



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



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



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



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



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


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


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


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


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


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


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


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


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



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



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 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



This restrict blood flow through the affected alveoli Diverts blood to

well oxygenated alveoli

An important mechanism for Balancing blood

flow and ventilation


vasoconstriction




Generalized hypoxia as in Exposure to high

altitude (>5000 – 7000 feet)

Hypoventilation Hypoxic

vasocosntriction can cause Increase in total

pulmonary resistance Pulmonary

hypertension


vasoconstriction



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



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 = 40PVCO2 = 46

PAO2 = 104

PACO2 = 40

VA / Q = 0.84

PaO2 = 98

PVO2 = 40



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



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 = ∞



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 = ∞


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


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


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


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


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

ventilation 27-apr-17 ventilation

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