Respiratory Physiology: Gas Exchange
Dr Shihab Khogali
Ninewells Hospital & Medical School, University of Dundee
 Understand the difference
between pulmonary ventilation
and alveolar ventilation, and the
significance of anatomical dead
space
 Understand the basic principles of
ventilation perfusion matching
 Understand the significance of
alveolar dead space
What is
This
Lecture
About?
 Know that the physiological dead
space = anatomical + alveolar
dead space
 Understand the four factors which
influence the gas transfer across
the alveolar membrane
 Know the non-respiratory
functions of the respiratory
system
See blackboard for detailed learning objectives
Understand the followings in relation to the four
factors which influence gas transfer across
membranes:
The Dalton’s Law of partial pressures. Know that
gases move across membranes by partial
pressure gradient
The role of diffusion coefficient on gas transfer
across membranes
The effect of membrane surface area and
membrane thickness on gas transfer, with the
Fick’s Law of diffusion
Some inspired air remains
in the airways (anatomical
dead space) where it is not
available for gas exchange
Pulmonary Ventilation =
tidal volume (ml/ breath) x
Respiratory Rate (breath/min)
= 0.5 L X 12 breath/min = 6
L/min under resting
conditions
Alveolar Ventilation is less
than pulmonary ventilation
because of the presence of
anatomical dead space.
Alveolar Ventilation = (tidal
volume – dead space
volume) x Respiratory Rate
= (0.5 – 0.15) x 12 = 4.2
L/min under resting
conditions.
Pulmonary Ventilation
& Alveolar Ventilation
Fresh air
from inspiration
Airway dead-space
volume (150 ml)
Alveolar air
After inspiration,
before expiration
Fig. 13-22, p. 472
Pulmonary Ventilation:
Is the volume of air breathed in and out per minute
Alveolar Ventilation:
Is the volume of air exchanged between the
atmosphere and alveoli per minute
This is more important as it represent new air available for
gas exchange with blood.
To increase pulmonary
ventilation (e.g. during
exercise) both the depth
(tidal volume) and rate of
breathing (RR) increase.
Pulmonary Ventilation
& Alveolar Ventilation
Fresh air
from inspiration
Airway dead-space
volume (150 ml)
because of dead space:
It is more
advantageous to
increase the depth
of breathing
Alveolar air
After inspiration,
before expiration
It is more advantageous to increase the Depth of Breathing
Ventilation Perfusion
The transfer of gases between the body and
atmosphere depends upon:
Ventilation: the rate at which gas is passing through the
lungs.
Perfusion: the rate at which blood is passing through
the lungs
Ventilation Perfusion
Both blood flow and ventilation vary from bottom to
top of the lung
Blood Flow
2
Flow
V/Q Ratio
1
Ventilation
Bottom
Lung Position
Top
The result is that
the average arterial
and alveolar partial
pressures of O2
are not exactly the
same. Normally
this effect is not
significant but it
can be in disease.
Alveolar Dead Space
 The match between air in the alveoli and the blood in the
pulmonary capillaries is not always perfect
 Ventilated alveoli which are not adequately perfused with
blood are considered as alveolar dead space
 In healthy people, the alveolar dead space is very small
and of little importance (note: the physiological dead
space = the anatomical dead space + the alveolar dead
space)
 The alveolar dead space could increase significantly in
disease
Ventilation Perfusion Match in the Lungs
Local controls act on the smooth muscles of
airways and arterioles to match airflow to blood
flow
Accumulation of CO2 in alveoli as a result of
increased perfusion decreases airway resistance
leading to increased airflow
Increase in alveolar O2 concentration as a result of
increased ventilation causes pulmonary
vasodilation which increases blood flow to match
larger airflow
Area in which blood flow (perfusion)
is greater than airflow (ventilation)
Helps
balance
Large blood flow
Helps
balance
Small airflow
CO2 in area
O2 in area
Relaxation of local-airway
smooth muscle
Contraction of local pulmonary arteriolar smooth muscle
Dilation of local airways
Constriction of local blood vessels
Airway resistance
Vascular resistance
Airflow
Blood flow
Area in which airflow (ventilation)
is greater than blood flow (perfusion)
Helps
balance
Helps
balance
Large airflow
Small blood flow
CO2 in area
O2 in area
Contraction of local-airway
smooth muscle
Relaxation of local pulmonary
arteriolar smooth muscle
Constriction of local airways
Dilation of local blood vessels
Airway resistance
Vascular resistance
Airflow
Blood flow
Note the Different Effects of O2
1. Partial Pressure
Gradient of O2 and
CO2
2. Diffusion
Coefficient for O2
and CO2
3. Surface Area of
Alveolar
Membrane
4. Thickness of
Alveolar
Membrane
Four Factors Influence The
Rate of Gas Exchange Across
Alveolar Membrane
 Gases move across cell
membranes etc by
pressure gradient
 The partial pressure of a
gas determines the
pressure gradient for that
gas
 The partial pressure of
gas (1) in a mixture of
gases that don’t react
with each other is:
 The pressure that gas (1)
would exert if it occupied
the total volume for the
mixture in the absence of
other components
 Thus if the total pressure
of the gas mixture is 100
kPa; and half of the
mixture is gas (1): the
partial pressure for gas
(1) is 50 kPa
What is Partial Pressure
of Gas?
Dalton’s Law of Partial Pressures
The Total Pressure exerted by
a gaseous mixture =
The sum of the partial pressures of
each individual component in
the gas mixture
Ptotal =
P1 + P2 +…+ Pn
The partial pressure of gas is:
The pressure that one gas in a mixture of
gases would exert if it were the only gas
present in the whole volume occupied by
the mixture at a given temperature.
Overview of
Respiration
Gases move from higher to
lower partial pressures (partial
pressure gradient)
Note units in the diagram (mmHg).
Here in UK you we use kPa
(kilopascals) but Americans and
American texts use mmHg.
To convert divide mmHg by 7.5.
Across Pulmonary Capillaries:
O2 partial pressure gradient
from alveoli to
blood =
Atmospheric air
Inspiration
Expiration
60 mm Hg (8 kP)
Net diffusion gradients
for O2 and CO2 between
the lungs and tissues
100 – 40 mmHg i.e. (13.3- 5.3 kP)
CO2 partial pressure gradient
Alveoli
from blood to
alveoli =
6 mm Hg (0.8 kP)
46 – 40 mmHg i.e. (6.1 – 5.3 kP)
Pulmonary
circulation
Across Systemic Capillaries:
O2 partial pressure gradient
from blood to
tissue cell =
> 60 mm Hg (8 kP)
100 – < 40 mmHg i.e. (13.3- < 5.3 kP)
Systemic
circulation
CO2 partial pressure gradient
from tissue cell to
blood =
> 6 mm Hg (0.8 kP)
> 46 – 40 mm Hg i.e. (> 6.1 – 5.3 kP)
Tissues
But the partial pressure gradient for CO2 is much smaller
than the partial pressure gradient for O2???
What offset the difference in partial
pressure gradient for CO2 and O2?
CO2 is more soluble in membranes than O2. The solubility
of gas in membranes is known as the Diffusion Coefficient
for the gas.
The diffusion coefficient for CO2 is 20 times
that of O2
 The lungs provide a very
large surface area with
thin membranes to
facilitate effective gas
exchange
Effect of surface Area & Membrane
Thickness on Gas Diffusion
Fick’s Law of diffusion
 The airways divides
repeatedly to increase the
surface area for gas
The amount of gas that moves
exchange
 The small airways form
outpockets (the alveoli).
This help increase the
surface area for gas
exchange in the lungs
 The lungs have a very
extensive pulmonary
capillary network
 Remember: the
pulmonary circulation
receives the entire
cardiac output
across a sheet of tissue in unit
time is proportional to the area of
the sheet but inversely proportional
to its thickness
The Respiratory Tree
Respiratory Membranes
 Alveoli: Thin-walled
inflatable sacs
• Function in gas
exchange
• Walls consist of a
single layer of flattened
Type I alveolar cells
 Pulmonary capillaries
encircle each alveolus
 Narrow interstitial
space
Four Factors Influence the Rate of Gas Transfer Across The Alveolar Membrane
Nonrespiratory Functions of Respiratory System
 Route for water loss and heat elimination
 Enhances venous return (Cardiovascular Physiology)
 Helps maintain normal acid-base balance (Respiratory and
Renal Physiology)
 Enables speech, singing, and other vocalizations
 Defends against inhaled foreign matter
 Removes, modifies, activates, or inactivates various
materials passing through the pulmonary circulation
 Nose serves as the organ of smell