Gas Exchange and Alveolar
Ventillation
Objectives
At the end of this session students should be able to
1.Define dead space, physiological and anatomical
2.Define partial pressure and fractional concentration
as they apply to gases in air.
3. List the normal airway, alveolar, arterial, and mixed
venous PO2 and PCO2 values.
3. List the normal arterial and mixed venous values for
O2 saturation, [HCO3-], and pH.
4. Be able to estimate the alveolar oxygen
partial pressure (PAO2) using the simplified form
of the alveolar gas equation
5.Name the factors that affect diffusive
transport of a gas between alveolar gas and
pulmonary capillary blood.
6. Understand diffusion limited and perfusion
limited gas exchange.
7.Define oxygen diffusing capacity.
Dead space
• Regions of the respiratory system that contain air but
are not exchanging O2and CO2with blood are
considered dead space.
• Anatomic dead space
• Airway regions that, because of inherent structure, are
not capable of O2and CO2exchange with the blood.
• Includes the conducting zone, which ends at the level
of the terminal bronchioles.
• The size of the anat VD in mL is approximately equal to
a person’s weight in pounds. (Thus a 150-lb individual has an
anatomic dead space of 150 mL.)
• Alveolar dead space(alv VD)
Refers to alveoli containing air but without
blood flow in the surrounding capillaries.
An example is a pulmonary embolus
Physiologic dead space
total dead space in the lung system
(anatVD+alvVD)
VENTILATION
• Total Ventilation/minute volume or minute
ventilation
total volume of air moved in or out (usually the
volume expired) of the lungs per minute.
Alveolar Ventilation
• Represents the air delivered to the respiratory
zone(include the alveoli, alveolar sacs,
alveolar ducts, and respiratory
bronchioles)per breath.
• Increases in the Depth of Breathing:
There will be equal increases in total and alveolar ventilation per
breath, since dead space volume is constant
• Increases in the Rate of Breathing
Increased rate causes increased ventilation of dead space and
alveoli.
A woman has a respiratory rate of 18, a tidal
volume of 350 mL, and a dead space of 100 mL.
What is her alveolar ventilation?
• a. 4.0 L
• b. 4.5 L
• c. 5.0 L
• d. 5.5 L
• e. 6.0 L
Gas laws
• Boyle's law: when temperature is constant,
pressure (P) and volume (V) are inversely
related
• Henry's law : that the concentration of a gas
dissolved in a liquid is proportional to its
partial pressure.
• Henry's law is used to convert the partial
pressure of gas in the liquid phase to the
concentration of gas in the liquid phase (e.g.,
in blood).
Where;
• CX =Concentration of dissolved gas (mL gas/100 mL blood) and not the gas
present in blood in bound form like bound to hemoglobin or plasma
protein.
• PX =Partial pressure of gas (mmHg)
• Solubility =Solubility of gas in blood (mL gas/100 mL blood/mmHg)
Dalton's law: the partial pressure of a gas in a
gas mixture is the pressure that the gas would
exert if it occupied the total volume of the
mixture in the absence of the other
components.
• Partial pressure of a gas in inspired air
COMPOSITION OF AIR
Ambient
Partial
pressure- air
gas
Humidifie Alveolar
d air
air
Expired
air
P O2
158mm
149mm
104mm
120mm
P CO2
0.3mm
0.3mm
40mm
28mm
P H2O
5.7mm
47mm
47mm
47mm
P N2
596mm
563mm
573mm
565mm
Alveolar–Blood Gas Exchange
Factors affecting alveolar pCO2
1. Metabolic Rate
Factors affecting alveolar pCO2
2. Alveolar Ventilation
Hyperventilation
• inappropriately
elevated level of
alveolar ventilation,
and pACO2 is
depressed.
Hypoventilation
•
inappropriately
depressed level of
alveolar ventilation,
and pACO2 is
elevated.
Factors affecting alveolar pO2
• The Alveolar Gas Equation
• The Effect of pACO2 on pAO2
The Effect of pACO2 on pAO2
RESPIRATORY MEMBRANE
1)fluid lining alveolus
2)alveolar epithelium
3)epith basement membrane
4)interstitial space
5)Capillary basement
membrane
6)capillary endothelial cell
alveolar–blood gas transfer: Fick law of
diffusion
1. Structural Features That Affect the Rate of
Diffusion.
A- area , T- thickness
2. Factors That Are Specific to Each Gas Present
D- diffusion coefficient, P1-P2 = pressure diff
FACTORS DETERMINING DIFFUSIONDIFFUSION COEFFICIENT
• DIFFUSION COEFFICIENT=S/√MW Depends on
• 1) Solubility in water 2)Molecular Weight
• Though CO2 has greater molecular weight than O2 it
is 20 times more soluble in water than O2.
CO2 > O2> N2
Diffusing Capacity of the
Respiratory Membrane
• In practice, surface area, thickness, and the
diffusion coefficient can be combined to yield
a constant that describes the lung’s diffusing
capacity (DL) for gas. Gas flow across the
barrier can then be estimated from:
Carbon Monoxide: A Gas that is Always
Diffusion Limited
• measuring the volume of carbon monoxide
absorbed in a short period and dividing this by
the alveolar carbon monoxide partial
pressure, one can determine accurately the
carbon monoxide diffusing capacity.
11. A patient inspired a gas mixture containing a
trace amount of carbon monoxide and then held
his breath for 10 sec. During breath holding, the
alveolar PCO averaged 0.5 mm Hg and CO uptake
was 10 mL/min. What is his pulmonary diffusing
capacity (DLCO)?
(A) 2.0 mL/min per mm Hg
(B) 5.0 mL/min per mm Hg
(C) 10 mL/min per mm Hg
(D) 20 mL/min per mm Hg
(E) 200 mL/min per mm Hg
DIFFUSION-LIMITED AND
PERFUSION-LIMITED GAS
EXCHANGE
Diffusion-limited
• means that the total
amount of gas transported
across the alveolar-capillary
is limited by the diffusion
process.
• In these cases, as long as
the partial pressure
gradient for the gas is
maintained, diffusion will
continue along the length of
the capillary. The
equilibrium is not achieved.
Perfusion-limited
• means that the total
amount of gas transported
across the alveolar/capillary
membrane is limited by
blood flow (i.e., perfusion)
through the pulmonary
capillaries. In perfusionlimited exchange, the
pressure gradient is not
maintained, and in this
case, the only way to
increase the amount of gas
transported is by increasing
blood flow. Equilibrium is
achieved.
Perfusion-Limited Gas Exchange
• the transport of N2O across the alveolar/pulmonary capillary barrier
– PAN2O is constant, and PaN2O is assumed to be zero at the
beginning of the pulmonary capillary.
– Thus, initially, there is a very large partial pressure gradient for
N2O between alveolar gas and capillary blood, N2O rapidly
diffuses into the pulmonary capillary. Because all of the N22O
remains free in blood, all of it creates a partial pressure. Thus,
the partial pressure of N2O in pulmonary capillary blood
increases very rapidly and is fully equilibrated with alveolar gas
in the first one fifth of the capillary.
– Once equilibration occurs, there is no more partial pressure
gradient and, therefore, no more driving force for diffusion. Net
diffusion of N2O then ceases, although four fifths of the
capillary still remain to be travelled by blood.
• O2 (under normal conditions)
Perfusion-limited O2 transport
• In the lungs of a normal person at rest, O2 transfer from
alveolar air into pulmonary capillary blood is perfusionlimited. PAO2 is constant at 100 mm Hg.
• At the beginning of the capillary, PaO2 is 40 mm Hg,
reflecting the composition of mixed venous blood. There is a
large partial pressure gradient for O2 between alveolar air
and capillary blood, which causes diffusion into the capillary.
As O2 is added to pulmonary capillary blood, PaO2 increases.
The gradient for diffusion is maintained initially because O2
binds to hemoglobin, which keeps the free O2 concentration
in blood and the partial pressure low. Equilibration of O2
occurs about one third of the distance along the capillary, at
which point PaO2 becomes equal to PAO2, and unless blood
flow increases, there can be no more net diffusion of O2.
Thus, under normal conditions, transport is perfusionlimited.
Diffusion-limited O2 transport
• In certain pathologic conditions (e.g., fibrosis) and during
strenuous exercise, transfer becomes diffusion limited.
• Fibrosis:
– In fibrosis the alveolar wall thickens, increasing the diffusion
distance for gases across the wall and decreasing DL which
slows the rate of diffusion of O2 and prevents equilibration of
O2 between alveolar air and pulmonary capillary blood. In
these cases, the partial pressure gradient for O2 is maintained
along the entire length of the capillary, converting it to a
diffusion-limited process (although not as extreme as in the
example of CO).
– At the end of the pulmonary capillary, equilibration has not
occurred between alveolar air and pulmonary capillary blood
(PaO2 is less than PAO2), which will be reflected in a
decreased PaO2 in systemic arterial blood and decreased
PvO2 in mixed venous blood.
• O2 diffusion along the length of the pulmonary capillary in
normal persons and persons with fibrosis. A, at sea level and
B, at high altitude.
O2 transport at high altitude
• Ascent to high altitude alters some aspects of the O2 equilibration process. At
high altitude, barometric pressure is reduced, and with the same fraction of
O2 in inspired air, the partial pressure of O2 in alveolar gas also will be
reduced.
• PAO2 is reduced to 50 mm Hg, compared the normal value of 100 mm Hg.
Mixed venous PO2 is 25 mm Hg (as opposed to the normal value of 40 mm
Hg). Therefore, at high altitude, the partial pressure gradient for O2 is greatly
reduced compared with sea level. Even at the beginning of the pulmonary
capillary, the gradient is only 25 mm Hg (50 mm Hg - 25 mmHg) instead of the
normal gradient at sea level of 60 mm Hg (100 mm Hg - 40 mm Hg). This
reduction of the partial pressure gradient means that diffusion of O2 will be
reduced, equilibration will occur more slowly along the capillary, complete
equilibration will be achieved at a later point along the capillary (two-thirds of
the capillary length at altitude, compared with one third of the length at sea
level).
• The final equilibrated value for PaO2 is only 50mmHg because PAO2 is only 50
mm Hg (it is impossible for the equilibrated value to be higher than 50 mm
Hg). The equilibration of O2 at high altitude is exaggerated in a person with
fibrosis. Pulmonary capillary blood does not equilibrate even by the end of
the capillary, resulting in values for PaO2 as low as 30 mm Hg, which will
seriously hamper O2 delivery to the tissues.
Examples
Diffusion-Limited Gas Exchange
• the transport of CO across the
alveolar/pulmonary capillary barrier
– net diffusion of CO into the pulmonary capillary
depends on the magnitude of the partial pressure
gradient, is maintained because CO is bound to
hemoglobin in capillary blood. Recall that only free,
dissolved gas causes a partial pressure. Thus, CO does
not equilibrate by the end of the capillary. In fact, if
the capillary were longer, net diffusion would continue
indefinitely, or until equilibration occurred.
• the transport of O2 during strenuous exercise
and in pathologic conditions such as emphysema
and fibrosis.