Gas Exchange and Pulmonary
Circulation
Learning Objectives
• Understand diffusion and the rate of diffusion.
• Understand gas pressure and partial pressure, and how
these factors determine the net direction of diffusion.
• Know the relevant partial pressures in atmospheric air,
alveolar air and expired air and why they differ.
• Know how the partial pressures of O2 and CO2 in
alveolar air depend on the ventilation rate.
• Know the basic anatomy of the respiratory membrane
and what factors determine the rate of diffusion
through this membrane.
• Understand the diffusing capacity and the factors that
increase it.
Pulmonary Gas Exchange
• This lecture will focus on the exchange of
gases between the pulmonary blood and
alveolar air, and the rate at which this occurs.
• The gases exchange by diffusion:
- O2 diffuses into the blood
- CO2 diffuses out of the blood
Diffusion
Gases diffuse from an area of high concentration to an area of low concentration.
It is based on the probability of freely moving molecules.
Gas Pressure
• Gas pressure is caused by the molecules
colliding with the surface.
• In the lungs, the gas molecules are colliding
with the surfaces of the respiratory passages
and alveoli.
• Higher concentrations of gas will produce
more collisions and cause a higher pressure.
• This idea of pressure applies to gases whether
in air or water.
Partial Pressure
• The total gas pressure is the pressure caused by all the gas
molecules colliding with the surface.
• The partial gas pressure is the pressure exerted by 1 gas
species alone. Written as PO2 (partial pressure of O2), PCO2
(partial pressure of CO2).
Atmospheric Air Partial Pressures
Nitrogen
597 mm Hg
78.62 %
Oxygen
159 mm Hg
20.84 %
Carbon
Dioxide
0.3 mm Hg
0.04 %
Water
3.7 mm Hg
0.5 %
Total
760 mm Hg
100 %
• The rate of diffusion of a gas molecule is directly proportional
to its partial pressure.
Solubility Coefficient
• The higher the solubility, the higher the
solubility coefficient and the lower the partial
pressure for a given concentration.
Partial Pressure of Water
•
•
•
•
Also called the vapor pressure.
The PH2O of atmospheric air is 3.7 mm Hg.
The PH2O at 37°C is 47 mm Hg.
Thus, the air we breathe gets humidified
during respiration. This occurs before the air
gets to the alveoli.
Direction of Diffusion
• The net diffusion is determined by the
difference between the partial pressures.
• If the partial pressure of O2 is greater in the
alveolar air than in the blood, the net
diffusion of O2 will be into the blood.
Comparing Atmospheric and Alveolar Air
• In the alveoli:
O2 is constantly being absorbed into the blood.
CO2 is diffusing into the alveolar air.
Air is humidified compared to atmospheric air.
Rate of Alveolar Removal
• The alveolar air is replaced slowly. During normal
ventilation, ~1/2 of the gas is removed in 17 sec.
• The slow replacement of alveolar air prevents sudden
changes in [blood gas].
Partial Pressure of O2 in Alveoli
• Alveolar PO2 depends on:
- The rate of O2 absorption into the blood.
- The rate of entry of new O2 during ventilation.
Why does the alveolar partial pressure of O2 not increase above 150 mm Hg?
Partial Pressure of CO2 in Alveoli
• Alveolar PCO2 depends on:
- The rate of CO2 excretion from the blood.
- The rate of removal of CO2 during ventilation.
Expired Air
Anatomy Reminder for Respiratory Membrane
Respiratory Membrane
• Gas exchange between the
alveolar air and pulmonary
blood occurs through the
membranes of the respiratory
bronchioles, alveolar ducts
and alveoli.
• The respiratory membrane has
a surface area of roughly 70
m2 or a 25 x 30-foot room.
• The total quantity of blood in
the pulmonary capillaries is
60-140 mL.
• Spreading 60-140 mL over a 25
x 30-foot room allows for rapid
respiratory exchange.
Respiratory Membrane
Factors Affecting Diffusion through the
Respiratory Membrane
• Thickness of the membrane.
• Surface area of the membrane.
• Diffusion coefficient.
• Difference in partial pressure.
Thickness and Surface Area of the
Membrane
• The thicker the membrane, the slower the
rate of diffusion. E.g., edema in the interstitial
space increases the distance gasses must
diffuse.
• If surface area decreases, the rate of diffusion
will decrease. E.g., emphysema causes
dissolution of alveolar walls.
Diffusion Coefficient
• The diffusion coefficient is proportional to the
solubility/MW.
• The greater the diffusion coefficient, the
greater the rate of diffusion.
• So, a small molecule that is highly soluble
diffuses fast (e.g., CO2). CO2 diffuses ~ 20 x
more rapidly than O2.
Differences in Partial Pressure
• As we have discussed, gases will diffuse from
areas of high partial pressure to areas of lower
partial pressure.
Diffusing Capacity
• Diffusing capacity is a
measure of how well a
gas diffuses across the
respiratory membrane.
• It is defined as the
volume of a gas that will
diffuse through the
membrane each minute
for a partial pressure
difference of 1 mm Hg.
Diffusing Capacity for O2
• Diffusing capacity for O2 is ~ 21 ml/min/mm
Hg in the average young man.
• Multiply this by the mean pressure difference
(11 mm Hg) and one obtains the amount of O2
diffusing through the respiratory membrane
each minute. In this example, 230 ml O2/min.
Effect of Exercise on Diffusing Capacity
• The diffusing capacity
increases because of:
- Dilating or opening
dormant capillaries.
- Improving the
ventilation/perfusion
ratio.
Varying Degrees of Alveolar Ventilation
and Capillary Blood Flow
• Even normally,
- Not all alveoli are equally ventilated.
- Blood flow through the capillaries is not the same
for all alveoli.
VA will be used to define alveolar ventilation.
Q will be used to define blood flow.
The VA/Q is the ventilation/perfusion ratio.
Zones of Pulmonary Blood Flow and VA/Q –
Top of Lungs
• Blood flow is low at the top
of the lungs, compared to
the bottom of the lungs.
• Also, alveolar ventilation is
low at the top of the lungs,
compared to that at the
bottom of the lungs, but
blood flow is decreased
more than ventilation.
• The VA/Q at the top of the
lungs is 2.5 x greater than
ideal.
Zones of Pulmonary Blood Flow and VA/Q –
Bottom of Lungs
• At the bottom of the
lungs, there is too little
ventilation relative to
the blood flow.
• The VA/Q at the bottom
of the lungs is 0.6 x
ideal.
VA/Q During Exercise
• During exercise, the blood flow to the upper
part of the lung increases and the VA/Q
reaches optimal values.