Model to facilitate studying oxygen
and carbon dioxide transport
KERMIT
A. GAAR, JR.
Department
of Physiology and Biophysics, School of Medicine, Louisiana State University
Medical Center, Shreveport, Louisiana 71130-3932
S6
1043-4046/91 $1.50 Copyright
0
1991 the American
Physiological
Society
Downloaded from http://advan.physiology.org/ by 10.220.33.2 on April 20, 2017
body do not change. In order for this steady-state condition to be maintained, there must always be a parity
between 1) the rate of lung ventilation, 2) the rate of
tissue perfusion, and 3) the rate of tissue utilization of
oxygen. If any one of these changes, even slightly, then
the concentrations of the gases in the body will change
also. Furthermore, the “rate of tissue perfusion” is meant
to include both the rate of oxygen delivery to and the
rate of carbon dioxide removal from the tissues. This is
determined by the product of blood flow rate and the
blood concentrations of oxygen or carbon dioxide. The
latter are determined mainly by the transport “capacity”
of the blood for these gases,which was mentioned above
in conjunction with blood hemoglobin concentration.
These and other factors can be studied with the 02/
CO2 Transport Model. This is an improved version of an
earlier model program used to study oxygen transport
that was originally written in the Applesoft BASIC lancomputer model; blood gases; cardiopulmonary
physiology
guage for use with the Apple II series of microcomputers
(1). Carbon dioxide transport was added to this new
revision, and a second version of the model has been
WHEN WE THINK of respiration, what usually comes to developed using QuickBASIC (Microsoft) for use with
mind is the movement of air into and out of the lungs. the IBM-PC and compatible microcomputers.
This is important, because it allows for oxygen absorpThe model program was constructed for the purpose
tion and carbon dioxide removal where the blood con- of facilitating the study and understanding of basic cartacts the respiratory membrane. However, it is no less diopulmonary principles of oxygen and carbon dioxide
important to think of respiration in terms of the carditransport. It is intended for use as a supplement to basic
ovascular system, which acts to transport the blood gases instruction, mainly by medical and graduate students at
between the lungs and the tissues.
an introductory level. This might be accomplished in
Transport of oxygen and carbon dioxide involves three
several different ways, depending on the requirements of
main areas: these gases are transported between the the students or instructors and the facilities available.
atmosphere and alveoli by lung ventilation, between One example would be to use the model to demonstrate
lungs and tissues via circulating blood, and between physiological simulations in a classroom setting. This
tissue capillaries and cells by the diffusion process (Fig.
approach is especially useful if microcomputer facilities
1). This would not be possible were it not for the presence are limited. However, if sufficient microcomputers are
of hemoglobin in the blood. Because oxygen is only
available for access by the students, then they can be
slightly soluble in the water of the blood, its combination
allowed to work their way through the exercises provided.
with hemoglobin increases the oxygen transport capacity
The instructor might also want to supplement these with
of blood -60-fold. Carbon dioxide is -20 times more additional exercises and/or study questions.
water soluble than oxygen, but its transport also depends
The physiological principles of the respiratory model
heavily on hemoglobin.
are based on the concept of a normal or “ideal” lung in
Under normal conditions, the amount of oxygen that
which there is no inequality of ventilation/perfusion
to
moves from the atmosphere to the tissues is determined
impair gas exchange, as is often found in diseased lungs.
by the rate at which the tissues use oxygen. This also In effect, the lung is modeled as a single large alveolus
determines the amount of carbon dioxide that moves in with one blood supply instead of the many smaller subthe opposite direction. In a normal resting individual,
units that actually exist. An analogous situation is conthe rate of oxygen use and carbon dioxide production is sidered to be present in the tissues where gas exchange
constant, and the concentrations of these gases in the occurs. In addition, the effects of pH, temperature, etc.,
GAAR, KERMIT A., JR. Model to facilitate studying oxygen
and carbon dioxide transport. Am. J. Physiol. 260 (Adv. Physiol.
Educ. 5): S6-S9, 1991.-A
simple model has been constructed
for microcomputer
(PC) simulations
involving basic cardiopulmonary principles
of oxygen transport.
Students can change
parameters such as metabolic rate, blood hemoglobin
concentration, barometric
pressure, air composition,
etc., and study
parameter effects on blood gas concentrations
and partial pressures. An important
feature of the model program is that there
are no negative feedback controls to maintain
homeostasis.
However, after a perturbation
has been introduced, adjustments
can be made to appropriate
variables to correct for abnormal
effects. For example, ventilation
rate and blood hemoglobin
concentration
might be adjusted to compensate for low atmospheric oxygen. Because these do not change automatically
in
the model program, learning is enhanced when the student has
to make the appropriate
adjustments needed to correct disturbances in the blood gases that follow a perturbation.
MODEL
OF
OXYGEN
AND
CARBON
.
Am
Capillaries
\
variables is shown in Fig. 2. A summary listing of the
values of the important model parameters and variables
can be printed at any time during a simulation.
For convenience, the model program is completely
menu driven. A simulation can be interrupted at any
time and a menu will appear on the screen. The menu
gives a list of different options from which a selection
can be made, as follows: 1) resuming the simulation
without changing parameter values, 2) changing parameter values and resuming the simulation, 3) printing
current data to the screen or a printer, 4) ending the
current simulation and starting over with a new simulation, and 5) quitting the simulation session.
Whenever option 2 is selected the operator is prompted
to make changes in the model parameters; an example
of this process is shown in Fig. 3. Adjustments can be
made to pulmonary ventilation, cardiac output, and the
rate of oxygen use by the tissues. (An important physiological concept that the model reinforces is the fact
that parity between these three basic factors is necessary
for the blood and tissue concentrations of the important
respiratory gasesto remain completely normal.) Another
factor that can be adjusted is the blood hemoglobin
\
Circulatory
Tissue Cells
1. Diagram
and carbon dioxide
FIG.
s7
TRANSPORT
TORR
of scheme used by body for transport
between atmosphere
and body tissues.
of oxygen
on oxygen-to-hemoglobin binding have been purposely
omitted, because they would have increased the model’s
complexity without significantly enhancing its heuristic
value.
IOO-
.........
PVC02
PaC02
pa02
. . .. . . .. . .. . ..._............_......._...
............................................................. pvo2
t
FEATURES
OF
THE
O&O2
TRANSPORT
MODEL
The OS/COPTransport Model program was formulated
using normal physiological values for an average adult
human individual. Physiological data have been preset
in the model program and do not have to be entered
initially when a simulation is begun. Therefore a simulation can start immediately when the model program is
run.
Complete documentation of the model and instructions for performing some special selected physiological
simulations have been prepared and are available for
distribution with the model program. For example, one
can examine the effects of a change in the barometric
pressure, such as occurs when one ascends a mountain
or descends underwater (deep-sea diving), or one can
study the effects of increased metabolism (exercise).
Other studies could be the effects of oxygen therapy on
acute heart failure or on acute respiratory failure.
The model has several convenient features to facilitate
these kinds of studies. One of the most important features is the presentation of important data in graphic
form. During a simulation the numerical values of arterial and venous blood oxygen concentrations and the
oxygen and carbon dioxide partial pressures are presented on the video monitor and updated with each
iteration while the model program is running. In addition, the arterial and venous blood oxygen and carbon
dioxide partial pressures are plotted graphically on the
screen as a function of time. A sample output of these
0-
1
I
I
5
10
15
I
I
20
Ca02
Cv02
Pa02
PvO2
17.98
12.98
55
39
25
Min
PVC02
PaC02
75
80
2. Graphical
output
of a simulation
as it would appear on a
monochrome
viaeo monitor.
I o proauce tnese results, pulmonary
ventilation
was lowered
to one-half
normal
at time indicated
by arrow.
Four curves shown changing
with time represent
4 blood gas tensions
(in Torr): P ao,, arterial
blood oxygen tension; Pvo,, venous blood oxygen
tension;
Pace,, arterial
blood carbon
dioxide
tension;
Pvco2, venous
blood carbon dioxide tension.
Corresponding
numerical
values shown
under graph are updated with each iteration.
These also include arterial
and venous blood oxygen concentrations,
Cao, and Cvo,, which are not
plotted on graph.
FIG.
*
1
-1
n-i
1
11
11
CHANGE
ALVEOLAR
CHANGE
INSPIRED
OXYGEN
PRESSURE
CHANGE
CARDIAC
OUTPUT
?
(Y
(Y
OR
OR
N)
N)
: N
: N
CHANGE
OXYGEN
?
(Y
OR
N)
: N
CHANGE
HEMOGLOBIN
(Y
OR
N)
: Y
HEMOGLOBIN
VENTILATION
1
DEMAND
HEMOGLOBIN
ARE
THESE
VALUES
?
NOW
( 15
CONCENTRATION
CORRECT
3
?
: N
3
CONCENTRATION
CONCENTRATION
NEW
RATE
?
(Y OR N)
(Y
OR
:
) G/DL
10
N)
: Y
FIG. 3. Facsimile
of type of output presented
on video screen when
option 2 is selected from previous
screen menu as described
in text. In
example
shown, only blood hemoglobin
was changed before resuming
simulation.
Downloaded from http://advan.physiology.org/ by 10.220.33.2 on April 20, 2017
c
/
DIOXIDE
S8
MODEL
OF
OXYGEN
AND
CARBON
DIOXIDE
TRANSPORT
of carbon dioxide transported.
The relative importance
of the above parameters
to
total carbon dioxide transport
can be seen graphically in
Fig. 4. At normal carbon dioxide production,
ventilation
has the greatest effect. Blood flow is not shown, but it is
about equal in effect to blood carbon dioxide transport
capacity, which is mainly determined by blood hemoglobin concentration.
For example, lowering either blood
hemoglobin concentration
or cardiac output to one-third
normal should cause Pv co, to increase from a normal
value of 45 Torr to -53-55 Torr. (Note also that although
Pvco, has risen, the total blood carbon dioxide concentration has fallen.) To summarize, because Pvco, is also
an important
indicator
of the body tissue PCO~ level,
total body stores of carbon dioxide can be expected to
increase whenever
blood hemoglobin
concentration
or
cardiac output falls below normal. This could be prevented, however, if there were a compensatory
increase
in ventilation.
Use of the model in this manner to examine the
relationship
between these variables will further illustrate and reinforce the concept that a parity must be
maintained to keep the blood and tissue gas levels from
changing appreciably.
AN EXAMPLE:
USING
THE
MODEL
CARBON
DIOXIDE
TRANSPORT
IMPLEMENTING
TO
STUDY
Carbon dioxide is formed as oxygen is metabolized by
the body tissues. Consequently,
tissue PCO~ depends on
the rate at which oxygen is used and on the rate at which
carbon dioxide is removed from the tissues by blood flow.
There are two mechanisms
that the cardiopulmonary
system can use to help compensate for an elevated rate
of carbon dioxide production
due to acutely increased
metabolism.
One is to increase cardiac output via the
body’s tissue blood flow autoregulation
phenomenon (2).
This carries the carbon dioxide away from the tissues as
fast as it is formed. The other mechanism is to increase
the rate of lung ventilation.
The effect of this is not so
obvious, but it works because tissue carbon dioxide levels
are also affected by the PCO~ of the arterial blood entering
the tissues. Furthermore,
arterial blood PCO~ is determined by the rate of alveolar ventilation
that removes
carbon dioxide from the pulmonary blood.
The tissues of the body contain large stores of carbon
dioxide, and an average adult produces several hundred
liters daily. It is especially important
that none of this
excess carbon dioxide be allowed to accumulate in the
body. An important
indicator
of the level of carbon
dioxide in the body is the PCO~ of the venous blood
(Pvco,) leaving the tissues. This is because it is in equilibrium with the tissue Pco~. The important
factors
affecting tissue Pco~, as indicated by Pvco2, are given by
equations found in the APPENDIX.
Namely, these are 1)
the rate of carbon dioxide formation, 2) the lung ventilation rate, 3) the tissue blood flow, and 4) the blood’s
capacity to transport carbon dioxide. The latter is mainly
determined by the blood hemoglobin concentration.
Because hemoglobin binds with both carbon dioxide and
with the hydrogen ions formed from carbonic acid, it is
responsible for as much as 80-90% of the total amount
WITH
SIMULATION
MODEL
SOFTWARE
System requirements
for running the 02/COZ Transport Model software depend on the microcomputer
and
peripheral devices available. Applesoft BASIC in ROM
TOTAL CO2
TRANSPORT
loo-
-
80
20 \SA= 1/2x
QA=~xN
0
1
0
I
20
1
I
40
BLOOD
1
PC02
-
I
60
TORR
I
80
I
N
1
100
4. Graph
showing
composite
dynamic
effects of alveolar
ventilation
rate (VA) and blood hemoglobin
concentration
([Hb])
on total
amount
of carbon dioxide
transported
by blood. Total CO, transport
means all of carbon
dioxide
in all its various
forms
that can be
transported
by blood. These data were obtained
with the model under
conditions
that both cardiac output and tissue oxygen uptake remained
unchanged
from normal.
FIG.
Downloaded from http://advan.physiology.org/ by 10.220.33.2 on April 20, 2017
concentration.
In addition, the oxygen pressure of the
inspired air can be changed in two different ways. One
way is to change the barometric
pressure, and the other
way is to change the inspired air oxygen concentration.
There are no automatic (reflex) adjustments
to breathing or blood flow built into the model program to counter
the effects of the changes described above. Instead, the
operator must supply any interventions
needed to correct
a disturbance,
which is part of the learning experience
provided by the model. For example, to keep blood gases
within a normal range during exercise, one must learn to
coordinate lung ventilation
with changes in cardiac output. After having tried this manually for several simulations, one can better appreciate the body’s mechanisms
that perform these functions automatically.
Another important
feature of the model program is
incorporated
into option 4. This option allows for beginning a new simulation with a new graph or leaving the
graph of the previous simulation on the screen for overlaying the new simulation. This is useful if one wants to
compare the results of two simulations.
For example, one
might study the effects of different types of interventions
to correct a particular disturbance
and have all recorded
on the same graph.
MODEL
OF
OXYGEN
AND
CARBON
(read-only memory) and a minimum
of 48 KB RAM
(random access memory) are required for Apple microcomputers and compatibles. For the IBM-PC
and other
compatible microcomputers,
MS-DOS or PC-DOS and
either a Hercules monochrome graphics card or a color
graphics card (IBM CGA, EGA, or VGA) is required.
Color graphics is preferred but not necessary.
Software for the model program is available on a
standard 5.25 in. diskette. Requests for this software
should specify either Apple or IBM format. Please send
requests to K. A. Gaar, Jr., Dept. of Physiology and
Biophysics, L. S. U. Medical Center, PO Box 33932,
Shreveport, LA 71130-3932 [FAX: (318)-674-60051.
APPENDIX
DIOXIDE
alent of the carbon dioxide-hemoglobin
dissociation curve. Substituting
rate of CO, production
p&To, = Pace, + f(C0, cap) x cardiac output
and assuming
ho,
equation
is
ho,
=
Cac0,
+
rate of CO, production
cardiac output
where Caco, and Cv co, are arterial and venous CO, concentrations, since Caco, = Pace, x f(C02 cap) and Cvco = Pvco x
f(COz,,,), where CO zcap is the blood carbon dioxide-carrying
capacity and Pa co, and Pvco, are arterial and venous Pco2,
respectively. These two expressions are functionally
the equiv-
PAco,
+
rate of CO, production
f(COs cap) x cardiac output
pwoz
from the alveolar ventilation
K x rate of CO, production
=
alveolar ventilation
rate
-Iis the rate of CO, production,
pvco, = vco2
rate of CO2 production
P&o, = alveolar ventilation rate xK
where PACT,is the alveolar partial pressure of carbon dioxide
and K is a constant.
The Fick equation (rearranged)
for carbon dioxide is
=
Received
rate of CO2 production
f(COa Cap) x cardiac output
then
K
alveolar
ventilation
+
Address
Louisiana
Shreveport,
equation
rate
1
f(COz cap) x cardiac
output
1
for reprint
requests:
Dept. of Physiology
and Biophysics,
State University
Medical
Center,
School
of Medicine
in
PO Box 33932, Shreveport,
LA 71130-3932.
22 February
1990; accepted
in final
form
24 September
1990.
REFERENCES
1. GAAR, K. A., JR. Oxygen
transport:
a simple model for study and
examination.
Physiologist
28: 412-415,
1985.
2. GAAR, K. A., JR. Oxygen
transport:
an analysis of short-term
blood
flow autoregulation.
Comp. Biomed.
Res. 20: 214-224,
1987.
Downloaded from http://advan.physiology.org/ by 10.220.33.2 on April 20, 2017
ventilation
that Pace, = PA~~~,then
Substituting
If ho2
The alveolar
s9
TRANSPORT