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RSPT 1207 – Cardiopulmonary Anatomy & Physiology
CARBON DIOXIDE TRANSPORT
Lecture Notes
Reference & Reading: Egan’s Chapter 10
I.
CO2 can be analyzed when blood is withdrawn from an artery and is called the
PaCO2.
a. Normal PaCO2 is 40 mmHg with a range of 35-45 mmHg
b. PaCO2 >45 mmHg is considered acidosis (acid)
c. PaCO2 <35 mmHg is considered alkalosis (base)
II.
Carbon Dioxide is carried in 3 distinct ways, in the:
a. Plasma
1. Carbamino compound
2. Dissolved CO2
3. Bicarbonate
b. Carried by the RBCs
1. Dissolved CO2
2. Carbamino Hemoglobin
3. Bicarbonate
III.
PLASMA CO2
a. Carbamino compound: the CO2 is carried bound to a plasma protein; is an
insignificant amount that accounts for 1% of CO2 in the body and cannot
be measured.
b. Dissolved CO2: CO2 is dissolved in the plasma of the blood in an artery
(PaCO2) or vein (PvCO2) the CO2 exerts a pressure that can be measured.
c. Bicarbonate: carried in the plasma. This amount is significant because the
relationship between carbonic acid to bicarbonate determines the pH of
the blood.
IV.
CO2 HYDROLYSIS
a. Hydrolysis: The combination chemically of carbon dioxide turns into
carbonic acid then to bicarbonate and hydrogen ions:
CO2 + H2O
H2CO3
HCO3- + H+
b. Which direction the reaction goes, depends on the accumulation of
substances on either end of the equation
c. When there is more CO2 (left of equation), the reaction moves to the right
creating more H ions.
d. If there are more H ions (right of equation), the reaction move to the left to
create more CO2
e. Clinically applied, this attempt to maintain balance is that CO2 “creates”
acid (H+). It also means that as H ions accumulate the pH becomes acidic
2
which eventually results in an increased CO2 a very volatile acid that can
removed by ventilating deeper and faster.
f. The ratio of carbonic acid to bicarbonate determines the pH of the blood.
g. The normal ratio of HCO3 to H2CO3 is 20:1. This results in a pH of 7.40.
h. As CO2 rises, more carbonic acid is created and the ratio changes with the
rising H2CO3. The pH will drop to more acidic levels.
i. As CO2 in the blood drops, the ratio will change with H2CO3 dropping.
The pH will rise to more alkalotic levels.
V.
HENDERSON-HASSELBALCH EQUATION
a. The Henderson-Hasselbalch equation describes the relationship between
the pH of the blood and the balance of bicarbonate and carbonic acid.
b. As HCO3- rises the pH rises. As H2CO3 rises, the pH drops.
c. The equation:
pH = 6.1 + log (HCO3/H2CO3)
or
pH = 6.1 + log [HCO3/(PaCO2 X .03)]
d. Clinically, H2CO3 is time consuming and costly to determine. PaCO2 X
0.03 is equivalent to the H2CO3 concentration. Therefore the HCO3- is
always a calculated value when arterial blood gas (ABG) analysis is
reported.
e. For every 1000 molecules of CO2 dissolved as a gas (PaCO2 ) only one
molecule is hydrolyzed
f. Because the relationship between PaCO2 and H2CO3 is so stable, the
PaCO2 can be measured and the carbonic acid can be derived from the
PaCO2.
g. Thus,
H2CO3 = PaCO2 X 0.03
h. The LOG chart for Henderson-Hasselbalch equation
i. Given a ABG analysis of :
pH
PaCO2 HCO352
22
Calculate the pH:
pH = 6.1 + log[22/(52 x .03)]
pH = 6.1 + log[22/(1.56)]
pH = 6.1 + log [14.1]
pH = 6.1 +1.15
pH = 7.25
PaO2
55
3
j.
A more realistic approach with the following given:
pH
7.22
PaCO2 HCO375
PaO2
42
Calculate the HCO3- :
7.22 = 6.1 + log[HCO3-/ (75 X .03)]
7.22 = 6.1 + log [HCO3 -/2.25]
Solving for HCO3-:
HCO3- = antilog (7.22-6.1) X 2.25
HCO3- = antilog (1.12) X2.25
HCO3- = 13.2 X 2.25
HCO3- = 29.7 or 30 mEq/L
VI.
PaCO2
a. Carbonic acid molecules and H ions are not actually measured, PaCO2 is
measurable. The PaCO2 is measured with an arterial blood gas analysis.
b. The normal PaCO2 will be 35-45 mmHg
c. An exception is when you have a patient with chronic lung disease, who will
have an increased PaCO2 with a normal pH. Patients in this population will
usually also has chronic hypoxemia (55-65 torr).
VII.
CARBON DIOXIDE CARRIED IN THE RBCs
a. Dissolved CO2: CO2 is dissolved in the water of the RBC; it exerts pressure
but it is insignificant and not measurable.
b. Carbamino hemoglobin: CO2 binds to the protein portion or globin of the
Hb.
c. Bicarbonate: CO2 is carried in the RBC’s plasma as carbonic acid; it carries
a large amount of CO2 because of the enzyme Carbonic anhydrase.
 Carbonic anhydrase: quickens the hydrolysis of CO2 into
HCO3 by 13,000 times. As CO2 rises the carbonic acid rises
and the RBC’s intracellular pH drops increasing CO2 loading
and decreases the RBC’s affinity for O2.
CA
CO2 + H2O
H2CO3
HCO3- + H+
d. The amount of CO2 in the blood is:
1. Bicarbonate
2. Carbamino compounds
3. Dissolved in plasma
VIII.
CHLORIDE SHIFT
a. Also known as the Hamburger Phenomenon
80%
12%
8%
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b. Because intracellular carbonic acid rises quickly, it immediately converts to
HCO3 and H+.
c. The H+ combines with reduced Hb (HHb) and the hydrogen ion is buffered.
d. The HCO3 moves out of the RBC (diffuses into plasma), there is an
imbalance of electric charges inside the RBC, causing Cl- to move from the
plasma into the RBC.
IX.
HALDANE EFFECT
a. CO2 has a dissociation curve as well and has a linear relationship.
b. The oxyhemeglobin saturation on the RBC (SaO2) affects the ability of the
hemoglobin to carry CO2.
c. The less O2 on the Hb, the more quickly CO2 loads.
d. The more deoxygenated blood enhances the loading of CO2
e. When pH or CO2 affects the O2 dissociation curve it is due to the Bohr
Effect. When SaO2 affects the ability of Hb to hold CO2 it is due to the
Haldane Effect.
X.
APPLYING O2 AND CO2 TRANSPORT
a. In the clinical setting we will be assessing the ability for a person to:
1. transport gases
2. maintain acid/base balance
b. This is evaluated using Arterial Blood Gases (ABGs).
c. The PaO2 and SaO2 help assess the ability to oxygenate the body.
Calculating the P(A-a)O2, the a/A ratio, and CaO2 will help to determine
the adequacy of oxygenation.
d. The pH and PaCO2 asses the body’s ability to ventilate. The major
stimulation to breathe is the presence of CO2 in the cerebral spinal fluid
e. The relationship between PaCO2 and HCO3- is viewed to determine the
ability of the lungs and kidneys to maintain acid/base balance.