TRANSPORT OF CARBON DIOXIDE
Carbon dioxide is transported by the
blood from cells to the alveoli.
Carbon dioxide is transported in the
blood in four ways:
1. As dissolved form (7%)
2. As carbonic acid (negligible)
3. As bicarbonate (63%)
4. As carbamino compounds (30%).
AS DISSOLVED FORM
Carbon dioxide diffuses into blood and
dissolves in the fluid of plasma forming a simple solution. Only
about 3
mL/100 mL of plasma of carbon dioxide is transported as dissolved state. It is
about 7% of total carbon dioxide in the blood.
AS CARBONIC ACID
Part of dissolved carbon dioxide in
plasma combines with the water to form carbonic acid. Transport of
carbon dioxide in this form is
negligible.
AS BICARBONATE
About 63% of carbon dioxide is
transported as bicarbonate. From plasma, carbon dioxide enters the
RBCs. In the RBCs, carbon dioxide
combines with water to form carbonic acid. The reaction inside RBCs
is very rapid because of the presence
of carbonic anhydrase. This enzyme accelerates the reaction.
Carbonic anhydrase is present only
inside the RBCs and not in plasma. That is why carbonic acid formation is at
least 200 to 300 times more in RBCs than in plasma.
Carbonic acid is very unstable. Almost
all carbonic acid (99.9%) formed in red blood corpuscles, dissociates into
bicarbonate and hydrogen ions. Concentration of bicarbonate ions in the cell
increases more and more.Due to high concentration, bicarbonate ions diffuse through
the cell membrane into plasma.
Chloride Shift or
Hamburger Phenomenon
Chloride shift or Hamburger phenomenon
is the exchange of a chloride ion for a bicarbonate ion across
RBC membrane. It was discovered by Hartog Jakob Hamburger
in
1892.
Chloride shift occurs when carbon
dioxide enters the blood from tissues. In plasma, plenty of sodium chloride is
present. It dissociates into sodium and chloride ions. When the negatively
charged bicarbonate
ions move out of RBC into the plasma,
the negatively charged chloride ions move into the RBC in order to
maintain the electrolyte
equilibrium (ionic balance). Anion exchanger 1 (band 3 protein), which
acts
like antiport pump in RBC membrane is
responsible for the exchange of bicarbonate ions and chloride
ions. Bicarbonate ions combine with sodium
ions in the plasma and form sodium bicarbonate. In this form, it is transported
in the blood. Hydrogen ions dissociated from carbonic acid are buffered by
hemoglobin inside the cell.
Reverse Chloride
Shift
Reverse chloride shift is the process
by which chloride ions are moved back into plasma from RBC shift. It
occurs in lungs. It helps in
elimination of carbon dioxide from the blood. Bicarbonate is converted back
into carbon dioxide, which has to be
expelled out. It takes place by the following mechanism:
When blood reaches the alveoli, sodium
bicarbonate in plasma dissociates into sodium and bicarbonate
ions. Bicarbonate ion moves into the
RBC. It makes chloride ion to move out of the RBC into the plasma, where it
combines with sodium and forms sodium chloride. Bicarbonate ion inside the RBC
combines with
hydrogen ion forms carbonic acid,
which dissociates into water and carbon dioxide. Carbon dioxide is then expelled
out.
AS CARBAMINO
COMPOUNDS
About 30% of carbon dioxide is
transported as carbamino compounds. Carbon dioxide is transported in
blood in combination with hemoglobin
and plasma proteins. Carbon dioxide combines with hemoglobin to
form carbamino hemoglobin or
carbhemoglobin. And it combines with plasma proteins to form carbamino
proteins. Carbamino hemoglobin and
carbamino proteins are together called carbamino compounds.
Carbon dioxide combines with proteins
or hemoglobin with a loose bond so that, carbon dioxide is
easily released into alveoli, where
the partial pressur of carbon dioxide is low. Thus, the combination of
carbon dioxide with proteins and
hemoglobin is a reversible one. Amount of carbon dioxide transported
in combination with plasma proteins is
very less compared to the amount transported in combination with
hemoglobin. It is because
the quantity of proteins in plasma is only half of the quantity of hemoglobin.
CARBON DIOXIDE
DISSOCIATION CURVE
Carbon dioxide is transported in blood
as physical solution and in combination with water, plasma
proteins and hemoglobin. The amount of
carbon dioxide combining with blood depends upon the partial
pressure of carbon dioxide. Carbon
dioxide dissociation curve is the curve that demonstrates the relationship
between the partial pressure of carbon dioxide and the quantity of
carbon
dioxide that combines with blood.
Normal Carbon Dioxide
Dissociation Curve
Normal carbon dioxide dissociation
curve shows that the carbon dioxide content in the blood is 48 mL%
when the partial pressure of carbon
dioxide is 40 mm Hg and it is 52 mL% when the partial pressure of
carbon dioxide is 48 mm Hg. Carbon
dioxide content becomes 70 mL% when the partial pressure is about 100 mm Hg
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