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Anatomy & Physiology (Open + Free)
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Unit 10:: The Respiratory System
Introduction to the
Respiratory Sy stem
Module 39 /
Respiratory Structures
and Functions
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Transport of Carbon Dioxide
Explain the m echanism s of gas
transport in the blood.
Forms of Carbon Dioxide
Carbon dioxide is carried in the blood in three forms: dissolved, attached to hemoglobin, and converted to
bicarbonate ions. Dissolved CO2 accounts for 7–10 percent of the carbon dioxide carried in the blood. This is
also the only form of carbon dioxide that diffuses from the tissues into the blood and from the blood into the
alveoli for expulsion from the body. Here, we will examine in depth the transfer of carbon dioxide using
hemoglobin or by the formation of bicarbonate.
Carbaminohemoglobin and Bicarbonate
Carbon dioxide can bind to any protein and form a carbamate compound. The protein found in the highest
concentration in red blood cells is hemoglobin, and 20–23 percent of the CO2 carried in the blood is bound to
hemoglobin in the form of carbaminohemoglobin. In the capillaries of the systemic tissues, CO2
molecules attach to the terminal amino acids of the alpha and beta chains of the hemoglobin molecule.
Deoxygenated hemoglobin (hemoglobin with no or less than the maximal oxygen bound, abbreviated HHb),
such as that found in metabolically active tissues, binds CO2 easily. In the capillaries of the lungs, the elevated
levels of oxygen found in alveoli force the carbon dioxide off the hemoglobin molecule and oxidize the protein,
freeing up hydrogen ions. Although some carbon dioxide is transported as carbaminohemoglobin, the
majority, about 70 percent, is dissolved in the blood as bicarbonate ions that arise from the reversible
reactions discussed below.
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Carbon dioxide in the presence of water can be reversibly converted to carbonic acid Carbonic acid is not very
stable and readily dissociates into a hydrogen ion and a bicarbonate ion. In fact, this is why carbonated
beverages are acidic. Carbon dioxide is added to the drink mixture under pressure and dissolves in the
beverage. When the CO2 has bubbled out of the beverage, it tastes flat because the acid is gone. The same
thing happens in red blood cells, except that red blood cells contain an enzyme called carbonic anhydrase
(CA), which is capable of facilitating one million reactions per second per enzyme molecule. Because of the
enzyme, most of the CO2 dissolved in the blood is quickly converted to carbonic acid which breaks down to
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form, hydrogen ions, and bicarbonate ions.
The chemical reaction for this process is the following:
CO2 (in the presence of CA) + H2 O ⇆ H2 CO3 ⇆ H++ HCO3
Where H2 O is water, CA is carbonic anhydrase, H2 CO3 is carbonic acid, H+ is a hydrogen ion, and HCO3 - is a
bicarbonate ion. The second part of the reaction, which produces the hydrogen and bicarbonate ions, does not
have an enzyme, but depends on the dissociation of the weak acid. This series of reactions provides buffering
for the blood.
Carbon dioxide production occurs in many tissues, especially muscle. The carbon dioxide amount of diffuses
from the tissue of origin into a systemic capillary and dissolves in the plasma. A small amount of the carbon
dioxide is transported this way. Most of the CO2 that diffused into the plasma diffuses into a red blood cell
and reacts with intracellular water molecules to produce hydrogen and bicarbonate ions. Remembering that
the reactions are reversible, it makes sense that the direction of the reaction sequences will be driven by
levels of accumulated products. The dissociation of carbonic acid is driven by the relative concentration of
carbonic acid compared to the relative levels of bicarbonate(carbonic acid’s conjugate base). A build-up of
bicarbonate in the RBCs would slow or halt the dissociation of carbonic acid. This build-up doesn’t usually
happen because, RBCs have a membrane channel that allows bicarbonate to leave the RBC and enter the
blood plasma. To maintain electric neutrality inside the RBC and in the plasma, every time a negative
bicarbonate ion leaves the red blood cell it is exchanged for a negative chloride ion from the plasma. This
exchange is called the chloride shift. The bicarbonate ion in the plasma becomes part of the blood’s
buffering system, maintaining blood pH within a narrow range. Deviation from this range compromises organ
function and can cause death. The hydrogen ion liberated from the conversion of CO2 to bicarbonate binds to
a a deoxygenated hemoglobin molecule causing it to become reduced. Deoxygenated hemoglobin easily picks
up a molecule of CO2 , creating carbaminohemoglobin. Hemoglobin is an important buffering agent for the
hydrogen ions produced from the conversion of carbon dioxide to bicarbonate ions. If this buffering did not
occur, the intracellular fluid of the red blood cell would become progressively more acidic, resulting in
deterioration of cell functions. Some CO2 from the tissues can be found as as bicarbonate ions and dissolved
CO2 in the plasma. The remainder of the carbon dioxide is attached to hemoglobin or it is still in the carbonic
acid form and will stay in the red blood cells.
Graph of partial pressure. This work by Cenv eo is licensed under a Creativ e Com m ons Attribution 3 .0 United States
(http://creativ ecom m ons.org/licenses/by /3 .0/us/).
When the blood enters the pulmonary capillaries, gaseous carbon dioxide in the plasma diffuses into the
alveoli. Some of the bicarbonate diffuses from the plasma into the red blood cells, and a chloride ion passes
back into the plasma, reversing the chloride shift that occurred in the capillaries in the systemic tissues. The
high partial pressure of oxygen in the alveoli causes the carbaminohemoglobin to dissociate into
deoxyhemoglobin, a hydrogen ion, and a molecule of carbon dioxide. The released CO2 is available for
diffusion. The free hydrogen ion combines with a bicarbonate ion and reforms carbonic acid. The carbonic acid
is converted back to carbon dioxide and water under the influence of carbonic anhydrase.