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Internal respiration is about
ensuring the transport of oxygen in the blood from the lungs to the cells,
and the transport of metabolic carbon dioxide from the tissue cells into the
blood and to the lungs. Once CO2 and
H2O enter the interstitial fluid (around the cells) as a consequence
of cellular respiration, they diffuse into the plasma of the blood. About 90 percent of the CO2 then
diffuses into the red blood cell. The
balance of about 10 percent remains dissolved in the plasma, the dissolved PCO2. The presence of CO2 in the red
blood cell is crucial to oxygen distribution.
Carbon dioxide is hydrated
(combines with H2O) to form carbonic
acid: CO2 + H2O ↔ H2CO3. The carbonic acid dissociates (breaks down) into hydrogen and bicarbonate ions: H2CO3 ↔
H+ + HCO3̄. The increased presence of hydrogen ions, H+,
means that the red blood cells become less alkaline, i.e. the pH of the fluid
(cytosol) in red blood cells
decreases. The bicarbonates, HCO3̄,
diffuse into the blood where they buffer acids, e.g. lactic acid. The amount of CO2
generated by tissues determines precisely how much carbonic acid is formed,
and thus the pH of the red blood cell, as well as the amount of bicarbonate
entering the plasma. The presence of
CO2 gas and the drop in pH within red blood cells, independently
and together, alter the spatial constitution (conformation) of the hemoglobin (Hb), which decreases its
affinity for oxygen, i.e., it more readily gives up its oxygen and raises
plasma PO2 level; this change is known as the Bohr Effect. Thus, hemoglobin
more readily distributes its O2 to the tissues that need it, while
simultaneously buffering the hydrogen ions generated by the dissociation of
carbonic acid (H2CO3) to restore normal pH in red blood
cells: HbO2 + H+ ↔ HHb + O2. Reduced pH and increased PCO2
not only predisposes hemoglobin to release its oxygen, but also to release nitric oxide (a gas), a potent
vasodilator. The result is increased
blood volume and flow, which increases oxygen and glucose supply to cells
that generate higher levels of CO2, cells with elevated
metabolism. Increased plasma PCO2
levels lead to increased (1) supply
of oxygen (more blood), (2) supply of glucose (more blood), (3) levels of PO2
(O2/ml blood), and (4) supply of bicarbonates for buffering
acids. Proper PCO2
regulation means that red blood cell chemistry reflects surrounding tissue
metabolism. Overbreathing reduces
dissolved PCO2, and thus decreases CO2 and carbonic
acid in red blood cells. This means
reduced hydrogen ion concentration, increased pH in red blood cells. The effect on hemoglobin is twofold: (1)
increased affinity for O2 (Bohr Effect), reducing the likelihood
of its release into the plasma, and (2) diminished release of nitric oxide,
resulting in vasoconstriction. This translates
into less oxygen (local hypoxia),
less glucose (local hypoglycemia),
and reduced buffering capacity for the tissues in need. Reduced nitric oxide also elevates plasma
platelet level, their aggregation, and “adhering” propensity, thus increasing
the likelihood of blood clotting. Click here to learn
about external respiration
and cellular
respiration. Copyrighted by Behavioral Physiology Institute, Santa Fe, New Mexico USA |