Page 298 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Respiratory Acid-Base Disorders 289

conscious factors (i.e., speaking, smelling, breath hold- extracellular compartments. Thus CO 2 diffuses rapidly
ing) can purposely alter respiration, ventilatory regula- from the tissues into red blood cells. In the blood, bicar-
tion is mainly controlled by the autonomic nervous bonate is formed through the following reaction:
system. As such, the CNS receives afferent inputs from
chest wall, airway and lung parenchyma receptors as well CA þ
CO 2 þ H 2 O $ H 2 CO 3 $ H þ HCO 3 ð3Þ
as chemoafferent information from arterial blood and
pH. Altogether, sensory information from mechanor-
The first reaction is quite slow in plasma but fast within
eceptors and chemoreceptors is integrated within
the erythrocyte due to the presence of carbonic anhydrase
brainstem breathing centers to generate the neural respi-
(CA), which quickly hydrates CO 2 to form carbonic acid.
ratory rhythm and pattern. The respiratory output is
As shown in Figure 11-2, carbonic acid spontaneously
further modulated by neurochemical signals (i.e., seroto- þ
dissociates into H and HCO 3 at intracellular pH.
nergic inputs) with subsequent contraction of effector
When the concentration rises, HCO 3 ions diffuse from
inspiratory muscles (e.g., diaphragm). Oxygen is then the red cells into the plasma. However, the cell membrane
taken into the lungs, alveolar ventilation results, and is relatively impermeable to cations (H ), and chloride
þ
blood gas levels of oxygen and carbon dioxide are (Cl ) ions diffuse into the red cells from plasma to main-

regulated as the respiratory control cycle continues.
tain electroneutrality (so-called “chloride shift”). In the
lungs, the shift of chloride out of red cells is facilitated
GAS DIFFUSION AND by the high intracellular concentration of chloride
TRANSPORT DURING (approximately 60 mEq/L) when compared with other
RESPIRATION cells. Most of the carbon dioxide (approximately 81%)
is subsequently transported to the lung as bicarbonate.
A small amount is transported still dissolved in plasma
CARBON DIOXIDE (approximately 8%), and some is combined with terminal
As oxygen is transported to and used by tissues, metabolic amino groups of blood proteins (approximately 11%), the
processes in the body normally produce approximately most important of which is carbaminohemoglobin. 78
15,000 mmol of carbon dioxide daily. The lungs are
responsible for excreting a great deal more carbonic acid OXYGEN
(H 2 CO 3 and dissolved carbon dioxide) each day than the Although respiratory blood-gas disorders primarily result
kidneys. 63 Hence, alveolar ventilation and carbon dioxide from alterations of CO 2 levels, consideration must be also
removal have a large influence in acid-base balance. be given to alterations in O 2 levels as patients with respi-
Dissolved carbon dioxide is approximately 20 to 24 times ratory acid-base disorders may also become hypoxemic.
more soluble than oxygen. It is so diffusible that we Oxygen transport is initiated by contraction of the dia-
can assume complete equilibration of PCO 2 across phragm with consequent movement of inspired gas down
membranes. As the tissues produce carbon dioxide, equi- the continually branching airways until the transitional
librium is rapidly achieved between intracellular and and respiratory bronchioles, alveolar ducts, and alveoli

Interstitial fluid Capillary

Hemoglobin
Hemoglobin-CO 2
CO 2 CO 2 CO 2 CO 2
H O
2
H CO 3
2
H
HCO 3 Hemoglobin-H
Cl
Hemoglobin
HCO 3
Cl
Cell Red blood cell
Figure 11-2 The chloride shift. Increased CO 2 from cell metabolism leaves plasma and enters red blood
cells, where it combines with hemoglobin and forms carbaminohemoglobin. The largest amount of CO 2
inside red blood cells is hydrated to form carbonic acid, which dissociates into bicarbonate and hydrogen ions.
Bicarbonate diffuses out of the red blood cells into plasma in exchange for chloride ions.