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(A) (B)
pH 7.4 pH <7.4
VetBooks.ir 100 Oxygenated blood 100 Oxygenated blood

80
80
leaving the lungs
leaving the lungs
Percent saturation 60 Percent saturation 60
40
40
Reduced blood
20
returning from tissues
returning from tissues 20 Reduced blood
0 0
0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140
Gaseous pressure of oxygen Gaseous pressure of oxygen
(mmHg) (mmHg)

Figure 19-15. (A) An oxygen–hemoglobin dissociation curve that illustrates the percent saturation of
hemoglobin by oxygen at different partial pressures of oxygen. (B) Effect of lowering the pH from 7.4 to
7.2 on the oxygen–hemoglobin dissociation curve.

Almost all of the carbon dioxide (93%) muscles of respiration can be consciously
that enters the blood in the systemic circu- controlled, as illustrated by voluntarily
lation diffuses into erythrocytes (Fig. 19‐16). holding the breath, normal ventilation is
Some of the carbon dioxide (23%) chemi- almost entirely reflexive.
cally combines with the hemoglobin in the Portions of the brainstem known as the
erythrocytes to form carbaminohemo- respiratory center are responsible for the
globin. When the erythrocytes carrying the regular intermittent rhythmic breathing at
carbaminohemoglobin reach the pulmo- rates appropriate for each species during
nary capillaries in the lungs, the reaction is eupnea (normal quiet breathing) and any
reversed so that the carbon dioxide can dif- changes to that rate or rhythm that must
fuse into alveoli to be expired. Most (70%) of occur. Four specific regions within the
the carbon dioxide that enters the erythro- pons and medulla have been identified,
cytes is converted to carbonic acid under each with a specific respiration‐related
the influence of the enzyme carbonic anhy- function. Neurons in the inspiratory region
drase. The carbonic acid rapidly dissociates are tonically active, firing at an inherent
into a hydrogen ion and a bicarbonate ion rhythmic rate by regular variations of their
in the erythrocyte. The hydrogen ion is membrane potentials. Expiratory‐related
buffered by hemoglobin in the erythrocyte, neurons are located in an adjacent area;
and the bicarbonate ion leaves the erythro- however, they do not discharge spontane-
cyte and enters the plasma. It is in this form ously and so are normally active only dur-
(bicarbonate ion in plasma) that most ing a forced expiration.
carbon dioxide is transported from the Stimulation of the inspiratory center
peripheral tissues by the blood to the lungs leads to contractions of the diaphragmatic
(Fig. 19‐16). Within the lungs, the reactions and intercostal muscles via neural connec-
are reversed so that carbon dioxide can be tions through the spinal cord and phrenic
reformed and be expired from the alveoli. and intercostal nerves, respectively.
Feedback circuits within the respiratory
Control of Ventilation center also relax these muscles and allow
for passive expiration.
Contraction and relaxation of skeletal The tonic activity of the inspiratory
muscle generates the forces to move air in center is regulated by neural input from a
and out of the lungs. Although the skeletal variety of sites. In resting animals, the most