nitric oxide to the periphery, the process could cause additional
vasodilation by systemic blood vessels. This would have effects
on both local blood ﬂ
ow and systemic arterial blood pressure.
Finally, the fetus has a unique form of hemoglobin
which has a higher afﬁ nity for oxy-
gen. This allows an increase in oxygen uptake in the placenta.
Therefore, although fetal arterial
is lower than that in the
air-breathing newborn, fetal hemoglobin allows adequate oxy-
gen supply to the developing organs.
Control of Respiration
Neural Generation of Rhythmical Breathing
The diaphragm and intercostal muscles are skeletal muscles
and therefore do not contract unless the nerves stimulate them
to do so. Thus, breathing depends entirely upon cyclical respi-
ratory muscle excitation of the diaphragm and the intercostal
muscles by their motor nerves. Destruction of these nerves
results in paralysis of the respiratory muscles and death, unless
some form of artiﬁ cial respiration can be instituted.
Inspiration is initiated by a burst of action potentials in the
spinal motor nerves to inspiratory muscles like the diaphragm.
Then the action potentials cease, the inspiratory muscles relax,
and expiration occurs as the elastic lungs recoil. In situations
such as exercise when the contraction of expiratory muscles
facilitates expiration, the nerves to these muscles, which were
not active during inspiration, begin ﬁ ring during expiration.
By what mechanism are nerve impulses to the respira-
tory muscles alternately increased and decreased? Control of
this neural activity resides primarily in neurons in the medulla
oblongata, the same area of the brain that contains the major
cardiovascular control centers. (For the rest of this chapter we
will refer to the medulla oblongata simply as the medulla.)
There are two main anatomical components of the
). The neurons of the
sal respiratory group (DRG)
re during inspira-
tion and have input to the spinal motor neurons that activate
respiratory muscles involved in inspiration—the diaphragm
and inspiratory intercostal muscles. The primary inspiratory
muscle at rest is the diaphragm, which is innervated by the
phrenic nerves. The
ventral respiratory group (VRG)
other main complex of neurons in the medullary respiratory
respiratory rhythm generator
is located in the
of neurons in the upper part of the
VRG. This rhythm generator appears to be composed of pace-
maker cells as well as a complex neural network which, acting
together, set the basal respiratory rate.
The lower part of the VRG contains nerves that ﬁ re both
during inspiration and expiration. The inspiratory neurons of the
Brainstem respiratory control centers responsible
for respiratory rhythm generation, activation of
inspiratory and expiratory nerves and muscles, and
for monitoring lung inﬂ ation via stretch receptors
and alveolar ventilation via changes in arterial blood
gas partial pressures.
Blood gas partial pressures
Spinal motor neurons
Ventral respiratory group
Dorsal respiratory group
Lung stretch receptors