36
Dynamics
of
the
Vascular System
2.4.1 Control
of
the Central Cardiovascular System
Arterial baroreceptors sense and monitor blood pressure at various sites
in the cardiovascular system. They are principally located at the aortic
arch and the carotid sinus. Responses from these baroreceptors, together
with those of chemoreceptors are
transmitted to the brain.
The
chemoreceptors which monitor the
CO2,
02,
and pH of the blood are
located principally at the aortic body and the carotid body.
There
are
also
mechanoreceptors in
the
heart
(i.e.
atrial
mechanoreceptive afferent fibers), as well as thermoregulatory receptors,
that initiate appropriate reflex effects on the overall cardiovascular
system.
Additionally, skeletal muscle contraction or changes in the
composition of the extracellular fluid of tissues can activate afferent
fibers which
are
embedded
in
the muscle or tissue to cause changes in
the cardiovascular system.
Sensory inputs are temporally and spatially integrated at regions that
contain
neurons in the
brain occupying
space known as the
cardiovascular center.
This
region is located at the medulla oblongata
and pons. The medullary cardiovascular center also receives inputs from
other regions of the brain, including the medullary respiratory center,
hypothalamus, and cerebral cortex. The output from the medullary
cardiovascular center feeds
into
sympathetic and parasympathetic
autonomic motor neurons that innervate the heart and the smooth muscle
of arterioles and veins, as well as to other brain neurons.
The autonomic nervous system consists of two principal trunks: the
sympathetic nervous system and the parasympathetic system. Stimulation
of sympathetic nerves increases the rate and force of contraction of the
heart and causes vasoconstriction which increases arterial blood pressure.
The stimulation of parasympathetic nerves, causes a decrease in arterial
blood pressure.
The opposing effects of these two systems on blood
pressure are sensed by two
functionally different regions
of
the
medullary cardiovascular center.
These are known as the pressor and
depressor
regions.
Stimulation of the
pressor
center
results in
sympathetic activation and an increase in blood pressure. Stimulation of
the depressor center results in parasympathetic activation and a decrease
