6
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
(+)
U
p
Cardiac
Stimulation
Vascular
Constriction
I
ANS
<
---------
Fast
Kidneys
Blood Volume-*
Slow
■ FIGURE 1.4 Feedback control of arterial pres-
sure (AP) by the autonom ic nervous system
(ANS)
and kidneys. A sudden fall in AP elicits a
rapid baroreceptor reflex that activates the ANS
to stim ulate the heart (increasing cardiac output)
and constrict blood vessels to restore AP. The
kidneys respond to decreased AP by retaining Na+
and water to increase blood volume, which helps
to restore AP. The (+) indicates the restoration of
arterial pressure following the initial fall in pressure
(i.e., a negative feedback response).
force for organ perfusion. As described in
Chapter 6, neural and hormonal (neurohu-
moral) mechanisms regulating cardiovascular
function are under the control of pressure sen-
sors located in arteries and veins (i.e., barore-
ceptors). These baroreceptors, through their
afferent neural connections to the brain, pro-
vide the central nervous system with informa-
tion regarding the status of blood pressure in
the body. A decrease in arterial pressure from
its normal operating point elicits a rapid baro-
receptor reflex that stimulates the heart to
increase cardiac output and constricts blood
vessels to restore arterial pressure (Fig. 1.4).
These
cardiovascular
adjustments
occur
through rapid changes in autonomic nerve
activity
(particularly
through
sympathetic
nerves) to the heart and vasculature. Negative
feedback control mechanisms, as this exam-
ple illustrates, can be defined as a process in
which a deviation from some condition (e.g.,
normal arterial pressure) leads to responses
(e.g., cardiac stimulation and vasoconstric-
tion) that diminish the deviation.
In addition to altering autonomic nerve
activity, a fall in arterial pressure stimulates
the release of hormones that help to restore
arterial pressure by acting on the heart and
blood
vessels;
they
also
increase
arterial
pressure by increasing blood volume through
their actions on renal function. In contrast to
the rapidly acting autonomic mechanisms,
hormonal mechanisms acting on the kidneys
require hours or days to achieve their full
effect on blood volume. Hormonal mecha-
nisms include secretion of catecholamines
(chiefly epinephrine) by the adrenal glands;
release of renin by the kidneys, which triggers
the formation of angiotensin II and aldos-
terone; and release of antidiuretic hormone
(vasopressin) by the posterior pituitary. Hor-
mones such as angiotensin II, aldosterone,
and vasopressin are particularly important
because they act on the kidneys to increase
blood volume, which increases cardiac output
and arterial pressure.
In summary, arterial pressure is monitored
by the body and ordinarily is maintained
within narrow limits by negative feedback
mechanisms
that
adjust
cardiac
function,
systemic vascular resistance, and blood vol-
ume. This control is accomplished by changes
in autonomic nerve activity to the heart and
vasculature, as well as by changes in circulat-
ing hormones that influence cardiac, vascular,
and renal function.
THE CONTENT OF THE
FOLLOWING CHAPTERS
This textbook emphasizes our current knowl-
edge of cellular physiology as well as the
classical biophysical concepts that have been
used for decades to describe cardiac and vas-
cular function. Chapter 2 describes the elec-
trical activity within the heart, both at the
cellular and whole organ level. Chapter 3
builds a foundation of cellular physiology by
emphasizing intracellular mechanisms that
regulate cardiac and vascular smooth mus-
cle contraction. These cellular concepts are
reinforced repeatedly in subsequent chap-
ters. Chapter 4 examines cardiac mechanical
function. Chapter 5 summarizes concepts of
vascular function and the biophysics of blood
flow in the context of regulation of arterial
and venous blood pressures. Neurohumoral
mechanisms regulating cardiac and vascular
function are described in Chapter 6. Chapter
7 describes the flow of blood within different
organs, with an emphasis on local regulatory
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