706
UNIT 4
Maintenance of the Body
19
Tree mechanisms bring this about:
Arteriolar vasodilation.
Decreased output from the vasomo-
tor center allows arterioles to dilate. As peripheral resistance
falls, so does MAP.
Venodilation.
Decreased output from the vasomotor center
also allows veins to dilate, which shiFs blood to the venous
reservoirs. Tis decreases venous return and CO.
Decreased cardiac output.
Impulses to the cardiac centers
inhibit sympathetic activity and stimulate parasympathetic
activity, reducing heart rate and contractile force. As CO
falls, so does MAP.
Baroreceptor Reflexes
When arterial blood pressure rises, it activates
barorecep-
tors
. Tese stretch receptors are located in the
carotid sinuses
(dilations in the internal carotid arteries, which provide the
major blood supply to the brain), in the
aortic arch
, and in
the walls of nearly every large artery of the neck and thorax.
When stretched, baroreceptors send a rapid stream of im-
pulses to the cardiovascular center, inhibiting the vasomotor
and cardioacceleratory centers and stimulating the cardio-
inhibitory center. Te result is a decrease in blood pressure
(Figure 19.9)
.
Stimulus:
Blood pressure
(arterial blood
pressure falls below
normal range).
1
Stimulus:
Blood pressure
(arterial blood
pressure rises above
normal range).
1
Baroreceptors
in carotid sinuses
and aortic arch
are stimulated.
2
Baroreceptors
in carotid sinuses
and aortic arch
are inhibited.
2
Impulses from baroreceptors
stimulate cardioinhibitory center
(and inhibit cardioacceleratory
center) and inhibit vasomotor
center.
3
Impulses from baroreceptors
activate cardioacceleratory center
(and inhibit cardioinhibitory center)
and stimulate vasomotor center.
3
CO and
R
return blood
pressure to
homeostatic range.
5
CO and
R
return
blood pressure to
homeostatic range.
5
Rate of
vasomotor impulses
allows vasodilation,
causing
R.
4b
Vasomotor
fibers stimulate
vasoconstriction,
causing
R.
4b
Sympathetic
impulses to heart
cause
HR,
contractility, and
CO.
4a
Sympathetic
impulses to heart
cause
HR,
contractility, and
CO.
4a
IMBALANCE
IMBALANCE
Homeostasis:
Blood pressure in normal range
Figure 19.9
Baroreceptor reflexes that help maintain blood pressure homeostasis.
CO
5
cardiac output;
R
5
peripheral resistance; HR
5
heart rate; BP
5
blood pressure.
previous page 740 Human Anatomy and Physiology (9th ed ) 2012 read online next page 742 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off