704
UNIT 4
Maintenance of the Body
19
constricts under sympathetic control, venous volume is re-
duced and blood is pushed toward the heart.
All three of these functional adaptations increase venous return,
which increases stroke volume (by the Frank-Starling mecha-
nism) and therefore increases cardiac output.
Maintaining Blood Pressure
List and explain the factors that influence blood pressure,
and describe how blood pressure is regulated.
Define hypertension. Describe its manifestations and
consequences.
Maintaining a steady flow of blood from the heart to the toes is
vital for organs to function properly. But making sure a person
jumping out of bed in the morning does not keel over from in-
adequate blood flow to the brain requires the finely tuned coop-
eration of the heart, blood vessels, and kidneys—all supervised
by the brain.
Central among the homeostatic mechanisms that regulate
cardiovascular dynamics are those that maintain blood pres-
sure, principally
cardiac output
,
peripheral resistance
, and
blood
volume
. If we rearrange the formula pertaining to blood flow
presented on p. 702, we can see how cardiac output (blood flow
of the entire circulation) and peripheral resistance relate to
blood pressure:
F
5
Δ
P/R
or
CO
5
Δ
P/R
or
Δ
P
5
CO
3
R
Clearly, blood pressure varies
directly
with CO and
R
. Addition-
ally, blood pressure varies directly with blood volume because
CO depends on blood volume (the heart can’t pump out what
doesn’t enter its chambers).
So in theory, a change (increase or decrease) in any of these
variables would cause a corresponding change in blood pres-
sure. However, what
really
happens in the body is that changes
in one variable that threaten blood pressure homeostasis are
quickly compensated for by changes in the other variables.
As we described in Chapter 18, CO is equal to
stroke volume
(ml/beat) times
heart rate
(beats/min), and normal CO is 5.0 to 5.5
L/min.
Figure 19.8
shows the main factors determining cardiac
output—venous return and the neural and hormonal controls.
Remember that the cardioinhibitory center in the medulla is “in
charge” of heart rate most of the time and, via the parasympathetic
vagus nerves, maintains the
resting heart rate
. During “resting” pe-
riods, venous return (end diastolic volume) largely controls stroke
volume. During stress, the cardioacceleratory center takes over,
activating the sympathetic nervous system and increasing both
heart rate (by acting on the SA node) and stroke volume (by en-
hancing cardiac muscle contractility, which decreases end systolic
volume). Te enhanced CO, in turn, increases MAP.
In the following discussion we explore factors that regulate
blood pressure.
Short-term regulation
by the nervous system
and bloodborne hormones alters blood pressure by changing
peripheral resistance and CO.
Long-term regulation
alters blood
volume via the kidneys. Figure 19.11 (p. 709) summarizes the
influence of nearly all of the important factors.
Despite the structural modifications of veins (large lumens
and valves), venous pressure is normally too low to promote
adequate venous return. For this reason, three functional adap-
tations are critically important to venous return:
The muscular pump.
Te
muscular pump
consists of skeletal
muscle activity. As the skeletal muscles surrounding the deep
veins contract and relax, they “milk” blood toward the heart,
and once blood passes each successive valve, it cannot flow
back
(Figure 19.7)
. People who earn their living in “stand-
ing professions,” such as hairdressers and dentists, o±en have
swollen ankles because blood pools in their feet and legs.
Indeed, standing for prolonged periods may cause fainting
because skeletal muscle inactivity reduces venous return.
The respiratory pump.
Te
respiratory pump
moves blood
up toward the heart as pressure changes in the ventral body
cavity during breathing. As we inhale, abdominal pressure
increases, squeezing local veins and forcing blood toward the
heart. At the same time, the pressure in the chest decreases,
allowing thoracic veins to expand and speeding blood entry
into the right atrium.
Sympathetic venoconstriction.
Sympathetic venoconstric-
tion reduces the volume of blood in the veins—the capaci-
tance vessels. As the layer of smooth muscle around the veins
Venous valve
(open)
Contracted
skeletal
muscle
Venous valve
(closed)
Vein
Direction of
blood flow
Figure 19.7
The muscular pump.
When contracting skeletal
muscles press against a vein, they force open the valves proximal
to the area of contraction and blood is propelled toward the heart.
Backflowing blood closes the valves distal to the area of contraction.
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