Maintenance of the Body
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
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-
. 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
Clearly, blood pressure varies
with CO and
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
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
(beats/min), and normal CO is 5.0 to 5.5
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
by the nervous system
and bloodborne hormones alters blood pressure by changing
peripheral resistance and CO.
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.
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
. 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.
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.
tion reduces the volume of blood in the veins—the capaci-
tance vessels. As the layer of smooth muscle around the veins
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.