585
CHAPTER FIFTEEN
Cardiovascular System
to low-gravity conditions. It is called orthostatic
intolerance. Normally, gravity helps blood cir-
culate in the lower limbs. In microgravity or no
gravity, blood pools in blood vessels in the cen-
ter of the body, registering on receptors there.
The body interprets this as excess blood, and in
response, signals the kidneys to excrete more
F
uid. But there really isn’t an increased blood vol-
ume. On return to earth, the body has a pint to
a quart less blood than it should, up to a 10% to
20% decrease in total blood volume. If blood ves-
sels cannot sufficiently constrict to counter the
plummeting blood pressure, orthostatic intoler-
ance results. To minimize the effect, astronauts
wear lower-body suction suits, which apply a vac-
uum force that helps draw blood into the blood
vessels of the lower limbs. Maintaining fluid
intake helps prevent dehydration.
W
hen the rescue team approached
the space shuttle
Atlantis
just after it
landed on September 26, 1996, they
brought a stretcher, expecting to carry o±
Mission
Specialist Shannon Lucid, Ph.D. The fifty-three-
year-old biochemist had just spent 188 days
aboard the Russian
Mir
space station (fig. 15I).
About 70% of astronauts cannot stand at all upon
reencountering gravity, but Lucid walked, albeit a
little wobbly, the 25 feet to the crew transporter.
The human body evolved under conditions
of constant gravity. When a body is exposed to
microgravity (very low gravity) or weightless-
ness for extended periods, changes occur. Lucid
was expected to require the stretcher because of
decreased muscle mass, mineral-depleted bones,
and low blood volume. The 400 hours that she
logged on the
Mir
’s treadmill and stationary bicy-
cle may have helped her stay in shape.
Lucid was poked and prodded, monitored
and tested, as medical researchers attempted to
learn how six months in space a±
ects cardiovas-
cular functioning, respiratory capacity, mood,
blood chemistry, circadian rhythms, muscular
strength, body F
uid composition, and many other
aspects of anatomy and physiology.
²eeling unsteady upon returning to earth is
one of the better-studied physiologic responses
15.4
CLINICAL APPLICATION
Space Medicine
Control of Blood Pressure
Blood pressure (BP) is determined by cardiac output (CO)
and peripheral resistance (PR) according to this relationship:
BP = CO
×
PR. Maintenance of normal blood pressure there-
fore requires regulation of these two factors
(f g. 15.36)
.
Cardiac output
depends on the stroke volume and heart
rate. Stroke volume, the amount of blood pumped in a sin-
gle beat, is refl
ected by the difference between
end-diastolic
volume
(EDV), the volume of blood in each ventricle at the
end of ventricular diastole, and
end-systolic volume
(ESV),
the volume of blood in each ventricle at the end of ventricu-
lar systole. Mechanical, neural, and chemical factors affect
stroke volume and heart rate.
Cardiac output is limited by the amount of blood returning
to the ventricles, called the
venous return.
Usually, however,
stroke volume can be increased by sympathetic stimulation,
which increases the force of ventricular contraction. Only
about 60% of the end-diastolic volume is pumped out in a
Blood cells and some plasma proteins increase blood
viscosity. The greater the blood’s resistance to fl
owing, the
greater the force needed to move it through the vascular sys-
tem, so blood pressure rises as blood viscosity increases and
drops as blood viscosity decreases.
The viscosity of blood normally remains stable. However,
any condition that alters the concentrations of blood cells or
speciF
c plasma proteins may alter blood viscosity. ±or exam-
ple, anemia may decrease viscosity and consequently lower
blood pressure. Excess red blood cells increase viscosity and
blood pressure.
PRACTICE
45
How is cardiac output calculated?
46
How are cardiac output and blood pressure related?
47
How does blood volume a±
ect blood pressure?
48
What is the relationship between peripheral resistance and blood
pressure? Between blood viscosity and blood pressure?
FIGURE 15I
Shannon Lucid’s 188-day
stay in space revealed to researchers much
about the body’s responses to microgravity
conditions. While aboard the space station
Mir,
Lucid conducted experiments on quail
embryos and growth of protein crystals.
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