circulation: the heart and blood vessels 129
What is blood pressure?
- Blood pressure is the fluid pressure blood exerts against the
walls of blood vessels. Heart contractions generate blood
pressure.
- Systolic pressure is the peak of pressure in the aorta while blood
pumped by the left ventricle is flowing into it.
- Diastolic pressure measures the lowest blood pressure in the
aorta, when blood is flowing out of it.
Take-Home message
blood pressure Fluid pres-
sure blood exerts against
blood vessel walls. It is
measured in millimeters of
mercury.
hypertension Chronically
elevated blood pressure.
the peak of pressure in the aorta while the left ventricle
contracts and pushes blood into the aorta. The second num-
ber, diastolic pressure, measures the
lowest blood pressure in the aorta,
when blood is flowing out of it and
the heart is relaxed.
Values for systolic and diastolic
pressure provide important health
information. Chronically elevated
blood pressure, or hypertension, can
be associated with various ills, such
as athero sclerosis (Section 7.8). The chart in Figure 7.12 lists
some major causes and risk factors. Hypertension can lead
to a stroke or heart attack. Each year it indirectly kills about
180,000 Americans, many of whom may not have had any
outward symptoms. Roughly 40 million people in the
United States are unaware that they have hypertension.
Personal blood pressure monitors are marketed as tools for
keeping tabs on blood pressure.
Abnormally low blood pressure is called
hypotension. This condition can develop
when for some reason there is not enough
water in blood plasma—for instance, if
there are too few proteins in the blood
to “pull” water in by osmosis. A large
blood loss also can cause blood
pressure to plummet. Such a
drastic decrease is one sign
of a dangerous condition
called circulatory shock.
n Heart contractions generate blood pressure, which changes
as blood moves through the systemic circuit.
Blood exerts pressure against the walls
of blood vessels
Blood pressure is the fluid pressure that blood exerts
against vessel walls. Blood pressure is highest in the aorta;
then it drops along the systemic circuit. The pressure typi-
cally is measured when a person is at rest (Figure 7.11). For
an adult, the National Heart, Lung, and Blood Institute
has established blood pressure values under 120/80 as the
healthiest (Table 7.1). The first number, systolic pressure, is
8 Blood
F i g u r e 7.11 Animated! Measuring blood pressure is one way
to monitor cardiovascular health. A hollow cuff attached to a
pressure gauge is wrapped around the upper arm. The cuff is
inflated to a pressure above the highest pressure of the cardiac
cycle—at systole, when ventricles contract. As air in the cuff is
slowly released, some blood flows into the artery in the arm. The
turbulent flow causes soft tapping sounds. When the tapping
starts, the gauge’s value is the systolic pressure, measured in
millimeters of mercury (Hg). This value measures how far the
pressure would force mercury to move upward in a narrow glass
column. Now more air is released from the cuff. Just after the
sounds grow dull and muffled, blood is flowing steadily, so the
turbulence and tapping end. The silence corresponds to diastolic
pressure at the end of a cardiac cycle, before the heart pumps
out blood.
Sheila Terry/SPL/Science Source
Systolic Diastolic
Normal 100–119 60–79
Hypotension Less than 100 Less than 60
Prehypertension 120–139 80–89
Hypertension 140 and up 90 and up
Table 7.1 Blood Pressure Values (mm of Hg)
- 5
F i g u r e 7.12 A variety of factors may cause hypertension.
- Smoking
- Obesity
- Sedentary lifestyle
- Chronic stress
- A diet low in sources of potassium and calcium, such as
fruits, veggies, and dairy
- Excessive salt intake (in some individuals)
- Poor salt management by the kidneys, usually due to disease
- Factors not related to lifestyle including advancing age,
male gender, and being of African descent
Risk Factors for Hypertension
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