B

blood pressure The force BLOODexerts against
the walls of the arteries as it travels through them,
as a combination of resistance and the HEART’s
pumping effort. A sphygmomanometer is the
device that measures blood pressure, reported in
millimeters of mercury (mm Hg). A typical blood
pressure reading reports the pressure at the peak
(systole, at ventricular contraction) and trough
(diastole, at ventricular filling) of the CARDIAC
CYCLE. The first number in a blood pressure read-
ing is the systolic measure and the second number
is the diastolic measure. These measures are inde-
pendently important as well as significant in com-
bination. Blood pressure is among the vital signs
health-care providers measure to assess general
health status.
Several mechanisms within the body, including
neurologic actions in the brainstem and hormonal
actions initiated in the KIDNEYS, regulate blood
pressure. Clusters of specialized NERVEcells in the
heart and major arteries, called baroreflex sensors,
continuously send biochemical signals to the regu-
latory mechanisms. These mechanisms are redun-
dant—that is, they overlap one another to respond
to physiologic changes such as fluid volume and
oxygen demand. These mechanisms increase
blood pressure by constricting arteries and arteri-
oles, raising the resistance blood encounters as it
flows through these blood vessels, and corre-
spondingly increasing the rate and force of the
heart’s contractions. They decrease blood pressure
through reverse actions, dilating arteries and arte-
rioles and decreasing the heart’s pumping force.
Blood pressure typically increases with exercise or
stress, reflecting increased METABOLISM. Higher
blood pressure pushes oxygen and NUTRIENTSmore
rapidly into the CAPILLARY BEDS, speeding the rate
at which these substances reach cells.

Blood pressure that is higher than is optimal for

cardiovascular health is HYPERTENSION; blood pres-

sure that is too low to adequately circulate blood

is HYPOTENSION. Most hypotension occurs as a SIDE

EFFECT of medications or neurologic conditions,

although some degree of hypotension is common

with cardiovascular slowing in aging. Researchers

believe age-related hypotension reflects distur-

bances of the baroreflexes. Cardiologists may pre-

scribe medications to constrict the arteries and

intensify the heart’s contractions when hypoten-

sion causes symptoms such as mental confusion or

SYNCOPE(fainting).

BLOOD PRESSURE VALUES

Classification Systolic Diastolic

healthy below 120 mm Hg below 80 mm Hg

prehypertension 120–139 mm Hg 80–89 mm Hg

stage 1 140–159 mm Hg 90–99 mm Hg

hypertension

stage 2 160 mm Hg and 100 mm Hg and

hypertension above above

Hypertension poses a significant threat to car-

diovascular health, raising the risk for HEART ATTACK,

RENAL FAILURE, and STROKE. Researchers do not fully

understand how hypertension develops, though

they do know the contributing factors the develop-

ment of it (salt intake, physical inactivity, OBESITY,

and DIABETES) as well as how to influence blood

pressure regulatory mechanisms to bring it under

control in most situations. Hypertension exists

when either systolic or diastolic pressure is ele-

vated. Health conditions that contribute to hyper-

tension include

• arteriosclerosis, atherosclerotic disease, and cig-
  arette smoking, each of which stiffens the arter-
  ies and narrows the arterioles

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