The Cardiovascular System: Blood Vessels
angiotensin II receptor blockers. Inhibiting ACE or blocking
receptors for angiotensin II suppresses the renin-angiotensin-
, which ac-
counts for 10% of cases, is due to identiﬁable conditions, for ex-
ample obstructed renal arteries, kidney disease, and endocrine
disorders such as hyperthyroidism and Cushing’s syndrome.
Treatment for secondary hypertension focuses on correcting
the problem that caused it.
In many cases,
, or low blood pressure (below
90/60 mm Hg), simply reﬂects individual variations and is no
cause for concern. In fact, low blood pressure is oFen associated
with long life and an old age free of cardiovascular disease.
Hypotension is usually a concern only if it leads to inadequate
blood ﬂow to tissues. ±or example, hypotension may cause diz-
ziness or fainting because of inadequate oxygen delivery to the
is one of the most important signs of
circulatory shock (p. 717) and a threat to patients undergoing
surgery and those in intensive care units.
is a temporary drop in blood pres-
sure resulting in dizziness when a person rises suddenly from
a reclining or sitting position. Elderly people are prone to or-
thostatic hypotension because the aging sympathetic nervous
system does not respond as quickly as it once did to postural
changes. Blood pools brieﬂy in the lower limbs, reducing blood
pressure and consequently blood delivery to the brain. Chang-
ing position slowly gives the nervous system time to adjust and
usually prevents this problem.
is a sign of a serious under-
lying condition. Addison’s disease (inadequate adrenal cortex
function), hypothyroidism, or severe malnutrition can cause
Check Your Understanding
Describe the baroreceptor reﬂex changes that occur to
maintain blood pressure when you rise from a lying-down to
a standing position.
The kidneys play an important role in maintaining MAP
by inﬂuencing which variable? Explain how renal artery
obstruction could cause secondary hypertension.
For answers, see Appendix H.
Blood Flow Through Body
Tissues: Tissue Perfusion
Explain how blood ﬂow is regulated in the body in general
and in speciﬁc organs.
Blood ﬂow through body tissues, or
, is in-
volved in (1) delivering oxygen and nutrients to tissue cells, and
removing wastes from them, (2) exchanging gases in the lungs,
(3) absorbing nutrients from the digestive tract, and (4) forming
Chronic hypertension is a common and dangerous disease.
An estimated 30% of people over age 50 are hypertensive. Al-
though this “silent killer” is usually asymptomatic for the ﬁrst
10 to 20 years, it slowly but surely strains the heart and damages
the arteries. Prolonged hypertension is the major cause of heart
failure, vascular disease, renal failure, and stroke. ²e higher the
pressure, the greater the risk for these serious problems.
Because the heart is forced to pump against greater resis-
tance, it must work harder, and over time the myocardium
enlarges. When ﬁnally strained beyond its capacity, the heart
weakens and its walls become ﬂabby. Hypertension also rav-
ages the blood vessels, accelerating the progress of atheroscle-
A Closer Look
on pp. 700–701). As the vessels become
increasingly blocked, blood ﬂow to the tissues becomes inad-
equate and vascular complications appear in the brain, heart,
kidneys, and retinas of the eyes.
Although hypertension and athero-
sclerosis are oFen linked, it is oFen diﬃcult to blame hyperten-
sion on any distinct anatomical pathology. Indeed, about 90% of
hypertensive people have
for which no underlying cause has been identiﬁed. ²is is be-
cause primary hypertension is due to a rich interplay between
your genes and a variety of environmental factors:
Hypertension runs in families. Children of hyper-
tensive parents are twice as likely to develop hypertension as
are children of normotensive parents, and more blacks than
whites are hypertensive. Many of the factors listed here re-
quire a genetic predisposition, and the course of the disease
varies in diﬀerent population groups.
Dietary factors that contribute to hypertension include
high intakes of salt (NaCl), saturated fat, and cholesterol, and
deﬁciencies in certain metal ions (K
, and Mg
Obesity causes hypertension in a number of ways
that are not yet well understood. ±or example, adipocytes
release hormones that appear to increase sympathetic tone
and interfere with the ability of endothelial cells to induce
Hypertension usually appears aFer age 40.
Particularly at risk are “hot reactors,” people whose
blood pressure zooms upward during every stressful event.
Nicotine, an important chemical in tobacco and
one of the strongest toxins known, causes intense vasocon-
striction not only by directly stimulating postganglionic
sympathetic neurons but also by prompting release of large
amounts of epinephrine and NE. Chemicals in cigarette
smoke also damage the tunica intima, interfering with its
ability to chemically regulate arteriolar diameter.
Primary hypertension cannot be cured, but most cases can
be controlled. Restricting salt, fat, and cholesterol intake, in-
creasing exercise and losing weight, stopping smoking, manag-
ing stress, and taking antihypertensive drugs can all help. Drugs
commonly used are diuretics, beta-blockers, calcium channel
blockers, angiotensin converting enzyme (ACE) inhibitors, and