484 Chapter 14
who eat a high-salt diet, but for the general population, a diet
that is too low in salt may also be unhealthy. However, dietary
potassium influences the relationship between dietary salt and
hypertension. A diet low in potassium increases the effect of
salt on blood pressure, while a diet higher in potassium—as
can be obtained by eating fruit and vegetables—may reduce
the ability of salt to cause hypertension.
Dangers of Hypertension
If other factors remain constant, blood flow increases as arte-
rial blood pressure increases. The organs of people with hyper-
tension are thus adequately perfused with blood until the
excessively high pressure causes vascular damage. Because
most patients are asymptomatic (without symptoms) until sub-
stantial vascular damage has occurred, hypertension is often
referred to as a silent killer.
Hypertension is dangerous for a number of reasons. One
problem is that high blood pressure increases the afterload, mak-
ing it more difficult for the ventricles to eject blood. The ven-
tricles must then work harder, leading to pathological growth
of their walls. This abnormal hypertrophy that can result from
hypertension, or from valve defects or obesity, increases the
risk of arrhythmias and heart failure. Hypertrophy due to these
causes differs from the normal left ventricular hypertrophy often
seen in well-trained athletes, which relieves wall stress and is
believed to be beneficial.
Additionally, high pressure may damage cerebral blood
vessels, leading to cerebrovascular accident, or “stroke.”
(Stroke is the third leading cause of death in the United States.)
Finally, hypertension contributes to the development of athero-
sclerosis, which can itself lead to heart disease and stroke as
previously described.
CLINICAL APPLICATION
Preeclampsia, formerly called toxemia of pregnancy, occurs in
up to 8% of women worldwide who are pregnant beyond their
twentieth week. It is characterized by the new onset of hyper-
tension, but differs from gestational hypertension by evidence
of damage to organs such as the liver and kidneys. Thrombo-
cytopenia (low platelet count) may occur, and abnormally large
amounts of proteins in the urine ( proteinuria ) may be present.
Urine normally has little protein, and the presence of proteinuria
indicates that plasma proteins are abnormally leaking through
the kidneys’ filtering units ( glomeruli ) into the urine. This lowers
the plasma protein concentration and oncotic pressure (sec-
tion 14.2), producing edema and swelling of the feet, legs, or
hands. The causes of preeclampsia are not well understood,
but it is believed to stem from dysfunction of the placenta, per-
haps involving vasoconstriction and hypoxia within the uterus/
placenta environment. If preeclampsia becomes severe, the
hypertension can cause seizures and stroke. The only cure for
preeclampsia is delivery of the baby.Clinical Investigation CLUES
Mark was diagnosed with essential hypertension, for
which he was prescribed an ACE inhibitor.- What is essential hypertension, and what are its
dangers? - By what mechanisms does an ACE inhibitor lower
blood pressure?
Treatment of Hypertension
The first form of treatment that is usually attempted is modi-
fication of lifestyle. This modification includes cessation of
smoking, moderation of alcohol intake, and weight reduction,
if applicable. It can also include programmed exercise and a
more moderate sodium intake. People with essential hyperten-
sion may have a potassium deficiency, and there is evidence
that eating food that is rich in potassium may help lower blood
pressure. There is also evidence that supplementing the diet
with Ca^2 1 may be of benefit, but this is more controversial.
If lifestyle modifications alone are insufficient, various
drugs may be prescribed. These may include diuretics that
increase urine volume, thus decreasing blood volume and pres-
sure. Drugs that block b 1 -adrenergic receptors (such as ateno-
lol) lower blood pressure by decreasing the cardiac rate and
are also frequently prescribed. ACE (angiotensin-converting
enzyme) inhibitors, calcium antagonists, and various vasodila-
tors ( table 14.10 ) may also be used in particular situations.
Another class of drugs, the angiotensin II-receptor block-
ers (ARBs), allows angiotensin II to be formed but blocks the
binding of angiotensin II to its receptors. This reduces angio-
tensin II–induced vasoconstriction and (via angiotensin II stim-
ulation of aldosterone secretion) salt and water retention. ACE
inhibitors and ARBs are currently the most widely prescribed
drugs for the treatment of hypertension. Newer drugs include
those that inhibit renin activity and in other ways reduce the
activity of the renin-angiotensin-aldosterone system.Circulatory Shock
Circulatory shock occurs when there is inadequate blood flow
and/or oxygen utilization by the tissues. Some of the signs of shock
( table 14.11 ) are a result of inadequate tissue perfusion; other signs
of shock are produced by cardiovascular responses that help com-
pensate for the poor tissue perfusion ( table 14.12 ). When these
compensations are effective, they (together with emergency medi-
cal care) are able to reestablish adequate tissue perfusion. In some
cases, however, and for reasons that are not clearly understood, the
shock may progress to an irreversible stage and death may result.Hypovolemic Shock
The term hypovolemic shock refers to circulatory shock that
is due to low blood volume, as might be caused by hemorrhage