Human Physiology, 14th edition (2016)

482 Chapter 14

14.7 HYPERTENSION, SHOCK, AND

CONGESTIVE HEART FAILURE

An understanding of the normal physiology of the cardio-

vascular system is prerequisite to the study of its patho-

physiology, or mechanisms of abnormal function. Studying

the mechanisms of abnormal cardiovascular function is

important medically and can improve our understanding of

normal physiology.

pressure in the brachial artery, where blood pressure measure-
ments are commonly taken, therefore increases to 120 mmHg
in this example. The rise in pressure from diastolic to sys-
tolic levels (pulse pressure) is thus a reflection of the stroke
volume.
The mean arterial pressure represents the average arte-
rial pressure during the cardiac cycle. This value is signifi-
cant because it is the difference between this pressure and
the venous pressure that drives blood through the capillary
beds of organs. The mean arterial pressure is not a simple
arithmetic average because the period of diastole is longer
than the period of systole. Mean arterial pressure can be
approximated by adding one-third of the pulse pressure to
the diastolic pressure. For a person with a blood pressure of
120/80, the mean arterial pressure would be approximately
80  1  1/3 (40)  5  93 mmHg.

Mean arterial pressure^5 diastolic pressure^1 1/3 pulse pressure

A rise in total peripheral resistance and cardiac rate
increases the diastolic pressure more than it increases the sys-
tolic pressure. When the baroreceptor reflex is activated by
going from a lying to a standing position, for example, the dia-
stolic pressure usually increases by 5 to 10 mmHg, whereas
the systolic pressure either remains unchanged or is slightly
reduced (as a result of decreased venous return). People with
hypertension (high blood pressure), who usually have elevated
total peripheral resistance and cardiac rates, likewise have a
greater increase in diastolic than in systolic pressure. Dehydra-
tion or blood loss results in decreased cardiac output, and thus
also produces a decrease in pulse pressure.
An increase in cardiac output, by contrast, raises the
systolic pressure more than it raises the diastolic pressure
(although both pressures do rise). This occurs during exercise,
for example, when the blood pressure may rise to values as high
as 200/100 (yielding a pulse pressure of 100 mmHg).

| CHECKPOINT

12a. Describe the relationship between blood pressure

and the total cross-sectional area of arteries,

arterioles, and capillaries. Describe how arterioles

influence blood flow through capillaries and arterial

blood pressure.

12b. Explain how the baroreceptor reflex helps to

compensate for a fall in blood pressure. Why will

a person who is severely dehydrated have a rapid

pulse?

13a. Describe how the sounds of Korotkoff are produced

and explain how these sounds are used to measure

blood pressure.

13b. Define pulse pressure and explain the physiological

significance of this measurement.

LEARNING OUTCOMES

After studying this section, you should be able to:

14. Describe the causes and dangers of hypertension.
    15 Describe the causes and dangers of circulatory
    shock.


15. Explain the events that occur in congestive heart
    failure.

Hypertension

Hypertension is blood pressure in excess of the normal range; it

may be defined as a systolic pressure of 140 mmHg or higher,

or a diastolic pressure of 90 or higher ( table 14.8 ). Hypertension

that is a result of (secondary to) known disease processes—such

as chronic renal failure or an adrenal tumor—is called secondary

hypertension. Of the hypertensive population, secondary hyper-

tension accounts for only about 5%. Hypertension that is the

result of complex and poorly understood processes is called

primary, or essential, hypertension. Newer evidence sug-

gests that cardiovascular risk begins to increase when a person’s

systolic blood pressure exceeds 115 mmHg or diastolic pres-

sure exceeds 75 mmHg. Although the medical goal for healthy

people is a blood pressure that does not exceed 120/80, a panel

of experts recently recommended that physicians not prescribe

blood pressure medications for patients over age 60 who are oth-

erwise healthy unless their blood pressure is 150/90 or higher,

nor for patients between the ages of 30 and 59 unless their blood

pressure exceeds 140/90.

Diseases of the kidneys and arteriosclerosis of the renal

arteries can cause secondary hypertension because of high

blood volume. More commonly, the reduction of renal blood

flow can raise blood pressure by stimulating the secretion of

vasoactive chemicals from the kidneys. Experiments in which

the renal artery is pinched, for example, produce hypertension

that is associated (at least initially) with elevated renin secre-

tion. These and other causes of secondary hypertension are

summarized in table 14.9.

Essential Hypertension

The vast majority of people with hypertension have essential

hypertension. Because their blood pressure is directly propor-

tional to cardiac output and total peripheral resistance, one