Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 483

appears to involve (1) an increased activity of sympathetic

nerves, stimulating vasoconstriction; (2) changes in the secre-

tion of paracrine regulators by the artery endothelium (such as

reduced secretion of nitric oxide, which promotes vasodilation,

and increased secretion of endothelin, which promotes vaso-

constriction); and (3) changes in the structure of the arteries

that result in increased resistance to flow.

The interactions between salt intake, sympathetic nerve activ-

ity, cardiovascular responses to sympathetic activity, responses to

paracrine regulators from the endothelium, and kidney function

make it difficult to sort out the cause-and-effect sequence that

leads to essential hypertension. Some scientists believe that kidney

function may be a “final common pathway” in essential hyperten-

sion, in the sense that properly functioning kidneys should be able

to lower the blood volume to compensate for elevated blood pres-

sure from any cause.

Newer evidence on dietary NaCl and blood pressure dem-

onstrates a complex relationship. A high-salt diet was espe-

cially well correlated with hypertension and cardiovascular

disease among older people and people with hypertension, but

less well correlated in younger and normotensive people. Low-

ering salt intake is still recommended for hypertensive patients

or both of these must be elevated. It is well established that
a diet high in salt is associated with hypertension. A possible
explanation for this association is that a high-salt diet causes
increased plasma osmolality, which stimulates ADH secretion.
Increased ADH then causes increased water reabsorption by
the kidneys, increasing blood volume and thereby increasing
cardiac output and blood pressure.
This sequence should be prevented by the ability of the
kidneys to excrete the excess salt and water. However, the abil-
ity of the kidneys to excrete Na^1 declines with age, in part due
to a gradual decline in the filtering ability of the kidneys (the
glomerular filtration rate, described in chapter 17). Also, there
may be inappropriately high levels of aldosterone secretion,
stimulating salt and water reabsorption. This is suggested by
the observation that some people with essential hypertension
(who should have low renin secretion) may have normal or
even elevated levels of renin, and thus increased production of
angiotensin II, which stimulates aldosterone secretion.
Although hypertension may initially be produced by a rise
in blood volume and thus cardiac output, after some time the
total peripheral resistance rises to raise the blood pressure as
cardiac output declines. The rise in total peripheral resistance

Table 14.9 | Possible Causes of Secondary Hypertension

System Involved Examples Mechanisms

Kidneys Kidney disease Decreased urine formation

Renal artery disease Secretion of vasoactive chemicals

Endocrine Excess catecholamines (tumor of adrenal medulla) Increased cardiac output and total peripheral resistance

Excess aldosterone (Conn’s syndrome) Excess salt and water retention by the kidneys

Nervous Increased intracranial pressure Activation of sympathoadrenal system

Damage to vasomotor center Activation of sympathoadrenal system

Cardiovascular Complete heart block; patent ductus arteriosus Increased stroke volume

Arteriosclerosis of aorta; coarctation of aorta Decreased distensibility of aorta

Table 14.8 | Blood Pressure Classification in Adults

Blood Pressure

Classification

Systolic Blood

Pressure

Diastolic Blood

Pressure Drug Therapy

Normal Under 120 mmHg and Under 80 mmHg No drug therapy

Prehypertension 120–139 mmHg or 80–89 mmHg Lifestyle modification;* no antihypertensive drug indicated

Stage 1 Hypertension 140–159 mmHg or 90–99 mmHg Lifestyle modification; antihypertensive drugs

Stage 2 Hypertension 160 mmHg or greater or 100 mmHg or greater Lifestyle modification; antihypertensive drugs

*Lifestyle modifications include weight reduction; reduction in dietary fat and increased consumption of vegetables and fruit; reduction in dietary sodium (salt);
engaging in regular aerobic exercise, such as brisk walking for at least 30 minutes a day, most days of the week; and moderation of alcohol consumption.
Source: From the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure: The JNC 7
Report. Journal of the American Medical Association; 289 (2003): 2560–2572.