AGING AND THE
VASCULAR SYSTEM
It is believed that the aging of blood vessels, especially
arteries, begins in childhood, although the effects are
not apparent for decades. The cholesterol deposits
of atherosclerosis are to be expected with advancing
age, with the most serious consequences in the coro-
nary arteries. A certain degree of arteriosclerosis is to
be expected, and average resting blood pressure may
increase, which further damages arterial walls.
Consequences include stroke and left-sided heart
failure.
The veins also deteriorate with age; their thin walls
weaken and stretch, making their valves incompetent.
This is most likely to occur in the veins of the legs;
their walls are subject to great pressure as blood is
returned to the heart against the force of gravity.
Varicose veins and phlebitis are more likely to occur
among elderly people.SUMMARY
Although the vascular system does form passageways
for the blood, you can readily see that the blood ves-
sels are not simply pipes through which the blood
flows. The vessels are not passive tubes, but rather
active contributors to homeostasis. The arteries and
veins help maintain blood pressure, and the capillaries
provide sites for the exchanges of materials between
the blood and the tissues. Some very important sites of
exchange are discussed in the following chapters: the
lungs, the digestive tract, and the kidneys.314 The Vascular System
BOX13–5 CIRCULATORY SHOCK
Stages of Shock
Compensated shock—the responses by the body
maintain cardiac output. Following a small hemor-
rhage, for example, the heart rate increases,
the blood vessels constrict, and the kidneys
decrease urinary output to conserve water. These
responses help preserve blood volume and main-
tain blood pressure, cardiac output, and blood flow
to tissues.
Progressive shock—the state of shock leads to
more shock. Following a severe hemorrhage, car-
diac output decreases and the myocardium itself is
deprived of blood. The heart weakens, which fur-
ther decreases cardiac output. Arteries that are
deprived of their blood supply cannot remain con-
stricted. As the arteries dilate, venous return
decreases, which in turn decreases cardiac output.
Progressive shock is a series of such vicious cycles,
and medical intervention is required to restore car-
diac output to normal.
Irreversible shock—no amount of medical assis-
tance can restore cardiac output to normal. The
usual cause of death is that the heart has been dam-
aged too much to recover. A severe myocardial
infarction, massive hemorrhage, or septicemia may
all be fatal despite medical treatment.Circulatory shock is any condition in which cardiac
output decreases to the extent that tissues are
deprived of oxygen and waste products accumulate.
Causes of Shock
Cardiogenic shock occurs most often after a
severe myocardial infarction but may also be the
result of ventricular fibrillation. In either case, the
heart is no longer an efficient pump, and cardiac
output decreases.
Hypovolemic shockis the result of decreased
blood volume, often due to severe hemorrhage.
Other possible causes are extreme sweating (heat
stroke) or extreme loss of water through the kid-
neys (diuresis) or intestines (diarrhea). In these
situations, the heart simply does not have enough
blood to pump, and cardiac output decreases.
Anaphylactic shock, also in this category, is a mas-
sive allergic reaction in which great amounts of
histamine increase capillary permeability and
vasodilation throughout the body. Much plasma is
then lost to tissue spaces, which decreases blood
volume, blood pressure, and cardiac output.
Septic shockis the result of septicemia, the
presence of bacteria in the blood. The bacteria and
damaged tissues release inflammatory chemicals
that cause vasodilation and extensive loss of plasma
into tissue spaces.