If more vasoconstriction occurs, blood pressure
will increase (the container has become even
smaller). This is what happens in a stress situation,
when greater vasoconstriction is brought about
by sympathetic impulses. If vasodilation occurs,
blood pressure will decrease (the container is
larger). After eating a large meal, for example, there
is extensive vasodilation in the digestive tract to
supply more oxygenated blood for digestive activi-
ties. To keep blood pressure within the normal
range, vasoconstriction must, and does, occur else-
where in the body. This is why strenuous exercise
should be avoided right after eating; there is not
enough blood to completely supply oxygen to exer-
cising muscles and an active digestive tract at the
same time.
4.Elasticity of the large arteries—when the left
ventricle contracts, the blood that enters the large
arteries stretches their walls. The arterial walls are
elastic and absorb some of the force. When the left
ventricle relaxes, the arterial walls recoil or snap
back, which helps keep diastolic pressure within
the normal range. Normal elasticity, therefore,
lowers systolic pressure, raises diastolic pressure,
and maintains a normal pulse pressure. (Pulse pres-
sure is the difference between systolic and diastolic
pressure. The usual ratio of systolic to diastolic to
pulse pressure is approximately 3:2:1. For example,
with a blood pressure of 120/80 mmHg, the pulse
pressure is 40, and the ratio is 120:80:40, or 3:2:1.)
5.Viscosity of the blood—normal blood viscosity
depends upon the presence of red blood cells and
plasma proteins, especially albumin. Having too
many red blood cells is rare but does occur in the
disorder called polycythemia vera and in people
who are heavy smokers. This will increase blood
viscosity and blood pressure.
A decreased number of red blood cells, as is seen
with severe anemia, or decreased albumin, as may
occur in liver disease or kidney disease, will
decrease blood viscosity and blood pressure. In
these situations, other mechanisms such as vaso-
constriction will maintain blood pressure as close to
normal as is possible.
6.Loss of blood—a small loss of blood, as when
donating a pint of blood, will cause a temporary
drop in blood pressure followed by rapid compen-
sation in the form of a more rapid heart rate and
greater vasoconstriction. After a severe hemor-
rhage, however, these compensating mechanisms
may not be sufficient to maintain normal blood
pressure and blood flow to the brain. Although a
person may survive loss of 50% of the body’s total
blood, the possibility of brain damage increases as
more blood is lost and not rapidly replaced.
7.Hormones—several hormones have effects on
blood pressure. You may recall them from Chapters
10 and 12, but we will summarize them here and in
Fig. 13–10. The adrenal medulla secretes norepi-
nephrine and epinephrine in stress situations.
Norepinephrine stimulates vasoconstriction, which
raises blood pressure. Epinephrine also causes vaso-
constriction, and increases heart rate and force of
contraction, both of which increase blood pressure.
Antidiuretic hormone (ADH) is secreted by the
posterior pituitary gland when the water content of
the body decreases. ADH increases the reabsorp-
tion of water by the kidneys to prevent further loss
of water in urine and a further decrease in blood
pressure.
Aldosterone, a hormone from the adrenal cor-
tex, has a similar effect on blood volume. When
blood pressure decreases, secretion of aldosterone
stimulates the reabsorption of Naions by the kid-
neys. Water follows sodium back to the blood,
which maintains blood volume to prevent a further
drop in blood pressure.
Atrial natriuretic peptide (ANP), secreted by the
atria of the heart, functions in opposition to aldos-
terone. ANP increases the excretion of Naions
and water by the kidneys, which decreases blood
volume and lowers blood pressure.DISTRIBUTION OF BLOOD FLOW
An individual’s blood volume remains relatively con-
stant within the normal range appropriate to the size
of the person. Active tissues, however, require more
blood, that is, more oxygen, than do less active tissues.
As active tissues and organs receive a greater propor-
tion of the total blood flow, less active organs must
receive less, or blood pressure will decrease markedly.
As mentioned previously, precapillary sphincters
dilate in active tissues and constrict in less active ones.
The arterioles also constrict to reduce blood flow to
less active organs. This ensures that metabolically
active organs will receive enough oxygen to function
properly and that blood pressure for the body as a
whole will be maintained within normal limits.
An example will be helpful here; let us use the
body at rest and the body during exercise. Consult
Fig. 13–11 as you read the following. Resting cardiacThe Vascular System 309