Function of the Adrenal Medullae
Stimulation of the sympathetic nerves to the adrenal medullae
causes large quantities of epinephrine and norepinephrine to be released into
the circulating blood, and these two hormones in turn are carried in the blood
to all tissues of the body. On the average, about 80 per cent of the secretion
is epinephrine and 20 per cent is norepinephrine, although the relative proportions
can change considerably under different physiologic conditions.
The circulating epinephrine and norepinephrine have almost the same
effects on the different organs as the effects caused by direct sympathetic
stimulation, except that the effects last
5 to 10 times as long because both of these hormones are removed from the
blood slowly over a period of 2 to 4 minutes.
The circulating norepinephrine causes constriction of essentially
all the blood vessels of the body; it also causes increased activity of the
heart, inhibition of the gastrointestinal tract, dilation of the pupils of the
eyes, and so forth.
Epinephrine causes almost the same effects as those caused by
norepinephrine, but the effects differ in the following respects: First,
epinephrine, because of its greater effect in stimulating the beta receptors,
has a greater effect on cardiac stimulation than does nor-epinephrine. Second,
epinephrine causes only weak constriction of the blood vessels in the muscles,
in com-parison with much stronger constriction caused by norepinephrine.
Because the muscle vessels represent a major segment of the vessels of the
body, this differ-ence is of special importance because norepinephrine greatly
increases the total peripheral resistance and elevates arterial pressure,
whereas epinephrine raises the arterial pressure to a lesser extent but
increases the cardiac output more.
A third difference between the actions of epineph-rine and
norepinephrine relates to their effects on tissue metabolism. Epinephrine has 5
to 10 times as great a metabolic effect as norepinephrine. Indeed, the
epinephrine secreted by the adrenal medullae can increase the metabolic rate of
the whole body often to as much as 100 per cent above normal, in this way
increasing the activity and excitability of the body. It also increases the
rates of other metabolic activities, such as glycogenolysis in the liver and
muscle, and glucose release into the blood.
In summary, stimulation of the adrenal medullae causes release of
the hormones epinephrine and nor-epinephrine, which together have almost the
same effects throughout the body as direct sympathetic stimulation, except that
the effects are greatly pro-longed, lasting 2 to 4 minutes after the
stimulation is over.
Value of
the Adrenal Medullae to the Function of the Sympa-thetic Nervous System. Epinephrine and
norepinephrineare almost always released by the adrenal medullae at the same
time that the different organs are stimulated directly by generalized
sympathetic activation. There-fore, the organs are actually stimulated in two
ways: directly by the sympathetic nerves and indirectly by the adrenal
medullary hormones. The two means of stimulation support each other, and either
can, in most instances, substitute for the other. For instance, destruction of
the direct sympathetic pathways to the different body organs does not abrogate
sympa-thetic excitation of the organs because norepinephrine and epinephrine
are still released into the circulating blood and indirectly cause stimulation.
Likewise, loss of the two adrenal medullae usually has little effect on the
operation of the sympathetic nervous system because the direct pathways can
still perform almost all the necessary duties. Thus, the dual mecha-nism of
sympathetic stimulation provides a safety factor, one mechanism substituting
for the other if it is missing.
Another important value of the adrenal medullae is the capability
of epinephrine and norepinephrine to stimulate structures of the body that are
not innervated by direct sympathetic fibers. For instance, the metabolic rate
of every cell of the body is increased by these hormones, especially by
epinephrine, even though only a small proportion of all the cells in the body
are innervated directly by sympathetic fibers.
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