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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