The sympathetic nervous system is part of the autonomic nervous system  which is the link between the central nervous system ( brain and spinal cord) and the organs, relaying information concerned with automatic(involuntary) control  and maintenance of a stable internal environment ( homeostasis).

The autonomic nervous system controls the glands of the body, and is in essence a link between nerves and hormones (the chemicals which effect physiological changes).

The majority of the organs it innervates receive dual innervation, which means that they are supplied by both parts of the autonomic nervous system, the sympathetic  and parasympathetic divisions, which broadly speaking have opposing effects.

The exception to this is the adrenal gland, in which the cells are directly supplied by messages from the sympathetic nervous system; in fact, the adrenal gland could be said to be a part of the ANS.

Other glands which do not receive both sympathetic and parasympathetic input are: tear ducts, which have only parasympathetic input; and sweat glands and blood vessels, which are controlled almost exclusively by the sympathetic nervous system.

This is of relevance when we consider the clinical effects of a hyperactive sympathetic nervous system.