APA Dictionary of Psychology

n. a hormone, secreted by the B cells of the islets of Langerhans in the pancreas, that facilitates the transfer of glucose molecules through cell membranes. Together with glucagon, it plays a key role in regulating blood sugar and carbohydrate metabolism. In the absence of sufficient concentrations of insulin, glucose accumulates in the blood and is excreted, as in diabetes mellitus (see also hyperglycemia). Excessive concentrations of insulin (resulting, e.g., from insulin overdosage or an insulin-secreting tumor) give rise to hypoglycemia.