The organs comprising the endocrine system can be divided into two broad categories: those that serve functions in addition to their role as hormone secretors and those that serve only to secrete hormones. There are relatively few endocrine tissues that also have non-endocrine functions: the gonads, the pancreas, the pyloric mucosa of the stomach, and the mucosa of the duodenum. The testes in the male secrete testosterone which stimulates spermatogenesis, development of the male secondary sex characteristics, and development of the male reproductive structures. The ovaries in the female secrete estrogen and progesterone. Estrogen stimulates the growth of the uterine lining, development of the female secondary sex characteristics, and development of the female reproductive structures. Progesterone readies the uterus for embryo implantation and maintains the pregnancy after implantation. The hormones secreted by the gonads are steroidal in their chemical composition; the adernal cortex is the only other organ in mammals to secrete steroidal hormones.
The beta cells of the islets of Langerhans in the pancreas secrete insulin while the alpha cells secrete glucagon. Insulin's net effect on the body is to reduce glucose concentration in the blood. Insulin stimulates glycogen formation and storage and glucose oxidation in liver and muscle cells as well as stimulating muscle and adipose tissue cells to absorb glucose and convert it into protein and fat, respectively. Glucagon acts in the opposite manner stimulating conversion of glycogen into glucose in liver and muscle cells in order to raise blood-glucose levels.
