The endocrine system is important in the development and maintenance of the nervous system. Some reflexes involve both systems. Structurally, the endocrine system comprises a number of separate organs and tissues scattered throughout the body. All the structures have in common the characteristic that they produce one or more hormones . Other structures not listed above also produce hormones, but are not included within the endocrine system.
Some hormones are stored, others undergo continuous turnover.
There is usually a level of release which can be either increased or decreased. Hormones are usually transported in the blood bound to plasma proteins or to proteins specific to the particular hormone. As hormones are transported in the blood stream, their molecules are able to reach where ever the blood circulates. Yet hormones are only able to act on cells which possess receptors specific to that particular hormone. Cells which possess receptors are termed target cells, which in combination may be referred to as target tissues, or target organs.