The endocrine system is predominantly a system of glands, hormone secreting tissues and cells, normally as part of an autonomic response. Examples of its importance include blood glucose levels, growth hormones and ovulation. The hormones that are secreted can only interact with cells that have the correct receptors. The endocrine system can be quite slow at producing a response; this is due to the hormones travelling a long distance in the blood, by diffusion.
The main two categories of tissues are endocrine, a ductless gland or single cell, and exocrine, a gland that releases the hormones through ducts. The pitutiatry gland is at the base of the brain and has an important role in controlling the endocrine system, by both releasing its own hormones and signaling other glands to do the same. Other glands include the thyroid, the adrenal gland and the pancreas.