The endocrine system is composed of the following endocrine glands; thyroid, parathyroid, pituitary, pineal and adrenal. The endocrine glands secrete hormones, which are messenger molecules, into the interstitial fluid in the bloodstream. These released hormones affect nearly every cell, organ, and function of the human body. The endocrine system influences emotional behaviour, growth, development, tissue and sexual function, metabolic rate and reproductive system. Other organs and tissues containing cells that secrete hormones are the stomach, placenta, small intestine, heart, skin, thymus, adipose tissue, kidneys, pancreas, liver, hypothalamus, testes and ovaries.

In general, the endocrine system responses are slower and therefore control processes that happen more slowly than the nervous system. For example, the endocrine system influences things like cell growth, whilst faster processes, like breathing are controlled by the nervous system.

The nervous and endocrine systems are different systems, yet are complementary systems and work together to help the body function properly.