Regulatory mechanisms are essential for multicellular organisms to survive and maintain their integrity in variable, harmful environments, and multicellular animals have been able to partially insulate themselves from dramatic changes in the external environment by generating and regulating their own cellular microenvironment. This ability is realized by the synergistic regulation of the human endocrine system and the autonomic nervous system. The most important endocrine gland in the body is the pituitary gland, followed by the thyroid and parathyroid glands, the adrenal glands and the pineal gland.