Endocrine system
The endocrine system is a messenger system that regulates distant target organs through feedback loops of hormones secreted by internal glands of an organism straight into the circulatory system. The hypothalamus is the neurological control centre for all endocrine systems in animals. The thyroid gland and the adrenal glands are the two primary endocrine glands in humans. Endocrinology is the study of the endocrine system and its diseases. An axis is a group of glands that communicate with one another in a specific order, such as the hypothalamic-pituitary-adrenal axis. In addition to the specialist endocrine organs indicated above, many other organs that are part of other body systems, such as bone, kidneys, liver, heart, and gonads, have secondary endocrine activities. The endocrine hormone erythropoietin, for example, is secreted by the kidney. Amino acid complexes, steroids, eicosanoids, leukotrienes, and prostaglandins are examples of hormones.