The human endocrine system, along with the exocrine and heterocrine glands helps in the control and co-ordination of the body. Both the nervous and the endocrine systems collectively form the neuro-endocrine system. The study of these two systems is called neuro-endocrinology. Thomas Addison is known as the father of endocrinology.
The human endocrine system consists of various endocrine glands present in different parts of the body. Hypothalamus is present in the basal part of the diencephalon while the pituitary gland, the master gland of the endocrine system is present below the hypothalamus. Pineal gland is present on the dorsal side of the forebrain. Thyroid gland, the largest endocrine gland, is located on the ventral and lateral sides of the upper part of the trachea, while parathyroid glands are four pea-shaped glands embedded wholly or partially in the dorsal surface of the thyroid gland. Thymus gland is located just above the heart; and pancreas, the second-largest endocrine gland is located in the loop of the duodenum. Adrenal glands are present on the upper surface of both the kidneys. The gonads are also endocrine glands. In males, the scrotal sac bears a pair of testis, while in females, the abdomen bears a pair of ovaries. The endocrine glands pour their secretions directly into the blood due to the absence of ducts. Hence, the endocrine glands are also known as ductless glands and the secretions of these glands are called hormones.
Hormones are chemical messengers secreted in trace amounts by glands or neurons. Each hormone usually affects the target cells and regulates a definite physiological effect, by binding to the hormone receptors found either on the surface of the cell or within its cytoplasm. The hormones are degraded by tissues and are excreted by the liver as bile and by the kidneys as urine.
Hormones are also secreted by the exocrine glands such as the salivary glands, the sweat glands and the sebaceous glands, which release their secretions through ducts, hence called duct glands. The pancreas and the gonads have both exocrine and endocrine properties, hence called heterocrine glands. In these glands, the exocrine part releases its secretions through ducts while the endocrine part releases its hormones directly into the blood. In addition to these glands and organs, hormones are also secreted by the gastro-intestinal tract, the liver, the kidneys and the heart.