·         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 system collectively form the neuro- endocrine system. The study of these two systems is called Neuro-endocrinology.

·         The endocrine system consist of a group of specialized glands called endocrine glands and the substances called hormones.

·         The endocrine glands pour their secretions directly into the blood due to 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 messenger secreted in trace amounts by glands or neurons. Each hormones 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.

·         Hormones may be polypeptide, steroids, amino acid derivative or biogenic amines. Protein hormones or peptide hormones produced by the hypothalamus, pituitary, parathyroid and pancreas. For example, Relaxin is a protein hormone produced from the ovary and a hormone from the placenta i.e. HCG is also another protein hormone. Steroid hormones are produced from the adrenal medulla and the pineal gland. The hormone of the thyroid gland called thyroxin is an iodinated derivative of an amino acid.  

·         Hormones are also secreted by the exocrine glands such as the salivary gland, the sweat gland and the sebaceous gland, which releases 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 it hormones secretions through ducts while the endocrine part releases its hormones directly into the blood.

·         Hormones are also secreted by the GI tract, the liver, the kidneys and the heart.

Endocrine Gland:

·         Thomas Addison is known as Father of Endocrinology.

·         The human endocrine system consists of various endocrine glands present in different parts of the body.

·         Endocrine glands does not have ducts, they are responsible for bloodstream to transfer hormones. These hormones are produced in the glands and used to regulate physiology and behaviour. They are chemicals which are produced in trace amounts that act as intercellular messengers.

·         Hypothalamus is present in the basal parts 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 side 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 (second largest endocrine glands is located on the loop of the duodenum.

·         Adrenal glands are present on the upper surface of both the kidneys. The gonads are also endocrine gland; in males the scrotal sacs bears a pair of testis, while in females the abdomen bears a pair of ovaries.

·         The function of various endocrine glands like thyroid, adrenal, ovary and testes are regulated by hormones from the pituitary gland.

·         The function of the pituitary glands is regulated by the hormones of the hypothalamus. The primary target of the hypothalamic hormone is the pituitary gland in turn is regulated by the hormones of the hypothalamus. The primary target of the hypothalamic hormone is the pituitary gland.

·         Hormones reaching their target organ through the blood stimulate the synthesis of specific proteins or other secretory products in the target cells. The hormones are recognized at specific recognition sites known as hormone receptors. These receptors are present either on the cell membrane or in the cytoplasm and nucleus of the cell. The synthesis of specific protein leads to other biological, biochemical and physiological actions of individual hormones.

Function of Endocrine Gland:

·         The main function of endocrine gland is to secrete hormone directly into the bloodstream. Hormones are chemical substance that affects the activity of target site in the body.

·         Hormones serve as messengers, controlling and coordinating activities throughout the body. Upon reaching a target site, a hormone binds to the receptor and transmit signal to the target site to perform specific function.

·         Hormones also influence the way the body uses and stores energy and control the volume of fluid and the levels of salts and sugar (glucose) in the blood.

·         Maintain a stable environment or homeostasis.

·         Control growth of body.

·         Helps in development of an organism.

·         Hormones of endocrine glands are involved in reproduction of organism.

·         Helps in response to stimuli.

·         Very small amount of hormone can trigger very large response in the body.

·         Each type of hormone influences only certain organs and tissues. Some hormones affect only one or two organs, whereas others hormones influences throughout the body.

·         For example, Thyroid Stimulating Hormone (TSH), produced in pituitary gland affects only the thyroid gland. Thyroid hormone produced in the thyroid gland affects cells throughout the body and is involved in function like regulating growth of cells, controlling the heart rates, affecting the speed at which calories are burned. Insulin, secreted by the islet cells of pancreas, affects the processing (metabolism) of glucose, protein and fat throughout the body.

·         Most hormones are derived from protein. Others are steroids, which are faaty substances derived from cholesterol.

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