Human Endocrine System
* Endocrinology is a branch of medicine dealing with the study of the endocrine glands and the actions of their hormones.
* The hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, pancreas, adrenal and gonads are the endocrine glands in our body.
* Endocrine glands are also called ductless glands due to the absence of ducts.
* Hormones are non-nutrient chemical messengers secreted in trace amounts by glands or neurons.
* Exocrine glands release their secretions through ducts and are called duct glands.
* Glands in the human body that have both exocrine and endocrine properties are known as heterocrine glands.
Hypothalamus, Pituitary Gland and Pineal Glands
* The hypothalamus is a part of the fore brain and appears as the floor of the diencephalon.
* Neurohormones are of two types- releasing hormones and inhibiting hormones.
* The pituitary is the smallest endocrine gland but it produces hormones which control the thyroid gland, adrenal cortex and gonads.
* The pituitary gland is divided anatomically into the sdenohypophysis and the neurohypophysis.
* The pars distalis produces the growth hormone (GH), prolactin (PRL), thyroid stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), luteinizing hormone (LH) and follicle-stimulating hormone(FSH).
* The pars intermedia secretes melanocyte-stimulating hormone or MSH.
* The pineal gland secretes a hormone called melatonin, which regulates the twenty-four rhythm of the body.
Thyroid and Parathyroid Glands
* The thyroid gland is a median endocrine gland located in the neck between the trachea and larynx.
* The thyroid gland is the largest endocrine gland and weighs 25.
* Thyroid follicular cells secrete triiodothyronin (T3) and tetraiodothyronin(T4).
* Iodine is essential for the synthesis of hormones in the thyroid gland.
* Hyposecretion of thyroid-stimulating hormones causes goiter and defective development of the foetus, resulting in cretinism.
* Hypersecretion of thyroid hormones causes Graves’ disease or exopthalmic goiter.
* Hyposecretion of parathyroid hormone causes a medical condition called parathyroid tetany.
* Hypersecretion of the parathyroid hormone causes a skeletal disorder called ostetitis fibrosa cystic.
Hormones Secreting Glands and Tissues
* The thymus gland is a lobular organ situated on the dorsal side of the heart and the aorta and secretes a hormone called thymosin.
*Thymosins play a vital role in the differentiation of T-lymphocytes, which provide cell mediated immunity.
* The adrenal glands are a pair of yellowish, triangular –shaped glands situated on the anterior part of the kidneys.
* The steroid hormones or corticoids secreted by the adrenal cortex are differentiated as mineralocorticoids and glucocorticoids.
* The adrenal medulla is a centrally located tissue in the adrenal gland and secretes two hormones, namely adrenaline and noradrenaline.
* Tissues present in the heart, kidneys and gastro-intestinal tract also secrete hormones. However, these tissues are not categorized as endocrine glands.
Heterocrine Glands
* The Pancreas, testis and ovaries are heterocrine organs.
* The Islets of Langerhans in the pancreas have Alpha cells or oxyphils, Beta cells, Delta cells and F cells.
* Alpha cells, Beta cells, Delta cells and F cells secrete glucagon, insulin somatostatin and pancreatic polypeptide hormones respectively.
* Glucagon is a hyperglycemic or diabetogenic hormone that acts on the hepatocyte cells of the liver and adipose tissues.
* Insulin is known as the earliest hormone or hypoglycemic hormone.
* The Degeneration of beta cells leads to deficiency of Insulin which causes insulindependent Diabetes mellitus.
* Testis secrete four types of androgens, of which, testosterone is the main androgen.
* Ovaries secrete two hormones; estrogen and progesterone.
Mechanism of Hormone Action
* On the basis of their chemical nature, hormones can be divided into many groups such as protein hormones, steroids, iodothyronines and amino-acid derivatives.
* Hormone produce their effects on target organs by binding to specific proteins called hormone receptors located in target cells.
* Non-steroid hormones bind to extracellular receptors and lead to the formation of a hormone-receptor complex.
* Steroid hormones bind to intracellular receptor in the nucleus and forms a hormone-receptor complex.