Chapter 15 Endocrine System

Endocrine System

Endocrine System

Endocrine system is associated organs and tissues of the body that collectively release substances known as hormones. Endocrine organs are known also as ductless glands because their secretions are released directly into the bloodstream, in contrast to exocrine glands, which discharge their secretions on such external or internal surface tissues as those of the skin, the lining of the stomach, or of the lining of the pancreatic ducts. The endocrine glands and their hormones regulate the growth, development, and function of certain tissues and coordinate many of the metabolic processes within the body.

Endocrinology

It is the study of the endocrine glands, the hormonal substances produced by these glands, their physiological effects, and the disorders and diseases that involve their malfunction.

Pituitary Gland

The pituitary gland, or hypophysis, consists of three lobes, the anterior lobe, the intermediate lobe, which in primates is present for only a short part of the life span, and the posterior lobe. It is situated at the base of the brain and has been called the master controlling gland of the body. The anterior and the posterior lobes of the pituitary secrete different hormones.

Anterior Lobes Hormones

       The anterior pituitary is the source of growth hormone, also called somatotropin, which promotes the development of body tissues, particularly of bone matrix and muscle, and influences carbohydrate metabolism.

•      Adrenocorticotropic hormone, or ACTH, which stimulates the adrenal cortex.
•     Thyroid-stimulating hormone, or thyrotropin, known as TSH. It stimulates thyroid hormone secretion.
•     Follicle-stimulating hormone (FSH), stimulates follicle maturation in females. And Stimulates sperm production in males.
•     Luteinizing hormone (LH), which stimulate the sex glands.
•    Prolactin, which, with other special hormones, influences milk production of the mammary gland. In addition.
•    Melanocyte-stimulating hormone, which regulates the intensity of pigmentation in pigmented cells.

Posterior Lobes Hormones

•       The hypothalamus,that part of the brain from which the pituitary gland arises. The posterior lobe of the pituitary also stores another hormone secreted by the hypothalamus
•      Antidiuretic hormone (one that controls the excretion of water) named vasopressin, which is stored in the posterior lobe of the pituitary gland.
•      Oxytocin, stimulates muscular contractions, especially of the uterus, and ejection of milk from lactating mammary glands.

Adrenal Glands

•   Adrenal gland consists of an inner part called the medulla and an outer part called the cortex. The two glands are situated above the kidney.

Adrenal Medulla Hormones

•      Epinephrine, also called adrenaline, and norepinephrine, which affect a number of body functions; for example, they stimulate cardiac action, increase the blood pressure, and affect constriction and dilation of blood vessels and musculature. Epinephrine (but not norepinephrine) raises the blood-sugar level.

Adrenal Cortex Hormones

•         Adrenal secretions regulate the salt and water balance of the body, influence the blood pressure, affect lymphatic tissue, influence the mechanisms of the immune system, and regulate carbohydrate and protein metabolism.
•      Glucocorticoids, which include cortisone and hydrocortisone.
•      Mineralocorticoids, which include aldosterone and other hormonal substances that are essential to the maintenance of life and to adaptation to stress.
•      Androgens, term embracing any of the male sex hormones, substances that induce and maintain secondary sex characteristics in males.

Thyroid Gland

•        The thyroid is a two-lobed gland situated in the neck.
•        Thyroid hormones thyroxine and triiodothyronine, increase oxygen consumption and stimulate the rate of metabolism, regulating the growth and maturation of body tissues and affecting physical and mental alertness.
•        Thyroid Hormone, also secretes a hormone known as calcitonin, which lowers the levels of calcium and phosphate in the blood and inhibits bone resorption.

Parathyroid Glands

•        The parathyroids are found near or embedded in the thyroid gland. The parathyroid hormone regulates blood levels of calcium and phosphorus and stimulates bone resorption.

Ovaries

The ovaries are the female gonads, which are paired, almond-shaped bodies situated on either side of the uterus. The ovarian follicles produce the ova, or eggs, and also secrete a group of hormones called estrogens , which are necessary for the development of the reproductive organs and of such secondary sex characteristics as the distribution of fat, widening of the pelvis, mammary growth, and pubic and axillary hair.

Progesterone, another hormone produced by the ovaries, has as its principal function the maintenance of pregnancy, which it accomplishes primarily by its effects on the lining of the uterus. Progesterone also acts in conjunction with estrogens in promoting the growth and elasticity of the female genital tract. The ovaries also elaborate a hormone called relaxin, which acts to relax the cervix during childbirth, thus making delivery easier.

Testes 

The male gonads, the testes, are paired, ellipsoid bodies suspended in the scrotum. The Leydig cells of the testes produce one or more male hormones called the androgens. The androgens, the most important of which is testosterone, stimulate the development of secondary sex characteristics, influence the growth of the prostate and seminal vesicles, and promote secretory activity of these structures. The testes also contain cells that produce sperm. 

Pancreas

The bulk of the pancreas consists of exocrine tissue that releases digestive juices into the duodenum. Distributed throughout this tissue are clusters of endocrine cells called the islets of Langerhans, which secrete insulin and another hormone known as glucagon. Insulin affects carbohydrate, protein, and fat metabolism, increasing the rate of sugar utilization and promoting formation of protein and storage of fat. Glucagon temporarily raises blood-sugar levels, apparently by releasing glucose from the liver.

Endocrine Cycle

The endocrine system exerts a regulatory action on the reproductive cycles, including the development of the gonads, their period of functional maturity, and their subsequent senescence, as well as the menstrual cycle and the gestation period. The cyclic pattern of estrus, the period during which fertile mating is possible in lower animals, also is regulated by hormones.

Puberty, the time of sexual maturation, is marked by an increase in the secretion of pituitary gonad-stimulating hormones or gonadotropins, which cause maturation of the testes or ovaries and increased secretion of sex hormones. The sex hormones, in turn, affect the accessory sex organs and general somatic and psychic development.

Puberty in the female is associated with the onset of menstruation and ovulation. Ovulation, the release of an ovum from an ovarian follicle, occurs approximately every 28 days at about the 10th to 14th day of the menstrual cycle in human beings. The first part of the cycle is marked by the actual menstrual period, averaging about three to five days, and by the maturation of the ovarian follicle under the influence of the follicle-stimulating hormone of the pituitary. After ovulation, the vacated follicle, under the influence of another pituitary hormone, forms an endocrine body known as the corpus luteum that secretes progesterone, estrogen, and, probably during pregnancy, relaxin. Progesterone prepares the uterine lining for pregnancy; if it does not occur, the corpus luteum regresses, and the uterine lining, deprived of hormonal support, breaks down, resulting in menstrual bleeding. The rhythmic pattern of menstruation is explained by the reciprocal inhibitory-stimulative relationship between the estrogens and the pituitary gonad-stimulating hormones.

If pregnancy occurs, the placental secretion of gonadotropins, progesterones, and estrogens maintains the corpus luteum and the uterine lining and prepares the mammary glands for milk production, or lactation. Secretion of estrogens and progesterone is high during pregnancy, reaching a peak just before childbirth. Lactation begins shortly after the delivery, presumably as a result of changes in hormone balance following separation of the placenta.

Disturbances of Endocrine Function

Disturbances in function of endocrine production may be classed as either hyperfunction (excess activity) or hypofunction (insufficient activity).

Hypofunction may result from congenital defects, cancer, inflammatory lesions, degeneration, anterior pituitary disorders affecting the target glands, trauma, or, in the case of thyroid disease, iodine deficiency. Hypofunction may also result from surgical removal of a gland or destruction from radiation therapy.

Hyperfunction of a gland may be caused by a hormone-secreting tumor that is benign or, less often, malignant. Hyperfunction of the anterior pituitary gland with overproduction of the growth hormone may result in gigantism or acromegaly, or, when excess adrenal-stimulating hormone is produced, in a group of symptoms known as Cushing's disease, including hypertension, weakness, plethora, bruising, and a curious type of obesity. Deficiency in anterior pituitary function leads to dwarfism sexual underdevelopment, weakness, and occasionally, severe emaciation.

Subnormal adrenal-cortical activity results in Addison's disease, whereas excessive activity may produce Cushing's syndrome or cause virilism in women or children. Disturbances in gonadal function affect mainly the development of primary and secondary sex characteristics.

Thyroid deficiency produces cretinism and dwarfism in the infant, and myxedema, marked by coarsening of the features and slowing of mental and physical reactions, in the adult. Excessive thyroid function (Graves' disease, toxic goiter) is characterized by protrusion of the eyes, tremor and sweating, increased pulse rate, cardiac palpitations, and nervous irritability.

Diabetes insipidus results from deficiency of the antidiuretic hormone, and diabetes mellitus from a defect in the production of the pancreatic hormone insulin, or in the body's response to it.

Addidson's Disease

Cushing's Disease

Myxedema