ENDOCRINE SYSTEM B.A. AMEYAW • Endocrine system is a collection of glands that produce and secrete hormones into the bloodstream • Gland is an organ or group of cells that is specialized for synthesizing and secreting certain fluid either for use in the body or for excretion • There are two main groups of glands; 1. Exocrine gland: a gland that discharges its secretion by means of a duct which opens onto an epithelial surface. Examples are sweat glands and sebaceous glands 2. Endocrine gland: a ductless gland that manufactures one or more hormones and secretes them directly into the bloodstream • Endocrine glands are referred to as ductless glands because hormones diffuse directly into the bloodstream • Hormone is a chemical messenger that is produced in one part of the body, passes into the bloodstream and is carried to other organs or tissues, where it acts to modify their structure or function • The study of the endocrine system and the diagnosis and treatment of its dysfunctions is called endocrinology Differences between endocrine system and nervous system Nervous system Composed of nerve cells Uses electrical impulses to send signals or messages via neurotransmitters Mode of transport is through neurones / nerve fibres Responsible for the regulation of short term processes / effect of message last for very short in seconds or minutes / it is involved in the immediate and short‐term maintenance of homeostasis Reacts rapidly to stimuli Response/action can be voluntary or involuntary Endocrine system Composed of glands Uses hormones to send signals or messages Mode of transport of the hormones is through the bloodstream Responsible for the regulation of longer term processes / effect of message last longer in minutes, hours and even days Reacts slowly to stimuli Response/action is always involuntary Target cells or effects are often localised (muscles cells, glands) Target cells or effects are widespread (cells throughout the body) Position of endocrine glands Endocrine glands and their hormones Hypothalamus • The hypothalamus and pituitary gland work as a unit in regulating some other endocrine glands activities • The hypothalamus makes up the lower region of the diencephalon and lies just above the brain stem. It is linked with the thalamus above and the pituitary gland below • One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland. Almost all hormone secretion by the pituitary gland is controlled by either hormonal or electrical signals from the hypothalamus • Hypothalamus synthesize and secrete the hypothalamic releasing and inhibitory hormones that control secretion of the anterior pituitary hormones Hormones Secreted by Hypothalamus Hormones Secreted by Hypothalamus Growth hormone-releasing hormone (GHRH) Growth hormone-inhibiting hormone (GHIH) or somatostatin Thyrotropin-releasing hormone (TRH) Physiological action on the anterior pituitary gland Stimulates the release of growth hormone Corticotropin-releasing hormone (CRH) Stimulates the release of adrenocorticotropin Gonadotropin-releasing hormone (GnRH) Stimulates the release of gonadotropins; folliclestimulating hormone (FSH) and luteinising hormone (LH) Inhibits prolactin secretion Stimulate the release of prolactin Prolactin-inhibiting hormone (PIH) Prolactin releasing hormone (PRH) Inhibits the release of growth hormone Stimulates the release of thyroid stimulating hormone Pituitary gland • The pituitary gland (hypophysis) is attached to the bottom of the hypothalamus by a slender stalk called the infundibulum • It is situated in a hypophyseal fossa (depression) called ‘sella turcica’, present in the sphenoid bone at the base of skull • It is the size of a pea, weighs about 500 mg, and with a diameter of approximately 1cm • Regardless of its small size, the pituitary gland regulates many body functions. It is often referred to as the master gland • The pituitary gland consists of two main parts: 1. Anterior pituitary gland (anterior lobe or adenohypophysis) 2. Posterior pituitary gland (posterior lobe or neurohypophysis) • The activity of the adenohypophysis is controlled by the release of hormones from the hypothalamus through hypothalamo-hypophyseal portal blood vessels (hypophyseal portal veins) • The activities of neurohypophysis is controlled by nerve stimulation from hypothalamus Anterior pituitary gland Hormones secreted by anterior pituitary gland Growth hormone (GH) Thyroid stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Prolactin (PRL) Follicle stimulating hormone (FSH) Luteinising hormone (LH) Physiological action Stimulates growth of body cells Stimulates thyroid hormone release Stimulates the adrenal cortex to secrete corticosteroid Stimulates growth of breast tissue and milk production Stimulates production of sperm in the testes, stimulates secretion of oestrogen by the ovaries, maturation of ovarian follicles, ovulation Stimulates secretion of testosterone by the testes, stimulates secretion of progesterone by the corpus luteum Posterior pituitary gland Hormones secreted by Physiological action posterior pituitary gland Oxytocin 1. Stimulate uterine contraction in childbirth 2. Stimulate release of breastmilk in response to suckling Antidiuretic hormone (ADH) 1.Increases water reabsorption by the kidney tubules (water returns to the blood) 2.Decreases sweating Thyroid gland • The thyroid gland is a butterfly‐shaped gland located in the front of the neck on the trachea just below the larynx Hormones secreted by thyroid gland Hormones secreted Physiological action by thyroid gland Thyroxine (T4) Increase metabolic rate in the body Tri-iodothyronine (T3) Increase metabolic rate in the body Calcitonin Decreases the reabsorption of calcium and phosphate from bones to blood Parathyroid glands • The parathyroid glands are small glands located on the back (posterior) of the thyroid gland • There are usually four of these glands • They secrete parathyroid hormone (PTH) in response to hypocalcemia. PTH controls the distribution of calcium in the body Adrenal glands • The two adrenal glands are found on top of each of the two kidneys • Structurally and functionally, each adrenal gland is composed of two major regions: • adrenal cortex • adrenal medulla Adrenal cortex • This is the outer part of each adrenal gland and it is made up of three distinct functional layers • Each layer is involved in the production of steroid‐based hormones collectively known as the corticosteroids (adrenocorticocoids). The layers are: • zona glomerulosa – produces the mineralocorticoids • zona fasciculata – produces the glucocorticoids • zona reticularis – produces adrenal sex hormones (gonadocorticoids) Hormones of the adrenal cortex Hormones secreted by adrenal cortex Mineralocorticoids (e.g. aldosterone) Glucocorticoids. Examples are: cortisol (hydrocortisone) corticosterone cortisone Physiological action 1. Regulation of salt and water balance in the body 1. They influence the metabolism of most body cells 2. They promote glycogen storage in the liver 3. During fasting, they stimulate the generation of glucose 4. They increase blood glucose levels 5. They are involved in providing resistance to stressors Hormones of the adrenal cortex Hormones secreted by adrenal cortex Sex hormones. Examples are: Male hormones (androgens) Female hormones (oestrogens) Physiological action 1. Androgens stimulate development of male sex organs and male secondary sexual NB: they are secreted in minimal amounts in characteristics both sexes by the adrenal cortex . Their effect is 2. Oestrogen stimulate often masked by hormones from the testes and development of female ovaries in late puberty and adulthood in males sex organs and female and females respectively. In females, the secondary sexual masculinisation effect of androgen secretion can characteristics become evident after menopause as levels of oestrogen from the ovaries decrease Adrenal medulla • Medulla is the inner part of adrenal gland • The cells of the adrenal medulla secrete epinephrine, norepinephrine, and dopamine which collectively are called catecholamines and they are sympathomimetic (having the effect of stimulating the sympathetic nervous system) • These hormones are secreted in stress situations and help prepare the body for “fight or flight.” • Epinephrine (adrenaline) has a greater effect on the heart and metabolic processes whereas norepinephrine (noradrenaline) has its influence on blood vessel diameter Hormones of the adrenal medulla Hormones secreted by Physiological action adrenal medulla Epinephrine (adrenaline) 1. Causes vasoconstriction in skin and viscera 2. Causes vasodilation in skeletal muscles 3. Increases heart rate and force of contraction 4. Increases the rate of breathing 5. Increases metabolic rate Norepinephrine 1. Causes widespread vasoconstriction in the skin, (noradrenaline) viscera, and skeletal muscles 2. Constriction of small blood vessels leading to an increase in blood pressure Dopamine It plays a small role in the fight-or-flight response; helps in vasoconstriction of blood vessels Pancreas • The pancreas is an elongated organ with its head lying close to the first part of the small intestine (the duodenum) and the body behind the stomach, the tail extends out towards the spleen • The pancreas is made up of exocrine tissue and the associated ducts • The tissue produces and secretes a fluid rich with digestive enzymes into the small intestine. Scattered throughout the exocrine tissue are endocrine cell clusters called islets of Langerhans (islets) • Each islet has three major cell types, each of which produces a different hormone: 1. alpha cells: they secrete the hormone glucagon which increases blood glucose level 2. beta cells: the most abundant of the three cell types and secrete the hormone insulin which decreases blood glucose level 3. delta cells: they secrete the hormone somatostatin which inhibits the secretion of both insulin and glucagon in addition to inhibiting the secretion of growth hormone from the anterior pituitary Pineal gland • The pineal gland is a small body attached to the third ventricle • It secretes the hormone called melatonin • Melatonin secretion is more in darkness than in daylight and that helps regulate the biological clock (sleep-wake cycles) • Melatonin is also believed to be associated with inhibition of growth and development of the sex organs before puberty, by preventing synthesis or release of gonadotropins Ovaries • The ovaries are located in the pelvic cavity, one on each side of the uterus Hormones secreted by ovaries Oestrogen Progesterone Inhibin Physiological action Determines secondary sex characteristics Prepares the body for pregnancy; involved in the menstrual cycle Inhibits release of FSH Testes • The testes are located in the scrotum, a sac of skin between the upper thighs Hormones secreted by testes Testosterone Inhibin Physiological action Determines secondary sex characteristics and plays minor role in spermatogenesis Inhibits release of FSH
