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Transcript
Chemical Coordination Endocrine Glands An endocrine gland is a gland that has no duct to carry its secretions. Rather, these are carried through the circulatory system. An endocrine gland produces an internal secretion that is discharged into the blood or lymph to various parts of the body. Exocrine Glands An exocrine gland is a gland which lies near the concerned organ and empties its secretions directly, or has a duct to carry its secretions to its target organ. Hormones Hormones are organic compounds manufactured by the endocrine glands from substances carried in the blood. They are also known as chemical coordinators or messengers. Hormones are mainly protein in nature, but some are steroids. They are carried by the blood to their target organ, stimulating it by functional activity to increase functional activity or to increase the secretions of another hormone. The glands secrete their hormones continuously but the quantity of secretion can be increased or decreased according to body needs. Control of hormone secretion occurs in several ways : Nerve cells produce chemicals, which are carried to the gland and cause secretion. The gland may secrete a hormone as a response to impulses from the autonomic nervous system. e.g. adrenal glands secreting adrenaline One gland produces a hormone that affects a second gland. The second gland secretes its hormone, which influences secretion of the first gland. This is called a feedback mechanism. e.g. release of glucagon or insulin from the pancreas Video Clip 1 : How Hormones Work The Endocrine System The ductless or endocrine glands of the body form the endocrine system. There are five main endocrine glands in the body. 1. The pituitary gland or the hypophysis 2. The thyroid gland 3. The adrenal or suprarenal glands 4. The pancreas 5. The testes in males or the ovaries or ovaria in females Video Clip 2 : The Endocrine System Figure 1 : The Endocrine System The Pituitary Gland The pituitary gland is a small, grey, rounded body and is located in a bony cavity of the skull, the sella turcica, 13 mm wide, beneath the brain and above the roof of the mouth. It is attached to the base of the brain by the infundibular stalk. The hypophysis is 1.3 cm by 1.0 cm by 0.5 cm in size and 0.55 g to 0.6 g in weight. Video Clip 3 : The Pituitary Gland and its Hormones The pituitary gland releases seven hormones. 1. GONADOTROPIN : Hormone gonadotropin is released from the anterior lobe of the pituitary gland – the adenohypophysis. It acts on the gonads – the testes in males and the ovaria in females. In females, gonadotropin stimulates the growth of Graafian follicles, and in males, it stimulates the growth of the germinal epithelium in the testes – thus causing the production and secretion of sex hormones. 2. THYROTROPIN : Hormone thyrotopin is released by the adenohypophysis and is also known as the thyroid stimulating hormone (TSH). It controls the production and the amount of hormone secreted by the thyroid gland, by altering the number, size, and rate of hormone production by the thyroid gland. 3. CORTICOTROPIN : Hormone corticotropin is also released by the adenohypophysis, and is known as the adrenocorticotrophic hormone (ACTH). It controls the production and the amount of hormone secreted by the cortices of the adrenal glands, by altering the number and degree of activity of the cells of the adrenal cortex. 4. SOMATOTROPIN : Hormone somatotropin is released by the adenohypophysis, and is also known as growth or somatotrophic hormone. It increases the rate of growth, and then maintains size once maturity has been attained. Hormone somatotropin exerts its influence mainly on the hard tissue of the body, although there is some effect on the soft tissue. It is secreted by the adenohypophysis throughout life even though most growth procedures are completed by adolescence. Oversecretion of hormone somatotropin causes overgrowth of the long bones, resulting in gigantism. Undersecretion of hormone somatotropin causes dwarfism. 5. PROLACTIN : Prolactin is a female hormone. It is released by the adenohypophysis during pregnancy and the whole period of milk production in the postpartum period. It is also known as the lactogenic hormone. This hormone stimulates the development of the breasts, ad production of milk in the mammary glands. 6. VASOPRESSIN : Vasopressin is secreted by the posterior lobe of the pituitary gland – the neurohypophysis, and is also known as the antidiuretic hormone (ADH). It is produced in the hypothalamus and is then carried to the neurohypophysis by the hypothalamohypophyseal tract for storage. This hormone causes an increase in the reabsorption of water from the glomerular filtrate by the kidney tubules so that less urine is excreted. It does this by altering the permeability of the kidney tubule epithelial cells. In this way, ADH plays a major part in osmoregulation and excretion. Deficient secretion of ADH causes less water to be reabsorbed from the glomerular filtrate and the consequent production of large amounts of copious urine – a condition called diabetes insipidus or water diabetes. Excessive secretion causes large amounts of water to be reabsorbed and consequently increases the concentration of urine. 7. OXYTOCIN : Hormone oxytocin is produced by the hypothalamus and is then carried to the neurohypophysis by the hypothalamo-hypophyseal tract for storage and secretion. This is a female hormone, which is released at the very end of pregnancy. It acts mainly on the unstriped muscle of the pregnant uterus, stimulating it to contract, thus inducing parturition, i.e. the birth of the baby. The Thyroid Gland The thyroid gland is a butterfly-shaped gland that lies in the anterior part of the neck immediately below the larynx and overlying the trachea. The thyroid gland secretes hormone thyroxine. The function of hormone thyroxine is to regulate metabolism in the tissues. It controls growth and development of the body by affecting energy release by mitochondria. Thyroxine steps up the rate of reaction of all cells, by increasing the secretion of enzymes. Video Clip 4 : The Thyroid Gland and its Hormones Lack of hormone thyroxine or hypothyroidism in children causes thyroid cretinism, which, if untreated, results in a mentally retarded dwarf. Hypothyroidism in adults causes myxoedema. In both cretinism and myxoedema, the skin and hair are coarse and dry; the metabolic rate is slow and so the patient is abnormally obese and sluggish. Excessive secretion of hormone thyroxine is called hyperthyroidism. It causes thyrotoxicosis, which is a condition marked by an increased metabolic rate. The individual is anxious, nervous, and restless, with a fast pulse rate. The skin is fine and moist. They eyes may or may not protrude from the orbits – a condition known as exophthalmos. The secretion of hormone thyroxine is completely regulated by hormone thyrotropin released by the pituitary gland. The Adrenal Glands There are two adrenal or suprarenal glands that lie one above and in front of the upper end of each kidney. The gland has two distinct regions – the outer part called the cortex and the inner part called the medulla. The adrenal medulla releases hormone adrenaline. This hormone is released during stress or emergency. It prepares the body for fight or flight. Video Clip 5 : The Adrenal Gland and its Hormones Effects of Adrenaline It increases the heartbeat and the rate and depth of breathing. It increases blood supply to the heart and skeletal muscles by vasodilation, and by the constriction of other arteries like those supplying the skin, etc. Activity of the gut, the kidneys and other organs not related energy release is lowered. Blood sugar level is heightened by increased breakdown of glycogen into glucose. The Pancreas The pancreas is a long gland that lies immediately beneath the stomach. It is both an endocrine and an exocrine gland. The endocrine part of the pancreas is known as the Islets of Langerhans. The islets are composed of -cells and -cells. The -cells release hormone insulin and the -cells secrete hormone glucagon. Hormone insulin causes the liver to convert extra glucose into glycogen. Hormone glucagon causes the breakdown of glycogen into glucose. Video Clip 6 : The Pancreas and its Hormones The Ovaries The ovaries or the ovaria are the female sex glands. There are tow ovaries in the female, one on each side of the pelvic cavity. The ovary produces two hormones – oestrogen and progesterone. Hormone oestrogen has two main functions. 1. It controls the development of secondary sexual characteristics in females. These are : a) Development of the mammary glands b) Development of pubic hair c) Onset of the menstrual cycle 2. It causes the lining of the uterus to thicken just before an ovum is released. Hormone progesterone promotes further thickening and vascularisation of the uterus after ovulation. It maintains the placenta during pregnancy and prevents the uterus from contracting until the baby is due to be born. It also helps in the development of the mammary glands. The production and secretion of hormones from the ovaries is controlled by hormone gonadotropin released by the pituitary gland. The Testes The testicles or the testes are the male sex glands. There are two testes in the male and are situated in the scrotum. The testis produces the male sex hormone testosterone. Hormone testosterone controls the development of the secondary sexual characteristics in males. Deepened voice Increased musculature Development of pubic hair Development of facial hair Production and secretion of testosterone is controlled by the pituitary hormone gonadotropin. Homeostasis & Feedback The mechanism in which the consistency of the internal environment of the body is maintained by not allowing the temperature and composition of body fluids to fluctuate beyond certain limits is known as homeostasis. The mechanism in which information about bodily functions is sent to the central nervous system and the endocrine system so that consistency of the internal environment of the body can be maintained is knows as feedback. Feedback Control in Homeostasis of Blood Sugar Level EXCESS DEFICIENT NORM If blood sugar level is above 150 mg/100 cm3 of blood DETECTORS Detectors present within the blood vessels detect changes in blood sugar level. CORRECTIVE MECHANISM The hypothalamus instructs the pancreas to release hormone insulin. The hypothalamus instructs the pancreas to release hormone glucagon. NEGATIVE FEEDBACK Insulin causes the liver to convert surplus glucose to glycogen. Thus, sugar level is lowered to normal. Glucagon causes the liver to convert stored glycogen into glucose. Thus, sugar level is brought back to normal. Lack of insulin causes sugar level to continue to rise, resulting in hyperglycaemia. Lack of glycogen causes blood sugar level to continue to fall, resulting in hypoglycaemia. The person collapses, loses consciousness and goes into coma, then dies. POSITIVE FEEDBACK If blood sugar level is below 90 mg/100 cm3 of blood Feedback Control in Homeostasis of Blood Thyroxine Level EXCESS DEFICIENT NORM If thyroxine level in blood is above 6 g/100 cm3 of blood If thyroxine level in blood is below 3 g/100 cm3 of blood DETECTORS Detectors present within the blood vessels detect changes in the level of thyroxine in blood. CORRECTIVE MECHANISM The pituitary gland stops the production of secretion of hormone thyrotropin. The pituitary gland increases the production and secretion of hormone thyrotropin. NEGATIVE FEEDBACK Lack of thyrotropin causes the thyroid gland to stop the production and secretion of hormone thyroxine. Thus, thyroxine level in blood is brought back to normal. Presence of thyrotropin accelerates production and secretion of thyroxine. Thus, thyroxine level is raised to normal. Thyroxine level continues to rise, resulting in hyperthyroidism. Body metabolism increases. Thyroxine level continues to fall, resulting in hypothyroidism. Body metabolism decreases POSITIVE FEEDBACK