Endocrine SystemDefinition:Theendocrine systemis a controlsystem of thebody, whichcontrolsvarious functions of the human body through secretions known as hormones, secreted by theendocrine glands.Endocrine Glands:Endocrine glandsare secretary tissues (glands) which secrete hormones directly into the blood stream. They are also called ductless glands. The study ofendocrine glandsis called endocrinology.

Exclusively endocrine: the pituitary,the thyroid,the parathyroid, thymus and the adrenals. Partially endocrine : The pancreas, gastric and duodenal epithelium, the gonads (testis in males and ovary in females) and placentain females.

ImportantEndocrine Glands& Their Functions (secretions)

Following are the importantendocrine glandsof the human body.i)Pituitary Glandii) Hypothalamusiii) Thymusiv) Pinealv)Thyroidvi) Parathyroidvii) Adrenalviii) Pancreasix) Testesx) Ovaries I) HypothalamusThe hypothalamus is located immediately below the thalamus at the centre ofthe brain, andcontrols many automatic functions of the body. the hormones produced by hypothalamus are of two types, the releasing hormones (which stimulate secretion of pituitary hormones) and the inhibiting hormones (which inhibit secretions of pituitary hormones).

hypothalamic hormone called Gonadotrophin releasing hormone (GnRH) stimulates the pituitary synthesis and release of gonadotrophins.

Somatostatin from the hypothalamus inhibits the release of growth hormone from the pituitaryFunction:The general functions of the hypothalamus are of extreme importance for the body, such as:-Pituitary glandregulation- Blood pressure regulation- Hunger and salt cravings- Feeding reflexes- Thirst- Body temperature regulation

II) Pituitary Gland: the master glandThepituitary glandis a small bean-shaped reddish gray organ, located near the hypothalamus (forebrain). It produces at least nine hormones which perform following functions:Functions:- Responsible for normal skeletal growth, milk secretion in mammary glands.-Controlsnormal functioning ofthyroidand adrenal glands- Stimulates the formation of the graffian follicles in the female ovary anddevelopment ofspermatozoa in male.- Stimulate the contraction of the uterine muscles during the final stage of pregnancy- Control the amount of urine secreted by the kidney.

Pituitary gland is divided anatomically into an adenohypophysis and a neurohypophysis.

Adenohypophysis consists of two portions, pars distalis and pars intermedia. The pars distalis region of pituitary, commonly called anterior pituitary, produces growth hormone(GH), prolactin(PRL), thyroid stimulating hormone(TSH), adrenocorticotrophic hormone(ACTH), luteinizing hormone(LH) and folliclestimulating hormone(FSH). Pars intermedia secretes only one hormone called melanocyte stimulating hormone(MSH).

Neurohypophysis (pars nervosa) also known as posterior pituitary, stores and releases two hormones calledoxytocin and vasopressin, which are actually synthesised by the hypothalamus and are transported axonally toneurohypophysis.

Oxytocin acts on the smooth muscles of our body and stimulates their contraction. In females, it stimulates a vigorous contraction of uterus at the time of child birth, and milk ejection from the mammary gland.

Vasopressin acts mainly at the kidney and stimulates resorption of water and electrolytes by the distal tubules and thereby reduces loss of water through urine (diuresis). Hence, it is also called as anti-diuretic hormone(ADH)

III) Pineal Gland:The pineal gland is a pine cone shaped gland of theendocrine system. Astructureof the diencephalon ofthe brain, the pineal gland produces several important hormones including melatonin.Function:The pineal gland is involved in several functions of the body including: Secretion of the Hormone Melatonin Regulation of Endocrine Functions Conversion of Nervous System Signals to Endocrine SignalsMelatonin plays a very important role in the regulation of a 24-hour (diurnal) rhythm of our body. For example, it helps in maintaining the normal rhythms of sleep-wake cycle, body temperature. In addition, melatonin also influences metabolism, pigmentation, the menstrual cycle as well as our defense capability.

IV) ThyroidGland:Thethyroidgland is paired organ located in upper region of the neck in front of windpipe. It produces the hormone thyroxin (T4), Triiodothyronine (T3) and calcitonin.Functions:- Plays an important role in regulating the body's metabolism- The T4 and T3 hormones stimulate every tissue in the body to produce proteins and increase the amount of oxygen used by cells.- The calcitonin hormone works together with the parathyroid hormone to regulate calcium levels in the body.

Iodine is essential for the normal rate of hormone synthesis in the thyroid. Deficiency of iodine in our diet results in hypothyroidism and enlargement of the thyroid gland, commonly called goitre. Hypothyroidism during pregnancy causes defective development and maturation of the growing baby leading to stunted growth (cretinism), Myxoedema, mental retardation, low intelligence quotient, abnormal skin, deaf-mutism, etc. In adult women, hypothyroidism may cause menstrual cycle to become irregular. Due to cancer of the thyroid gland or due to development of nodules of the thyroid glands, the rate of synthesis and secretion of the thyroid hormones is increased to abnormal high levels leading to a condition called hyperthyroidism which adversely affects the body physiology. This condition causes marked increase in the metabolic rate, rapid heart beat, shortness of breath and the eyes protrude out together with goitre in the neck.

Thyroid hormones play an important role in the regulation of the basal metabolic rate. These hormones also support the process of red blood cell formation. Thyroid hormones control the metabolism of carbohydrates, proteins and fats. Maintenance of water and electrolyte balance is also influenced by thyroid hormones.

Thyroid gland also secretes a protein hormone called thyrocalcitonin (TCT) which regulates the blood calcium levels. Calcitonin regulates the calcium and phosphate levels in the blood. If the calcium level in blood is high more calcitonin is secreted and the calcium ions are moved from the blood to the bones making them harder. The reverse happens when the calcium level in the blood is low making the bones soft.

V) Parathyroid Gland:The parathyroid glands are embedded in thethyroidgland. In humans, four parathyroid glands are present on the back side of the thyroid gland, one pair each in the two lobes of the thyroid gland . The parathyroid glands secrete a peptide hormone called parathyroid hormone(PTH). The secretion of PTH is regulated by the circulating levels of calcium ions.Functions:-Controlsthe concentration of calcium (Ca ion) and phosphorus (phosphate) in the blood.- Stimulates the absorption ofcalcium and magnesiumfrom the gut.- Increases tubular reabsorption of calcium, magnesium and hydrogen ions.

VI) Thymus:The thymus gland, which is shaped like a pyramid, is a specialized organ of the human immune system. It is also one of the important glands in the human body. It is a pinkish gray colored organ that is situated in the upper thoracic region, under the sternum or the breast bone. This gland secretes the peptide hormones called thymosins. Thymosins play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity. In addition, thymosins also promote production of antibodies to provide humoral immunity. Thymus is degenerated in old individuals resulting in a decreased production of thymosins. As a result, the immune responses of old persons become weak.Function:The thymus gland is very active, when the individual is a child. It plays a crucial role in developing and improving a child's immunity system. The main function of the thymus gland is to produce and process lymphocytes or T cells. Causes Feeling of Sleepiness Influences Sexual Development-It begins to atrophy after puberty.

VII) Adrenal Gland:Theadrenal glandcurve over the top of each kidney in abdomen. The adrenals (ad: adjacent, renal; kidney) are a pair of glands situated like caps one above each kidney. Each adrenal consists of two parts: a central medulla and a peripheral cortex.

The adrenal medulla secretes two hormones called adrenaline or epinephrine and noradrenaline or norepinephrine. These are commonly called as catecholamines. Adrenaline and noradrenaline arerapidly secreted in response to stress of any kind and during emergency situations and are called emergency hormonesor hormones of Fight or Flight. These hormones increase alertness, pupilary dilation, piloerection (raising of hairs), sweating etc. Both the hormones increase the heart beat, the strength of heart contraction and the rate of respiration. Catecholamines also stimulate the breakdown of glycogen resulting in an increased concentration of glucose in blood. In addition, they also stimulate the breakdown of lipids and proteins. Increases blood supply to the muscles while decreasing blood supply to the visceral organs. It releases more glucose into the blood from the liver.

The adrenal cortex secretes two categories of hormones: glucocorticoids and mineralocorticoids.

(a) Glucocorticoidse.g. cortisone In response to stress it raises blood glucose through action of the liver including deamination of amino acids. During starvation and prolonged fasting the required glucose is partly provided through this hormone. It adapts the body to stresses such as extreme heat or cold, burns, infections, etc.

Some of the cortical hormones behave like sex hormones. Overgrowth of adrenal cortex in young children causes premature sexual maturity. Overgrowth of adrenal cortex in mature female results in the development of male characters such as beard and deep voice. Overgrowth of adrenal cortex in mature males results in the development of some feminine characters such as enlargement of breasts.- Small amounts of androgenic steroids are also secreted by the adrenal cortex which play a role in the growth of axial hair, pubic hair and facial hair during puberty.

(b) Mineralocorticoidse.g. aldosteroneThis hormone is concerned with water retention. It increases reabsorption of sodium and chloride ions in kidneys.Function:- Adrenal glands are responsible for secreting hormones essential for carrying out some important metabolic processes. Besides, they also help the body in dealing with physical and mental stress.aldosterone helps in the maintenance of electrolytes, body fluid volume, osmotic pressure and blood pressure.

VIII) Pancreas:Pancreas is an endocrine as well as an exocrine gland. It has special groups of cells called Islets of Langerhans, which consists of three kinds of cells alpha cells producing glucagon, beta cells producing insulin and gamma cells producing somatostatin.

(i) Glucagon. Glucagon is a peptide hormone, and plays an important role in maintaining the normal blood glucose levels. Glucagon acts mainly on the liver cells (hepatocytes) and stimulates glycogenolysis resulting in an increased blood sugar (hyperglycemia). In addition, this hormone stimulates the process of gluconeogenesis which also contributes to hyperglycemia. Glucagon reduces the cellular glucose uptake and utilisation. Thus, glucagon is a hyperglycemic hormone.

(ii) Insulin. It performs two principal tasks; Promotes glucose utilization by the body cells. Stimulates deposition of extra glucose in the blood as glycogen in the liver. Non-secretion or under secretion of insulin causes diabetes mellitus (hyperglycemia, meaning more than normal sugar in blood.A diabetic person, has higher glucose in blood; excretes a great deal of urine loaded with sugar; feels thirsty because of loss of water through too much urination; loses weight and becomes weak. In some cases, the patient even loses the eyesight.

Oversecretion of insulin causes hypoglycaemia or low blood sugar. The brain may enter a state of coma if the level of sugar in blood becomes too low.

(iii) Somatostatin also called Growth Hormone-Inhibiting Hormone (GHIH) inhibits secretion of insulin as well as glucagon.

IX) Gonads (testis and ovary)Testes in males possess two kinds of cells : the sperm-producing germinal cells and the hormone-producing interstitial cells. The hormones produced are called androgens and the commonest one among them is testosterone.The testosterone estimulates the development of the male characters during which the body at puberty starts developing facial hair, and their voice cracks and deepens. Androgens play a major stimulatory role in the process of spermatogenesis (formation of spermatozoa). Androgens act on the central neural system and influence the male sexual behaviour (libido). These hormones produce anabolic (synthetic) effects on protein and carbohydratemetabolism.

Ovaries in females produce two kinds of hormonesestrogen and progesterone. Estrogenis secreted from the follicles of the ovary and stimulates the development of breasts and fat deposition on the hip in a mature woman. Estrogen prepares the wall of the uterus for receiving the fertilized egg.Progesterone is secreted by the corpus luteum (follicle left after the release of ovum). It brings about the final changes in the uterus for the retention and growth of the foetus during pregnancy.

X) PlacentaPlacenta of a pregnant woman produces certain hormones. One such hormone is human chorionic gonadotropin (HCG), which maintains the activity of corpus luteum in secreting progesterone continuously.

XI) Hormones from stomach and intestine(i) Gastrinis the hormone secreted by the mucus membrane of the pyloric end of the stomach. It stimulates the gastric glands to secrete gastric juice. Gastrin acts on the gastric glands and stimulates the secretion of hydrochloricacid and pepsinogen.(ii) Secretin is the hormone secreted by the inner lining of the duodenum. It stimulates the production of pancreatic juice while the hormone. Secretin acts on the exocrine pancreas and stimulates secretion of water and bicarbonate ions.(III) cholecystokinin:- CCK acts on both pancreas and gall bladder and stimulates the secretion of pancreatic enzymes and bile juice, respectively.

XII) heartThe atrial wall of our heart secretes a very important peptide hormone called atrial natriuretic factor(ANF), which decreases blood pressure. When blood pressure is increased, ANF is secreted which causes dilation of the blood vessels. This reduces the blood pressure.

XIII) Kidney:- The juxtaglomerular cells of kidney produce a peptide hormone called erythropoietin which stimulates erythropoiesis (formation of RBC).

Gastric inhibitory peptide:- GIP inhibits gastric secretion and motility.

Non-endocrine tissues Several other non-endocrine tissues secrete hormones called growth factors. These factors are essential for the normal growth of tissues and their repairing/regeneration.

THE FEEDBACK MECHANISM (CONTROL OF HORMONAL SECRETION)The amount of hormone released by an endocrine gland is determined by the bodys need for the particular hormone at any given time. The product of the target tissue exerts an effect on the respective endocrine gland. This effect may be positive (secrete more) or negative (secrete no more or slow down). This can be explained by taking the example of thyroid gland.

Feed back mechanism of thyroid activity Hypothalamus releases a hormone TSH-RH (TSH- Releasing Hormone) which instructs the anterior pituitary to release TSH (thyroid stimulating hormone). The TSH stimulates thyroid to release thyroxine. If the level of thyroxine in blood increases, the pituitary stops the release of TSH. If the level of thyroxine becomes still higher, then the inhibition of the release of thyroxine takes place not only at the level of the pituitary but also at the level of hypophysis to inhibit the release of TSH-RH. When the level of thyroxine falls in the blood, the thyroid gets stimulated to secrete more of it. What is happening here is that the starting point of an activity receives back the information whether to continue or increase, or to slow down or even stop.

PHEROMONESTHE CHEMICAL MESSENGERS AT SOCIAL LEVELPheromones are the secretions given out by an individualinto the environment, which bring about a specific response in other members of the same species. Some of the examples of the pheromones are as follows: Common ants march on the floor or walls in a trailon an invisible path laid down by a secretion from their bodies. It helps them to reach the destination one after another, as well as to return correctly to their own nest. When disturbed honey bees give out an alarm pheromonefrom their sting at the back and mandibles in the mouth. This alerts the inmates of the hive to face the attack. Females of a particular moth gives out a scent which can attract a male from as much distance as 3-4 kilometers. Introduction of a male mouse into a group of female mice shortens oestrus cycle (cycle of development of eggs in the ovary and ovulation). Introduction of a strange male mouse of a different strain disturbs to the extent that the newly pregnant females abort their foetuses. The source of pheromone of the strange male mouse is in its urine.