MODULE 6 COURSE MANUAL CHRISTINA LYNE christina@aromalyne.com
THE ENDOCRINE SYSTEM The endocrine system has many functions: it works closely with the nervous system and plays an important part in homeostasis, coordinates growth and development and affects reproduction and metabolism. The endocrine system is composed of ductless glands which produce hormones, the body s chemical messengers. Each gland produces specific hormones. WHAT IS A HORMONE? A hormone is a chemical messenger, made of protein, which is secreted directly into the bloodstream by a particular gland. They are produced in the gland and are then transported to the area / organ they control or affect. Hormones evoke a response in other cells or tissues of the body that are located far away. Some hormones have a slow action over a period of years, while others produce a quick response. They are produced a little at a time when they are needed and are not secreted continuously. STRUCTURE OF ENDOCRINE GLANDS An endocrine gland is a ductless gland which produces hormones. Ductless means that there is no separate canal or tube to transport the hormones to the blood. They are not physically connected to each other and may be widely separated from each other. Endocrine glands are groups of secretory cells surrounded by an extensive network of capillaries that allow the hormones to diffuse from the secretory cells into the bloodstream. The main endocrine glands are as follows: 1 pituitary gland 1 hypothalamus 1 pineal gland 1 thyroid gland 4 parathyroid glands 2 adrenal glands the islets of Langerhans (in the pancreas) 1 thymus gland 2 ovaries (female) 2 testes (male)
WHAT DO HORMONES AND THE ENDOCRINE SYSTEM DO? Hormones and the Endocrine System affect the behaviour and function of different areas of the body and of the body overall. For example, hormones are responsible for correct growth, changes in puberty, the menstrual cycle, pregnancy, the menopause, responses to stress and danger and the proper functioning of the kidneys and digestive system. Below is an overview of the physiology of the Endocrine System: Hormone Producing Cell Feedback Signal (usually negative feedback) Blood Disposal of hormone by degradation and/or excretion Desired Response (Target Cell) The amount of hormone released by the endocrine gland or tissue is determined by the body s need for the hormone at any given time. This is the basis to which the endocrine system operates. Hormone producing cells have available to them information from sensing and signaling systems that permit them to regulate the amount and duration of hormone release. Secretion is regulated so that there is no over/under production of the hormone. Once the hormone is released it is carried by the blood to target tissues and organs that contain target cells (cells which respond to the hormone). All cells are target cells for one or more hormones but not all cells respond to a particular hormone. Target cells contain receptors that bind the hormone so that it can produce an effect. Once the target cell responds to the hormone, the response must be recognised by the secretory cell by a feedback signal. WHAT HAPPENS TO THE HORMONE ONCE IT HAS DONE ITS WORK? Once a hormone has done its work, it is rendered inactive by its target cells. It is carried to the liver. There it is deactivated and broken down. It is then eliminated.
THE INDIVIDUAL ENDOCRINE GLANDS THE PITUITARY This is the master gland. It is a pea-sized mass made up of two parts: a larger anterior lobe and a smaller posterior lobe. Both lobes rest in a depression in the sphenoid bone behind the nose and between the eyes. It is attached to the hypothalamus by a stalk-like structure. It releases at least nine hormones that have important effects on the body. Some of these control body functions directly; others trigger different hormones to produce hormones of their own. THE HYPOTHALAMUS The hypothalamus is situated at the base of the brain above the pituitary and is connected to the pituitary by nerve fibres. It secretes hormones (called releasing factors ) which are transmitted to the pituitary. The pituitary gland then produces hormones which control other endocrine glands. The hypothalamus and the pituitary gland work together to co-ordinate and control hormone production.
The hypothalamus: Controls body temperature Motivates sexual behaviour Stimulates appetite and thirst Determines mood and emotions Regulates sleep Monitors hormone levels in the blood THE PINEAL GLAND This tiny little gland is found in the centre of the brain. Its function isn t fully understood, but it is believed to be associated with the inhibition of growth and development of the reproductive organs before puberty. It responds to sunlight. It secretes a hormone called melatonin that controls our body rhythms such as sleeping and waking. It is also believed to affect our appetite, fertility and growth. THE THYROID GLAND This gland is made up of two lobes and is located in the front of the neck, just below the larynx. It is under the control of the pituitary gland. The function of the thyroid is to take iodine, found primarily in seaweed and sea water fish, and convert it into thyroid hormone. Thyroid cells are the only cells in the body that can absorb iodine. Every cell in the body depends upon thyroid hormones for regulation of their metabolism. Unlike other glands, it can store the hormones it produces. It also secretes calcitonin. THE PARATHYROID GLANDS There are four glands, two either side behind the thyroid. They produce the hormone parathormone. The sole purpose of these glands is to control calcium levels within the blood. They also control how much calcium is in the bones, and therefore, how strong and dense the bones are. As the blood filters through the parathyroid glands, they detect the amount of calcium present. If too much calcium is present, the hormone calcitonin is released from the thyroid, preventing the removal of calcium from the bones. If calcium levels are insufficient, parathormone will trigger calcium to be released from bones and into the blood. Calcium is also the primary element which causes muscles to contract and is important in blood clotting. THE ADRENAL GLANDS There are two adrenal glands, each one located on the top of each kidney. Each gland is triangular in shape and is made up of two regions: an inner medulla and an outer cortex. Each region produces different hormones.
Adrenal Cortex: Mineralcorticoids help to control the correct balance of water, sodium and potassium in the body. The hormone responsible for most mineral corticoid activity is aldosterone. It inhibits the amount of sodium excreted in the urine into the blood. Sodium reabsorption is also accompanied by retention of water and therefore aldosterone is also involved in the regulation of blood volume and blood pressure too. Sex Corticoids comprise the female sex hormones oestrogen and the male hormones androgens. They are produced in small amounts and help to control the changes in males and females in late puberty and early adulthood. Glucocorticoids are essential to life, regulating metabolism and responses to stress. The main hormone secreted by the adrenal cortex is cortisol. It is stimulated by ACTH (adrenocorticotrophic hormone) from the anterior pituitary and by stress. Cortisol has several functions: Gluconeogenesis - cortisol causes the breakdown of proteins which are then released as amino acids into the blood stream. The liver then uses these amino acids to make glucose. This same process also raises blood sugar levels (hyperglycaemia) that provides the brain with glucose. Lipolysis - causes the breakdown on triglycerides into fatty acids and glycerol for energy production. It promotes the absorption of sodium and water from the renal tubes and causes an anti-inflammatory action. Adrenal Medulla The medulla is completely surrounded by the cortex. It produces the hormones adrenaline and noradrenaline. Both these hormones are released into the blood stream when the sympathetic nervous system is stimulated. They are structurally very similar and together are responsible for the fight or flight response by: Increasing heart rate & blood pressure Decreasing rate of digestion Diverting blood to essential organs, including the brain, heart and skeletal muscles. Dilating pupils Adrenaline has a greater effect on the heart whereas noradrenaline has more influence on blood vessels.
THE PANCREAS This gland is located a little below the stomach, between the duodenum and the spleen. It has two main functions: 1. to produce pancreatic endocrine hormones 2. to produce pancreatic digestive enzymes The cells which make up the islets of Langerhans are found in clusters scattered throughout the pancreas. Blood glucose levels are controlled by the opposing actions of insulin and glucagon: Insulin decreases blood glucose levels. If there is too much sugar in the blood, insulin is released causing the liver and muscles to store glucose (in the form of glycogen). Glucagon increases blood glucose levels. If there is insufficient sugar in the blood, or the body needs a rush of sugar for energy (an athlete about to run around the track) glucagon will cause the liver and muscles to release glucose into the bloodstream to restore levels. THE THYMUS GLAND The thymus lies behind the sternum and extends upwards into the root of the neck. It is made of lymphoid tissue. It produces several hormones, including thymosin, that are important in early life to assist with growth. It gradually degenerates after puberty. It plays an important role in immunity and is involved in the development of T-lymphocytes which help the body to fight viruses and infections. THE OVARIES These are the female glands and are found either side of the pelvic cavity. Each is attached to the upper part of the uterus by a short ligament, is about the size of an almond and is pinky grey in colour. They produce: Oestrogens which are responsible for the secondary sex characteristics in the female. Progesterone which stimulates the lining of the uterus to thicken for possible implantation of an embryo and to prepare the mammary glands for milk production. During pregnancy large amounts of oestrogens and progesterone are produced by the placenta preventing ovulation and menstruation and maintaining the thick lining of the uterus.
THE TESTES These are the male glands and are found in the groin, in the scrotum. The testes produce the hormone testosterone responsible for male sexual characteristics and for the development of secondary sexual characteristics such as the deepening of the voice and beard growth. LOCAL HORMONES A number of body tissues not normally described as endocrine glands secrete substances that do not travel to remote organs but act in tissues nearby (locally). They are: Histamine This is released as part of the inflammatory response, increasing capillary permeability and causing vasodilation. It also causes contraction of smooth muscle of the bronchi and alimentary tract and stimulates the secretion of gastric juice. Serotonin This substance is present in platelets - It is a local vasoconstrictor. When platelets come into contact with a damaged blood vessel, their surface becomes sticky and they attach themselves to the damaged wall. They then release serotonin which restricts the blood vessel, thereby reducing blood flow through it. In the brain - it plays a role in the brain and nerve function. It has an effect on mood. Serotonin leads to feelings of happiness / relaxation / sleepiness. It is concentrated in certain areas of the brain - the hypothalamus and mid brain contain large amounts. In the intestinal wall - it causes intestinal secretion. The stomach is surrounded by cells which release serotonin. This causes intestinal peristalsis. Erythropoietin This hormone is made in the kidneys and increases the rate of red blood cell formation (erythropoiesis). HOMEOSTATIC FEEDBACK MECHANISMS Many endocrine glands are linked to neural control centers by homeostatic feedback mechanisms. The two types of feedback mechanisms are negative feedback and positive feedback. Most endocrine glands are under the control of negative feedback mechanisms. Negative feedback mechanisms act like a thermostat in the home. As the temperature falls, the thermostat detects the change and triggers the central heating
to turn on and heat the house. Once the temperature reaches its thermostat setting, the heating turns off. An example of negative feedback is the regulation of the blood calcium level. The parathyroid glands secrete parathyroid hormone, which regulates the blood calcium amount. If calcium decreases, the parathyroid glands sense the decrease and secrete more parathyroid hormone. The parathyroid hormone stimulates calcium release from the bones and increases the calcium uptake into the bloodstream from the collecting tubules in the kidneys. Conversely, if blood calcium increases too much, the parathyroid glands reduce parathyroid hormone production. Both responses are examples of negative feedback. Positive feedback mechanisms have the effect of amplifying the stimulus and increasing the release of the hormone until a particular process is complete and the stimulus ceases. An example of positive feedback can be found in childbirth. The hormone oxytocin stimulates and enhances labour contractions. As the baby moves towards the birth canal, pressure on the receptors within the cervix send messages to the brain to produce oxytocin. Oxytocin travels to the uterus through the bloodstream, stimulating the muscles in the uterine wall to contract stronger. The contractions intensify and increase until the baby is outside the birth canal. When the stimulus to the pressure receptors ends, oxytocin production stops and labour contractions cease. THE INTER-RELATIONSHIP BETWEEN THE ENDOCRINE AND NERVOUS SYSTEMS The hypothalamus is the link between the nervous system and the endocrine system. It produces hormones which regulate hormone secretion by the pituitary gland. The pituitary gland then produces hormones which control other glands. The hypothalamus is connected by nerve fibres to many areas of the brain and receives information from them about the emotions and about conditions in other parts of the body. What is the difference between Endocrine and Exocrine Glands? Exocrine glands are glands that secrete their products through the ducts, and discharge it into the external environment, to organs or the outside the body. Exocrine glands differ from endocrine glands, because they have ducts that deliver the products in the superficial part of the body, such as the skin, or in the inner part where they are necessary, such as the pancreatic juice that is carried into the intestine to aid digestion. The glands that are found in the body are mostly exocrine glands. Examples of exocrine glands are sweat, saliva and mammary glands, as well as oil and enzymes. Exocrine glands manufacture and release hormones. These hormones are transported to the surrounding area, and to the blood. A specific receptor is then needed for that hormone to do its specified job.
The Endocrine system is one of the body s most important systems, especially with the control of the body s functions. This is how the body communicates and coordinates with the nervous system, reproductive system, pancreas, liver, kidneys and fat to maintain balance, or homeostasis, with reproduction, growth and development, and energy levels and responses to external stress and injury. Endocrine glands produce hormones that can be used inside the body. The endocrine transmits the hormonal messages to cells by secreting them into the blood and extracellular fluid. A receptor is needed in order to receive the message transmitted. The target points may be cells, tissues or organs. Endocrine glands are ductless, therefore the secreted hormones are released into the interstitial spaces that surround the cells. The hormones are delivered to the nearest capillaries, and spread throughout the body. The responses are delayed because hormones must first travel through the blood to reach the target organs. The duration is longer because the kidneys filter the blood. THE CHANGES IN OUTPUT OF SPECIFIC HORMONES THAT OCCUR DURING PUBERTY, THE MENSTRUAL CYCLE, PREGNANCY AND MENOPAUSE Puberty (male) This occurs between the ages of 10-14 years. LH (luteinising hormone) from the anterior pituitary gland stimulates the increase in production of testosterone. This hormone is responsible for the development of the body to sexual maturity. Puberty (female) Between the ages of 12-14 years, young girls are at the stage where their internal organs have matured sufficiently and they can bear children. The ovaries are stimulated by the hormones FSH (follicle stimulating hormone) and LH (luteinising hormone) secreted by the anterior pituitary. Increased secretion of these hormones stimulates the ovaries to secrete oestrogens, responsible for the secondary sexual characteristics. The breasts begin to develop and the uterine tubes, uterus and vagina grow. The Menstrual Cycle This event occurs every 26-30 days throughout the childbearing period (approx. 36 years). The hypothalamus secretes LHRH (luteinising hormone releasing hormone) which stimulates the anterior pituitary to release FSH and LH. Together, these hormones promote ovulation and stimulate the ovaries to produce oestrogens and progesterone. Oestrogens and progesterone stimulate and prepare the uterus to receive, nourish and protect a fertilised ovum (egg). If the ovum is not fertilised a new cycle begins.
Pregnancy For pregnancy to occur, the ovum is fertilised. During the first 3-4 months, the ovaries continue to secrete high levels of oestrogens and progesterone to thicken the lining of the uterus and to prepare the breasts for lactation. Progesterone inhibits uterine contraction. Prolactin levels increase during pregnancy in preparation for lactation, but high levels of oestrogens and progesterone prevent this from actually happening. The ovaries and the placenta also secrete inhibin, which prevents the secretion of follicle stimulating hormone (thus preventing egg production). At the end of the pregnancy progesterone levels fall so that uterine contractions can happen. Oxytocin (posterior pituitary gland) is responsible for stimulating uterine contraction. The ovaries and placenta now secrete relaxin to help the cervix to dilate and to relax the various joints and ligaments involved in labour. Levels of oestrogens and progesterone decrease after the birth, allowing lactation to commence. Menopause (female) The menopause usually occurs between the ages 45-55 years. And this marks the end of the childbearing period. The ovaries become less responsive to FSH and LH and ovulation and the menstrual cycle become irregular and eventually stop altogether. In some, this can happen suddenly or over a period of years - sometimes as long as 10 years! It is caused by a progressive reduction in oestrogen levels. Menopause (male) In males, there is no period comparable to the female menopause. However, at about the age of 55 years, the production of testosterone declines and with it reduced muscle strength, fewer viable sperm and decreased sexual desire occurs.
OTHER BODY TISSUES THAT HAVE AN ENDOCRINE FUNCTION These include: The heart The kidneys The skin The liver HEART The heart has an intrinsic system whereby the cardiac muscle is automatically stimulated to contract without the need for a nerve supply from the brain. However, the intrinsic system can be stimulated by circulating chemicals such as hormones. The hormones adrenaline and noradrenaline, secreted by the adrenal medulla, have the same effect as sympathetic stimulation i.e. they increase the heart rate. Thyroxine increases heart rate. KIDNEYS The formation of new red blood cells is called erythropoiesis. The primary stimulus to increased erythropoiesis is a condition called hypoxia i.e. deficient oxygen supply to body cells. Hypoxia increases the formation of red blood cells by stimulating the hormone erythropoietin, produced mainly by the kidneys. SKIN Sex hormones influence sebaceous glands and hair follicles. Androgen hormones (male hormones) in women and testosterone in men have a direct influence on sebaceous secretion and the production of acne. When sebaceous gland activity increases with the production of excess sebum then acne is formed. In later life, oestrogens (which antagonize the effect of androgens) and male hormones slow down in production, which in turn influences sebaceous secretions and creates a drier skin. LIVER The liver is an extremely active organ with many functions. Two of these functions have an endocrine function: Carbohydrate Metabolism converts glucose to glycogen in the presence of insulin and then converts glycogen back to glucose in the presence of glucagon
The above changes are important in the regulation of the blood glucose level. After a meal, the blood in the portal vein has a high level of glucose content and insulin converts some of it to glycogen for storage. Glucagon converts this glycogen back to glucose as required, to maintain the blood glucose level. Inactivation of hormones - these include insulin, glucagon, cortisol, aldosterone, thyroid and sex hormones.
The Glands Their Hormones Their Actions The Hypothalamus gland Corticotrophin releasing hormone (CRH) Thyroid stimulating hormone releasing hormone (TRH) Follicle stimulating hormone releasing hormone (FSHRH) Luteinizing hormone releasing hormone (LRH) Growth hormone releasing hormone (GHRH) Prolactin inhibiting hormone (PIH) Prolactin releasing hormone (PRH) Melanocyte inhibiting hormone (MIH) Targets the adrenal glands and triggers the adrenals to release ACTH Targets the thyroid where it functions to synthesize and release thyroid hormones T3 and T4 Targets the ovaries and the testes where it enables the maturation of the ovum and of the spermatozoa Targets the ovaries and testes and its receptors are in cells which promote ovulation and increase progesterone synthesis and release Targets the anterior pituitary to release growth hormones to most body tissues, increase protein synthesis and increase blood glucose Targets the anterior pituitary to inhibit milk production at the mammary gland Targets anterior pituitary to release milk production at the mammary gland Targets skin pigment cells (melanocytes) to regulate pigmentation
The Pituitary gland (Anterior lobe) Human growth hormone (HGH) Thyroid Stimulating Hormone (TSH) Adrenocorticotrophin (ACTH) Follicle-stimulating hormone (FSH) Prolactin (PRL) Luteinising hormone (LH) Melanocyte-stimulating hormone (MSH) Regulates height and growth Stimulates normal development and secretion of the thyroid Stimulates the adrenal cortex and releases corticosteroids Stimulates ovaries to produce oestrogens and to ovulate Stimulates milk production Stimulates production of oestrogens, progesterone and testosterone Stimulates release of melanin (Posterior lobe) Oxytocin (OT) Vasopressin - anti-diuretic (ADH) Stimulates uterine contraction, triggers milk production Reduces urine production and raises blood pressure The Thyroid gland Thyroxine Calcitonin (CT) Stimulates tissue metabolism, growth and development Maintenance of calcium and phosphorous balance The Parathyroid glands Parathormone (PTH) Stimulates calcium reabsorption in the kidneys, activates Vitamin D
The Pineal gland Melatonin Controls body rhythms The Adrenal glands Mineral Corticoids (mainly Aldosterone) Glucocorticoids (mainly Cortisol) Sex hormones (Androgens & Oestrogens) Adrenaline & Noradrenaline Regulates sodium/potassium balance in the body Regulates metabolism Plays a role in immunity and inflammation, helps to resist long-term stress. Stimulates sexual development & maturity, ovulation, pubic hair Prepares the body for fight or flight response. The Thymus Thymosin Helps fight viruses/infections The Pancreas Insulin Glucagon Somatostatin Helps glucose enter the cells, thus reducing the level of glucose in the blood. Raises glucose levels in the blood by releasing glucose from the liver Inhibits insulin and glucagon release, slows absorption of nutrients from the gastrointestinal tract The Ovaries Oestrogens Progesterone Responsible for female characteristics e.g. breast growth, widening of hips, pubic hair. Together with Progesterone, they regulate the female reproductive cycle. Helps maintain
pregnancy. Together with Oestrogen it regulates the female reproductive cycle. The Testes Testosterone Responsible for male characteristics e.g. changes at puberty - voice breaking, pubic & facial hair, sperm production, stimulates sex drive.