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Terminology & definitions Hormones: organic chemical messengers secreted directly into the blood by an endocrine gland. Hormones cause target organs to perform a specific function. Hyperthyroidism: Increased levels of thyroxin in the blood due to over-activity of the thyroid. Hypothyroidism: Decreased levels of thyroxin in the blood due to under-activity of the thyroid. Homeostasis: is the maintenance of a relatively balanced and constant internal environment by means of the negative feedback mechanism. Negative feedback mechanism: when there is an increase from normal, a corrective mechanism causes a decrease and vice versa to maintain a balanced system. Thermoregulation: a regulatory process to keep a constant body temperature in homoeothermic animals like birds and mammals. Hypothermia: when the body becomes so cold that the internal mechanisms cannot keep the internal core body temperature constant. Hyperthermia: when the body becomes too hot and the internal mechanisms are unable to cool the internal core body temperature down sufficiently. Key Concepts / Diagrams The Endocrine system works in conjunction with the Nervous system. The endocrine system is responsible for chemical coordination, regulating the functioning of all the organs in the body. The endocrine glands produce organic chemical messengers called hormones. All endocrine glands are ductless so hormones are secreted directly into the blood. Each gland has a rich supply of blood to transport the hormones to the target organs. Hormones generally consist of proteins and fats, but some (like the sex hormones) consist of fats only. Hormones control the activities of a target organ, but do not themselves perform the activity. Hormones work together as an integrated system where they either stimulate or inhibit organs. Processes are regulated by chemical co-ordination to ensure normal growth, development and functioning of all the systems in a coordinated manner. This coordination ensures the control of: tissue fluid nutrients blood pressure body temperature blood glucose levels osmoregulation (water) oxygen and carbon dioxide levels salt concentrations (electrolytes) 66
Endocrine glands in the body and the hormones that they secrete: Exocrine glands Exocrine glands are glands that secrete substances into ducts that lead into cavities in the body or lead directly to the external environment. Examples are sweat glands, mammary glands, the liver, salivary glands and the pancreas. Endocrine gland table (Learn Well): Hormone Gland Location Function Effect of under- and oversecretion Growth hormone (somatotrophin) Anterior lobe of the pituitary gland Base of the brain and attached to the hypothalamus For growth, repair and replacement of cells Oversecretion: Children giantism Adults acromegaly Undersecretion: Children pituitary dwarf Adults premature senility Follicle stimulating hormone (FSH) Anterior lobe of the pituitary gland Base of the brain and attached to the hypothalamus In males: stimulates spermatogenesis In females: stimulates the development of the follicle for process of ovulation No over- or undersecretion effects Luteinising hormone (LH) Anterior lobe of the pituitary Base of the brain and In males: stimulates the No over- or undersecretion effects 67
gland attached to the hypothalamus synthesis of the hormone testosterone by the Leydig cells in the testes In females: LH stimulates the release of the secondary oocyte from the Graafian follicle and then the development into the corpus luteum Thyroidstimulating hormone (TSH) Anterior lobe of the pituitary gland Base of the brain and attached to the hypothalamus Stimulates the production of thyroxin by the thyroid gland Oversecretion: goitre Undersecretion: lack of production of thyroxin thyroid gland is understimulated Antidiuretic hormone (ADH) Neurosecretory cells of the hypothalamus (ADH is stored in the posterior lobe of the pituitary gland) In the centre of the brain Regulates osmoregulation in the kidneys (in the distil convoluted tubules and the collecting tubules) Oversecretion: water retention and swelling (oedema) Undersecretion: dehydration Thyroxin Thyroid gland Below the larynx in the neck region Regulates the basal metabolic rate of the cells in the body Affects growth and functioning of the heart and the nervous system Stimulates growth and differentiation of tissue in a foetus and in children Regulates the body temperature when stimulated by the Oversecretion: goitre Undersecretion: Children cretinism Adults myxodema 68
hypothalamus Aldosterone Cortex of the adrenal gland Above the kidney Helps the uptake of sodium ions in the loop of Henle in the kidneys Oversecretion: oedema (water retention) Undersecretion: Addison s disease Adrenalin (fight-and-flight hormone) Medulla of the adrenal gland Above the kidney Prepares the body to deal with stress: Increase in heartbeat rate Increase in breathing rate Increase in blood pressure Increase in muscle tone Increase in blood sugar levels Decrease in blood supply to the skin and digestive system Causes pupils to dilate There are no known over- or undersecretion effects. The sympathetic and parasympathetic nervous system controls the release of adrenalin Glucagon Islets of Langerhans alpha cells Endocrine cells of the pancreas Controls the increase in the blood sugar level by causing the conversion of glycogen to glucose No over- or undersecretion effects Insulin Islets of Langerhans beta cells Endocrine cells of the pancreas Controls blood sugar by causing the conversion of glucose into glycogen Inhibits the functioning of glucagon Oversecretion: obesity Undersecretion: diabetes Testosterone Gonads: testes Leydig cells in the testes of males located in the scrotum at the bottom Testosterone is responsible for the secondary sexual characteristics Oversecretion: aggression. In females, it results in secondary sexual characteristics similar to that in males 69
of the pelvis in males like a deeper voice, pubic hair, hair on face Necessary for the normal development of sperm Activates genes in the cells of Sertoli to promote the differentiation of the spermatogonia Undersecretion: lack of secondary sexual characteristics and lack of healthy sperm production Oestrogen Gonads: ovaries Located in the lower abdominal region with each ovary located within the pelvic bones (in line with the balland-socket joints of the femurs) Oestrogen promotes the development of the secondary sexual characteristics in females like breasts, the thickening of the endometrium (uterus) and the female body shape Necessary for the process of ovulation Oestrogen inhibits the secretion of FSH by the anterior pituitary gland so that only one follicle is produced during ovulation High oestrogen levels will trigger the secretion of luteinising hormone (LH) Oversecretion: may cause cancer Undersecretion: menstruation cycle is affected, ovulation may be prevented leading to infertility, onset of menopause may occur Progesterone Gonads: ovaries Located in the lower abdominal Progesterone prepares the Undersecretion: during pregnancy, will cause a spontaneous 70
region with each ovary located within the pelvic bones (in line with the balland-socket joints of the femurs) endometrium of the uterus for implantation once fertilisation of the egg cell has occurred Necessary for the production of the mucus plug to prevent sperm or other substances from entering the uterus during pregnancy Decrease in progesterone levels causes menstruation Progesterone improves memory and cognitive ability miscarriage Prolactin Anterior lobe of the pituitary gland Base of the brain and attached to the hypothalamus Stimulates the mammary glands to produce milk Counteracts the effect of dopamine which is responsible for sexual arousal Oversecretion: can cause impotence and loss of libido Homeostasis Homeostasis is the maintenance of a relatively constant internal environment by automatic control mechanisms. Cells will function normally, regardless of the external environment. The nervous system controls all the systems in the body either directly or indirectly. Homeostasis of the endocrine system occurs through the negative feedback mechanism. Feedback systems are important because they ensure that all the systems are interrelated and work together. When there is an increase from normal, a corrective mechanism will cause it to decrease and vice versa. This ensures that a balance is maintained within the body. 71
Control of Thyroxin: 72
Blood glucose control: The blood glucose level is controlled by the Islets of Langerhans in the pancreas. Diabetics take insulin by injection. It is absorbed into the bloodstream at a much faster rate. If taken orally, there is a chance that digestive enzymes will denature insulin because it is a hormone and therefore consists of proteins. 73
Differences between the Endocrine System and the Nervous System Endocrine system Nervous system 1. Made up of glands 2. Produces hormones 3. Hormones transported by the blood 4. Effects are slower and more general 5. Hormones control long-term changes (e.g. growth) Thermoregulation 1. Made up of nerves 2. Produces nerve impulses 3. Impulses transmitted along the nerves 4. Effects are very quick and very specific 5. Nerve impulses control short-term changes (e.g. sneezing, lifting your arm) Body temperature must be kept constant in homoeothermic organisms like birds and mammals. Optimal body temperature in humans is between 36,6 ºC and 37 ºC. If the temperature is too high, the proteins will denature and be unable to function. If the temperature is too low, the proteins will become inactive the body temperature must be kept constant. The hypothalamus stimulates various systems to regulate the body temperature. When the body is hot, the hypothalamus will stimulate the pituitary to release more ADH and less thyroxin. When the body is cold, the opposite will take place. Hot day The body gets hot: You need more water in the blood to make more sweat to cool you down More ADH is released by the pituitary gland to keep more water in the blood Blood vessels near the surface of the skin dilate (vasodilation), so more blood reaches the sweat glands More sweat evaporates, cooling the skin and the blood in the vessels near the surface of the skin This means that you will produce more concentrated urine (less water in the urine) The hairs in the skin lie flat so no heat is trapped between the skin and the hairs The pituitary gland inhibits the thyroid so less thyroxin is released causing a decrease in cell metabolism, so less energy is released body cools down Cold day The body gets cold: You need less water in the blood as you do not need to produce sweat Less ADH is released by the pituitary gland to keep less water in the blood Blood vessels near the surface of the skin constrict (vasoconstriction), so less blood reaches the sweat glands No sweat evaporates, so there is no cooling of the skin and the blood in the surface vessels This means that you will produce less concentrated urine (dilute urine) The hairs in the skin are pulled upright by the erector muscles to trap warm air (goose bumps) The pituitary gland stimulates the thyroid so more thyroxin is released, causing an increase in cell metabolism. More energy is released and causes shivering body warms up Hypothermia (HypO Ohhh, it is Cooooold): occurs when the body is exposed to cold and the internal mechanisms cannot keep the internal core body temperature constant. Preventative measures: keep the body warm, keep moving, exercise, dress warmly, add more clothes and blankets, drink hot drinks, warm the body near a fire or heater, stay in a warm, protected environment etc. 74
Hyperthermia: occurs when the body becomes too hot and is unable to cool the internal core temperature down sufficiently. This results in dehydration, heat stroke and heat exhaustion. Preventative measures: the body can be cooled by changing the external temperature, for example cold compressions on the neck and forehead, swimming, taking a cold shower or bath, taking extra layers of clothes off, staying in the shade, wearing a hat, moving into an air conditioned room etc. Thermoregulation animal adaptations Various species of animals have behavioural, physical and physiological adaptations to deal with thermoregulation: Panting dogs. Insulation: fat layers under the skin e.g. blubber of marine mammals like whales and seals; trapped air under the fur of their bodies e.g. rabbits and cats; thicker winter coat of fur e.g. bears and squirrels and birds ruffle their feathers and trap the air between the feathers and skin. Body size: the metabolic rate of a small animal is faster than that of a larger animal. The smaller the animal, the larger the body surface in relation to mass - smaller animals lose heat to a cold environment more rapidly than the larger animal. Larger animals can retain more heat. Heat exchange mechanism: counter-current exchange mechanism involves two blood vessels that are in close contact with blood flowing in opposite directions, for example an artery and a vein. The body temperature may rise or fall, but the temperature of the thoracic cavity and brain will stay constant. Body orientation: reptiles orientate their bodies to the sun, depending on whether they need to warm up or cool down. Water conservation: kangaroo rat and camels. Behaviour patterns o Hibernation: e.g. squirrels, bats, bears, rodents, hedgehogs, marsupials and rattlesnakes. o Aestivation: e.g. ladybeetles, some species of snails, land crabs, desert tortoises, crocodiles, salamanders, some amphibians and lungfish. o Burrowing o Nocturnal behaviour: animals come out at night e.g. bats, bush-babies and genets. 75
X-ample Questions QUESTION 1: (Taken from Study and Master Grade 12, 2003) Study the diagram and answer the questions that follow: 1.1. Identify the organs marked A and B. (2) 1.2. Identify the hormones numbered 1 and 2. (2) 1.3. Name the exact structures that secrete hormones numbered 1 and 2 respectively. (2) 1.4. How do the hormones mentioned in question 1.2 reach the target organ A? (1) 1.5. Which organ (A or B) monitors the blood sugar levels? (1) 1.6. Name the abnormality that may develop from an under-secretion of the hormone numbered 1. (1) 1.7. What effect does the secretion of adrenalin have on the normal blood sugar level? (1) 76
QUESTION 2: (Taken from Study and Master Grade 12, 2003) Study the diagram and answer the questions that follow: 2.1. Identify the endocrine glands marked A and B. (2) 2.2. Which interaction is depicted by this diagram? (1) 2.3. Name the hormones numbered 1 and 2. (2) 2.4. Name the specific function performed by hormone number 2 as shown at point X in this interaction. (1) 2.5. What effect will: a) an over-secretion, and b) an under-secretion of hormone number 2 have on the production of hormone number 1? (2) 2.6. Name the hormone secreted by endocrine A necessary for normal growth and development. (1) 2.7. List FOUR specific characteristics of hormones. (4) 77
QUESTION 3: (Taken from BIOS BEST 2007 HG) Animal Responses to the Environment: Study the diagram of the human skin and answer the questions that follow: 3.1. Provide labels for structures A, B, D and E. (4) 3.2. State ONE function of C and D respectively. (4) 3.3. Describe the role played by the skin in regulating blood temperature on a cold day. (9) X-ercise Complete the following table. Gland Hormone Function Adrenal Medulla 1 Prepares the body for action in an emergency situation Pituitary gland 2 Stimulates the production of thyroxin (Anterior lobe) Thyroid Thyroxin 3 4 Somatotrophin Regulates growth and development of muscle tissue Beta cells of the 5 Causes the decrease of blood sugar levels Islets of Langerhans Neurosecretory 6 Regulates water levels in the blood cells of the Hypothalamus Alpha cells of the 7 8 Islets of Langerhans 9 10 Necessary for the normal development of sperm cells 78
Answers to X-ercise: 1. Adrenalin 2. TSH 3. Regulates the basal metabolic rate 4. Pituitary gland (anterior lobe) 5. Insulin 6. ADH 7. Glucagon 8. Causes an increase in blood sugar levels 9. Testes / Male gonads 10. Testosterone 79