Endocrine System: Human Endocrine System and Homeostasis - PDF

CHAPTER 7: THE HUMAN ENDOCRINE SYSTEM AND HOMEOSTASIS Introduction Various human mechanisms enable us to respond and react to the outside environment so as to maintain a constant internal environment. Our responses are controlled by the nervous and endocrine systems and to an extent, the immune sy...

CHAPTER 7: THE HUMAN ENDOCRINE SYSTEM AND HOMEOSTASIS Introduction Various human mechanisms enable us to respond and react to the outside environment so as to maintain a constant internal environment. Our responses are controlled by the nervous and endocrine systems and to an extent, the immune system. The working together of these systems helps to maintain stability within the organism and so protects us. The human nervous system responds via:  a rapid response to stimuli  the use of electrical impulses and neurotransmitters The endocrine response is controlled by the endocrine glands situated throughout the body. This response is:  a slower response that has a long-lived effect  as the endocrine glands produce and release specific hormones into the bloodstream.  An effector organ is targeted, and  a response is initiated.  When endocrine organs are either over- or under stimulated, endocrine disorders are observed. Key terminology a system responsible for chemical co-ordination and endocrine system regulation of various activities in the body a process of maintaining a constant internal environment homeostasis (blood and tissue fluid) within the body. chemical messengers in the body. they travel in the hormones bloodstream and cause an effect elsewhere in the body operate in the human body to detect changes or negative feedback imbalances in the internal environment and to restore balance 199  osmoregulation regulation of the water balance in the internal environment a measure of the concentration of solutes (e.g. salt, osmotic pressure glucose) present in a solution; this may determine whether a cell loses or gains water to work in opposite ways; if one hormone causes an antagonistically increase of a substance, the other hormone will cause a decrease of that substance, e.g. insulin and glucagon the control of the body temperature to keep it as close to thermoregulation 37°C as possible relates to an organism that generates heat internally endothermic through a metabolic process to maintain a constant body temperature vasoconstriction narrowing of blood vessels vasodilation widening of blood vessels heat loss when sweat changes into water vapour on the evaporation surface of the skin conduction transfer of heat between objects which are in contact convection as warm air rises it is replaced by cooler air radiation heat transfer between two objects which are not in contact Homeostasis is the tendency of an organism or cell to regulate its internal conditions, usually by a system of feedback controls, so as to stabilize health and functioning, regardless of the outside changing conditions. The difference between the important secretory glands Mammals produce secretions from exocrine and endocrine glands. The main differences between the two are listed in Table 1 below. Examples of each are given together with their secretions. Table 1: Exocrine and endocrine glands – the main differences Exocrine glands Endocrine glands  have ducts  ductless  secretions released into a cavity or  hormones released into the on a surface bloodstream  Examples: salivary glands (saliva),  Examples: pituitary (ADH), thyroid sweat glands(sweat) (TSH), pancreas (insulin) Note: The pancreas is both an endocrine and an exocrine gland. As an exocrine 200 gland it secretes digestive enzymes into the small intestine. As an endocrine gland it secretes insulin and glucagon into the bloodstream. Figures 1 and 2 give examples of exocrine and endocrine glands with their secretions. chemical secretions blood in capillaries skin surface hormones secreted chemicals into blood produced by gland Figure 1: Exocrine gland Figure 2: Endocrine gland Endocrine glands The endocrine system has many important endocrine glands that secrete hormones into the bloodstream where they are transported to their target site (such as an organ). These hormones do not work in isolation and often interact with other hormones in a sequence of events. They can work to produce a common effect, or they can work antagonistically. Hormones: Hormones are organic compounds that act as messengers in the body. Most hormones are proteins with some being steroids (lipids). They are only needed in small amounts and they give a more lasting response than a nerve response. Hormones may be over-secreted or under secreted resulting in certain disorders. The basic functions of hormones are:  reproduction, growth and development  maintenance of the internal environment (by stimulating or inhibiting the functioning of cells / organ)  regulation of metabolism by controlling production, usage and storage of energy 201 Figure 3 shows the location of the human endocrine glands and the hormones produced there. The function of the hormones is discussed later. hypothalamus pituitary gland ADH – antidiuretic GH – growth hormone hormone TSH – thyroid stimulating hormone FSH, LH and prolactin thyroid gland thyroxin adrenal gland adrenalin pancreas aldosterone glucagon insulin ovary (female only) oestrogen testes (only males) progesterone testosterone Figure 3: Endocrine glands and the hormones they secrete Introduction to the endocrine system: https://www.youtube.com/watch?v=SHgNaomqlRA The hypothalamus and pituitary gland The hypothalamus is a small area of the human brain, located above and connected to the pituitary gland (see Figure 4). It produces and secretes important hormones and is a link between the nervous and the endocrine systems. The hypothalamus plays an important role in controlling vital functions in the human body. In this text the focus is on the anti-diuretic hormone (ADH), produced by the hypothalamus. 202  cerebrum hypothalamus pituitary gland cerebellum brain stem detail hypothalamus pituitary gland posterior lobe anterior lobe Figure 4: Location of hypothalamus and pituitary gland in the human brain, with detail of link between the two glands. Table 2 below lists the hormones released by the hypothalamus and the pituitary gland. The function and target organ/s or gland/s are also given. Table 2: Endocrine glands, the hormones secreted and their function and target Hormones Target organ / Functions of hormone secreted gland hypothalamus  ADH  kidneys  stimulates the re-absorption of H2O (antidiuretic into the tubules hormone), also  protects the body against called dehydration vasopressin pituitary gland (anterior lobe)  thyroid gland  TSH (thyroid  stimulates growth of thyroid gland stimulating  stimulates thyroid gland to secrete hormone) the hormone thyroxin 203   ovaries  stimulates development of follicles  stimulates the ovaries to produce the hormone oestrogen  stimulates the development of ova  FSH (follicle (eggs) in the female stimulating  testes hormone)  stimulates the testes to produce spermatozoa  stimulates ova maturation  ovaries  stimulates ovulation (release of the  LH (luteinising ovum) hormone)  stimulates the production of the  testes male hormone testosterone  mammary glands  stimulates milk production and  prolactin secretion  Bone and muscle cells  GH (growth  Stimulates the growth of long bones hormone) and skeletal muscles Other important endocrine glands that make up the endocrine system are:  the adrenal glands,  the ovary in females and the testes in males,  the pancreas, and  the thyroid gland. All the endocrine glands are essential for ensuring healthy metabolism. 204 Adrenal glands These two triangular shaped glands are situated on top of each of the kidneys (see Figure 5). The adrenal glands have a central medulla region and an outer cortex. The two hormones produced and secreted from the adrenals, that we will discuss, are adrenalin and aldosterone (see Table 3). adrenal gland capsule medulla cortex blood vessels kidney Figure 5: Location and structure of one of the adrenal glands Table 3: Adrenal hormones, target organs / areas and their functions Target areas / organs Functions adrenalin (the ‘fight or flight’ hormone)  heart  increased heart rate and blood supply to cardiac muscles  liver  stimulates the liver to increase conversion of glycogen into glucose  skeletal muscles  increased blood supply to skeletal muscle  eye muscles  stimulates pupil dilation  skin  decreased blood supply to ‘less vital’ organs (digestive system and the skin)  lungs  increased breathing rate  body cells  increased metabolic rate aldosterone  kidneys  regulates the salt (sodium / Na+ and potassium / K+) concentration in the blood; works together with ADH to achieve this 205 The reproductive glands The hormones released by the human reproductive glands (see Figures 6A & 6B) are important in reproduction and for stimulating the secondary characteristics at onset of puberty. In females the ovaries are stimulated by FSH from the pituitary gland. The ovaries themselves release oestrogen and progesterone which play different roles in the menstrual cycle. In males the cells of Leydig in the testes secrete the male hormone testosterone which stimulates sperm production and maturation. fallopian tube testes ovary endometrium Figure 6A: Ovary Figure 6B: Testes Table 4: Hormones released by the human reproductive glands and their functions Ovaries Testes Oestrogen Testosterone  promotes the thickening of the  stimulates the development of the endometrial wall male sex organs and the secondary  promotes the development of the characteristics during puberty female secondary sexual  promotes the maturation of the characteristics at puberty sperm  promotes fertility Progesterone  promotes further thickening and vascularisation of the endometrial wall  maintains implantation of embryo during pregnancy 206 Activity 1: Endocrine glands and their hormones The diagram shows some of the human endocrine glands. Name the glands labelled A to G, and list the hormone/s produced by each gland. (20) A B C D E F G Activity 2: Effects of a hormone heart heart beats faster gut arteries to gut hormone A become narrow arteries to muscles widen muscle Effect of a certain hormone on the heart and blood supply to certain body parts 207  1. Give the name of hormone A. (1) 2. State the position of the gland that secretes hormone A in the human body.(1) 3. Explain the importance of the narrowing of the arteries to the gut under emergency conditions. (4) 4. Name the part of the human eye that is also affected by hormone A. (1) 5. Explain the influence of hormone A on the part named in question 4. (3) (10) Negative Feedback A negative feedback mechanism is an interaction between two hormones in which one hormone stimulates an increase in another hormone which then inhibits the first hormone, thus restoring balance. The following is the general sequence of events in a negative feedback mechanism:  an imbalance is detected by the receptor  a control centre is stimulated  the control centre responds  a message sent to target organ/s which are the effectors  the effector responds  it opposes (reverses) the imbalance  balance is restored Figure 7: Homeostasis mechanism 208 As illustrated in Figure 7, an imbalance triggers a stimulus to the receptors at 2. When stimulated, the receptors generate an impulse that is transmitted to the control centre at 3. The control centre responds by sending an impulse to the effector which is the target organ at 4. The target organ then reverses the imbalance and balance is restored at 5. Homeostatic control of the internal environment The tissue fluid which surrounds our cells constitute the internal environment. The conditions within cells depend on the conditions within the internal environment. When faced with changes from either the external or internal environment the human body controls this effect and homeostasis is achieved. Without homeostasis, organs, systems and ultimately, the whole organism, may be negatively affected. The important variables and homeostatic mechanisms that will be discussed are the:  maintenance of water levels  maintenance of glucose levels  maintenance of salt levels  regulation of the CO2 concentrations  regulation of thyroxin levels  regulation of body temperature It is important to understand the reasons why certain factors must be controlled and how this is carried out. The main mechanisms for maintaining homeostasis Table 5 below is a summary of six important homeostatic controls humans possess to ensure stability within their internal environment. Table 5: Homeostatic control of internal environment Stimulus Reason for importance of regulation Effector/s All metabolic reactions require a balance of water Kidneys Water and salt concentrations in the blood and surrounding and the skin tissue fluid. The presence of dissolved salts in the blood and tissue fluid determines the osmotic pressure which Salts Kidneys may contribute to the cell losing or gaining water by osmosis affecting the balance of these fluids 209  The presence of CO2 produced during cellular respiration affects the pH of the blood and tissue CO2 Lungs fluid. Enzymes are extremely sensitive to pH variations. The concentration of glucose in the body needs to Liver and Glucose be controlled to ensure that energy levels can be pancreas maintained, and metabolic functioning can continue. Cellular metabolism is regulated by the hormone Thyroxin thyroxin. Slight variations in the amount of thyroxin Thyroid (T4) can have a severe effect on the metabolic rate of an gland individual. An increase or decrease from normal body temperature can affect metabolism due to the effect Temperature on enzyme activity. High temperatures denature Skin enzymes whereas low temperatures slow down enzyme activity. Negative feedback in the control of water balance – OSMOREGULATION The maintenance of the water balance is very important to ensure that body metabolism continues. The homeostatic control of water and salt levels in blood and tissue fluid is carried out in a negative feedback system known as osmoregulation. pituitary gland secretes more ADH permeability of collecting ducts and distal convoluted tubules increases A water level decreases more water reabsorbed into blood normal water levels in blood water level increases less water reabsorbed into blood B more water is lost Pituitary gland stops secreting ADH permeability of collecting ducts and / secretes less ADH distal convoluted tubules decreases Figure 8: Negative feedback control of water levels in blood and tissue fluid 210 Figure 8 above and Table 6 below show how the body regulates: A Levels of water when there is a drop below normal (dehydration), and B Levels of water when there is an increase above normal (overhydration) Table 6: Osmoregulation A – decreased water level B – increased water level Dehydration - when the water levels in Overhydration is when water levels in the blood and tissue fluids are low the blood and tissue fluids are high. May be due to excessive exercise, hot May be due to cooler temperatures, temperatures, increased sweating or little exercise with no sweating and an decreased water intake. excessive intake of water. Low levels of water in the blood and Water levels in the blood and tissue tissue fluid is detected by receptor cells fluid are high, and this is detected by (osmoreceptors), in the hypo- the osmoreceptors in the hypo- thalamus of the brain. thalamus. Impulses are sent to the pituitary Impulses are sent to the pituitary gland and antidiuretic hormone gland and less anti-diuretic hormone (ADH) is released. (ADH) is released. The hormone is transported in the blood to the effector organ, the kidney. The Collecting ducts and distal permeability of the collecting duct convoluted tubules in the kidney and the distal convoluted tubule is become less permeable. increased. More water is reabsorbed and passed Less water is reabsorbed into the into the blood. blood. The blood becomes more dilute and More water is lost, more dilute urine is concentrated urine is excreted. excreted. Water levels in the blood and tissue fluid return to normal and homeostasis is achieved. Negative feedback in the regulation of salt concentration The osmotic pressure in the blood and tissue fluids is affected by the presence of solutes. Glucose and salts make up the solutes. Sodium ions (Na+) and potassium ions (K+) are salts that are regulated in a negative feedback system. Figure 9 and Table 7 show how the body regulates the: 211  A levels of salt concentration in the blood when they drop below normal, and B levels of salt concentration in the blood when they rise above normal. adrenal gland secretes more reabsorption of sodium ions aldosterone increases A salt level in the blood increases salt level in the blood decreases back to normal normal salt levels in blood salt level in the blood increases salt level in the blood decreases B back to normal Adrenal gland stops secreting aldo- reabsorption of sodium ions into sterone / secretes less aldosterone the blood decreases Figure 9: Negative feedback control of salt concentration in the blood Table 7: Homeostatic control of salt concentrations A – low salt level B – high salt level Low salt levels in blood and tissue When the salt levels in the blood and fluids. tissue fluids are increased. Receptor cells in the kidney detect Receptor cells in the kidney will detect decreased sodium ion levels. an increased presence of sodium ions. The adrenal gland in the kidney The adrenal gland stops releasing secretes the hormone aldosterone. aldosterone. Aldosterone stimulates the reabsorption of sodium ions from the Sodium ions will not be reabsorbed. filtrate and back into the blood. Less sodium is excreted in the urine. More sodium is excreted in the urine. The salt concentration of the blood and tissue fluid returns to normal and homeostasis is maintained. The regulation of water and salt levels in humans: https://www.youtube.com/watch?v=BugGCBAk_Os 212 Negative feedback regulating of carbon dioxide concentrations Carbon dioxide levels in the blood affect the pH of the blood and this can have an effect on metabolic processes. CO2 is one of the end products of cellular respiration. CO2 dissolves in water forming carbonic acid. The more carbon dioxide there is in the blood, the more acidic the blood becomes. Changes in pH influence enzyme activity. Figure 10 and Table 8 show the bodies response to an increase in CO2 levels in the blood. 1. Stimulus increased blood CO2 levels decreased pH, decrease in O2 6. Negative feedback blood CO2 levels fall, pH of blood returns to normal 2. Receptors low pH detected by chemoreceptors 5. Response heart rate increases more CO2 removed from body 3. Control centre impulses sent to medulla oblongata 4. Effector respiratory muscles – diaphragm and intercostals: increase the rate and depth of breathing; heart muscles – increase heart rate Figure 10: Negative feedback regulation of carbon dioxide levels in the blood Table 8: Homeostatic control of blood CO2 levels High concentrations of CO2 lead to the formation of carbonic acid. As a 1 result, the pH of the blood will drop (the blood becomes more acidic). 2 Chemoreceptors in the carotid artery are stimulated by the drop in pH. 3 Impulses are sent to the medulla oblongata. Breathing and heart muscles are targeted. 4 Diaphragm and intercostal muscles contract increasing the rate and depth of breathing. 5 Heart rate increases. More CO2 moves to the lungs to be exhaled therefore blood CO2 levels 6 return to normal. Homeostasis is maintained. 213 Negative feedback mechanism regulating blood glucose levels Carbohydrates are very important short-term energy storage molecules in all living organisms. In cellular respiration glucose is broken down to release energy which is stored in ATP. The endocrine gland that secretes hormones that regulate the blood glucose levels is the pancreas. Pancreatic hormones, insulin and glucagon, work antagonistically (in opposite ways) to maintain blood glucose levels. endocrine: insulin and glucagon blood vessel secreted into bloodstream pancreatic duct exocrine: digestive enzymes secreted into pancreatic duct duodenum Figure 11: The pancreas showing its function as both an endocrine and exocrine gland Blood glucose levels need to be maintained. Table 9 and Figure 12 show how: A high blood glucose levels are regulated, and B low blood glucose levels are regulated by the liver and pancreas. Table 9: Regulation of blood glucose levels A – high blood glucose B – low blood glucose Increased blood glucose levels are Decreased blood glucose levels are detected by the Islets of Langerhans in detected by the Islets of Langerhans in the pancreas. the pancreas. The Islets of Langerhans respond by Glucagon is secreted by the Islets of secreting insulin into the bloodstream. Langerhans into the blood-stream. 214  Insulin is transported to the liver which Glucagon is transported to the effector is the effector organ. organ, the liver. Enzymes in the liver catalyse the Glycogen is broken down into free conversion of excess glucose into glucose. Glucose is released into the glycogen. Glycogen is a storage bloodstream. carbohydrate. Glucose levels in the blood return to Glucose levels are increased to normal. normal levels. In people with normal pancreatic functioning glucose levels are maintained and homeostasis is achieved. A pancreas insulin stimulates glucose uptake by cells liver glucose high blood glycogen glucose level (e.g. after eating) normal blood glucose level B glucose blood blood glucose vessel level drops glucose liver glycogen pancreas glucagon Figure 12: Regulation of blood glucose by the liver and pancreas 215 Negative feedback in the regulation of thyroxin levels The basal metabolic rate (BMR) is the rate at which the body’s cells use energy when at rest. Basic vital functions need to be maintained. The hormone, thyroxin, produced in the thyroid gland:  stimulates the body to increase the metabolic rate when required  plays a vital role in the functioning of the heart and digestive system  is important in skeletal and brain development  is involved in maintenance of muscle tone Iodine is an essential element needed in the production of thyroxin. A good source of iodine in our daily diet is sea salt or iodised table salt. hypothalamus pituitary gland The hypothalamus and the pituitary in the brain control normal secretion of thyroid hormones, which in turn thyroid control metabolism. gland Figure 13: Location of thyroid gland in the neck and the connection to the pituitary gland Two glands are involved in the control of thyroxin levels:  pituitary gland – which releases thyroid stimulating hormone (TSH)  thyroid gland – which releases thyroxin Homeostatic regulation of thyroxin levels is illustrated in Figure 14 and detailed in Table 10. 216  A – low thyroxin levels B – high thyroxin levels normal pituitary gland thyroid gland thyroxin levels releases less releases less TSH thyroxin thyroxin level thyroxin level increases decreases thyroxin level thyroxin level increases decreases thyroid gland pituitary gland releases more releases more normal thyroxin TSH thyroxin levels Figure 14: Flow diagram showing regulation of thyroxin levels (A – low levels, B – high levels) Table 10: Homeostatic control of thyroxin A – low thyroxin level B – high thyroxin level When levels of thyroxin that fall below When levels of thyroxin increase above normal, this is detected by the pituitary normal, this is detected by the pituitary gland. gland which is then inhibited. This causes the pituitary gland to Less TSH is secreted from the pituitary secrete more TSH. gland. TSH is transported via the bloodstream Lower secretions of TSH result in the to the thyroid gland which stimulates thyroid gland releasing less thyroxin. increased secretion of thyroxin. The level of thyroxin is increased back The level of thyroxin is decreased back to normal. to normal. If we apply this to the negative feedback mechanism that restores thyroxin levels when it is too low, the appropriate steps can be made specific as follows: Step 1: The thyroxin level in the blood decreases Step 2: The pituitary is stimulated Step 3: Pituitary gland increases its secretion of TSH Step 4: TSH is transported by the blood to the thyroid gland Step 5: TSH stimulates the thyroid gland to increase its secretion of thyroxin Step 6: The thyroxin level in the blood increases Step 7: Thyroxin levels return to normal 217 Activity 3: A negative feedback mechanism The diagram below represents the interaction between two important endocrine glands. The hypothalamus is found at the base of the brain while the gland labelled B is present towards the front of the neck. hypothalamus A B C 1. Provide a label for gland B. (1) 2. Name hormone C. (1) 3. State one function of hormone A. (1) 4. Describe the negative feedback mechanism that operates when the level of hormone C is higher than normal in the blood. (5) (8) Endocrine system disorders In certain situations, hormone secretions are disrupted, and this affects homeostasis. Endocrine glands can either secrete too little (hyposecretion) or too much (hypersecretion) hormone. If this continues, a person would be diagnosed with an endocrine disorder. 218 Pituitary gland disorders Table 11: Pituitary gland disorders Acromegaly Dwarfism Gigantism (Figure 15) hypersecretion of GH hyposecretion of GH hypersecretion of GH Cause too much GH secreted too little GH produced too much GH secreted after puberty during childhood during childhood  enlargement of the  well-proportioned but  long bones and hands and feet of short stature connective tissue  additionally:  retarded growth and grow very fast Symptoms enlargement of the delayed puberty.  person may grow up forehead, jaw and to 2,1 to 2,5 m tall nose  heart and other organs also enlarge, causing high blood pressure Figure 15: Gigantism Thyroid gland disorders The continued production of too much thyroxin is known as hyperthyroidism (hyper = high). Continued low levels of thyroxin leads to hypothyroidism (hypo=low). 219 Table 12: Thyroid gland disorders Hyperthyroidism Graves’ disease (Figure 16A) Goitre (Figure 16B) Examples autoimmune disease – which Causes occurs when the immune system goitre – condition linked to elevated attacks the thyroid and causes it to thyroid activity overproduce the hormone thyroxin  increased metabolic rate Symptoms  bulging eyes  increased cardio-vascular activity  weight loss  increased anxiety  fast metabolism  swollen thyroid gland in neck hypothyroidism Cretinism (Figure 16C) Myxoedema (Figure 16D) Examples  caused by lack of thyroxin from  caused by underactive thyroid Cause birth gland in adulthood  physical, mental retardation  mental, physical tiredness Symptom  low metabolic rate  increase in dermal fat  roughening of skin Pancreas disorders – e.g.: Diabetes A person with continued (chronic) high glucose levels is said to be hyperglycaemic. Diabetes mellitus is a disease associated with high blood glucose levels. 220 The long-term effect of diabetes is damage to all the blood vessels which eventually leads to circulatory problems and multiple organ damage. Wounds on the skin do not heal well. The eyes are negatively affected, leading to poor vision and eventually blindness.  Type 1 diabetics might inject insulin as a treatment. There are sophisticated insulin pumps available which are directly attached to the patient and are effective in controlling glucose levels.  Type 2 diabetics can in most instances control their sugar levels by watching their diet, losing excess weight and exercising. Table 13: Representation of non-diabetic and both Type 1 and Type 2 diabetics Non-diabetic Type 1 Diabetic Type 2 Diabetic Normal blood Type 1 diabetes – pancreas not Type 2 diabetes – glucose levels: producing the hormone, insulin, insulin is available, but 80-100 mg/ml of necessary for controlling the the body is unable to glucose levels in humans control the glucose blood levels Table 13 and Figures 17A to 17B illustrate the normal insulin glucose interaction, and what happens in a Type 1 or a Type 2 diabetic. immune cells destroy beta cells in the pancreas pancreas cannot secrete insulin more glucose in blood cells cannot absorb and use glucose as there is no insulin Figure 17A: Type 1 diabetic – no insulin 221  pancreas pancreas secretes less insulin more glucose in blood too little insulin available to promote glucose absorption by cells Figure 17B: Type 2 diabetic – less insulin produced Activity 4: Research task on endocrine disorders You will need to research an endocrine disorder caused by the hypersecretion or hyposecretion of an endocrine hormone. You will present your information in either a PowerPoint Presentation or a poster format. The following needs to be covered: 1. What hormone is involved with this disorder? 2. Where is the hormone produced? 3. What are the target organs/structures of the hormone? 4. How is the secretion of the hormone regulated/controlled? 5. What is the normal function of the hormone? 6. How does the hormone contribute to homeostasis? 7. What are the causes of hypersecretion or hyposecretion 8. What are the symptoms and effects of hypersecretion or hyposecretion? 9. What are the treatments for under or over activation of the hormone pathway? Your teacher will provide you with a mark scheme. N.B.: Citation of at least 3 references. 222 Regulation of body temperature – THERMOREGULATION Endotherms are animals that maintain a constant body temperature independent of the environment and primarily include the birds and mammals. We as humans are endothermic. Our bodies can maintain a constant body temperature of approximately 36,8°C even when outside temperatures are very high or very low. It is vital that this happens to ensure that all metabolic functions continue. If body temperature were to drop below 36°C, metabolic processes will slow down and if it were to be raised above a safe level of 37,5°C, enzymes become denatured (cannot function effectively) and many body functions will be disabled. The way in which the body manages to control its internal core temperature when external environmental conditions change is called thermoregulation. It is one of the important homeostatic mechanisms in the human body. The human skin as thermoregulator The human skin is well adapted for thermoregulation. It is the largest organ in the body (see Figure 18 below) and has thermoreceptors which respond to either hot (Ruffini’s corpuscles) or cold (Krause’s end bulb). Heat can be lost from the body by evaporation, radiation, conduction and convection. sweat pores epidermis blood vessels dermis sweat gland receptors Figure 18: Cross section through human skin 223 The body’s thermoreceptors transmit messages to the temperature regulating centre in the hypothalamus. Both conscious (fanning in hot weather and adding clothing in cold weather) and subconscious reactions (increased sweating in heat and shivering in cooler weather – see Figure 19) occur in response to the change in environmental temperature. These reactions enable the core body temperature to remain constant. In a cold environment In a hot environment heat loss capillaries constrict capillaries dilate (vasoconstriction) (vasodilation) near near skin surface to skin surface to lose conserve heat heat to environment heat loss sweat secretion sweat glands begin stops to secrete sweat, causing heat loss by evaporation Figure 19: Homeostatic regulation of the body temperature Conscious = behavioural = voluntary subconscious = physiological = involuntary Table 14 below details the process of thermoregulation in hot and cooler conditions. Table 14: Thermoregulation Hot environment Cold environment  body temperature increases  body temperature decreases  warmed blood passes through the  slightly colder blood passes through hypothalamus the hypothalamus  impulses are sent to blood vessels in  impulses are sent to blood vessels in the skin and to sweat glands the skin and to sweat glands  vasodilation – blood vessels dilate  vasoconstriction – blood vessels and more blood flows closer to become narrow and less blood flows surface of the skin close to surface of the skin  more heat lost to outside air by  less heat is lost to outside air radiation and conduction 224   sweat glands activated and  sweat glands inactive; no sweating sweating occurs occurs  sweat evaporates – this has a  no evaporation of sweat cooling effect on the skin  surface blood capillaries lose heat to  heat is not lost as easily to the the environment and the body cools outside atmosphere and the body down warms up Extension: Extension:  The basal metabolic rate (BMR) also  The adrenal and the thyroid glands slows down and less body heat is release adrenalin and thyroxin produced. respectively. Both these hormones increase metabolic rate and more body heat is produced.  The hypothalamus stimulates the skeletal muscles to contract. Involuntary shivering occurs, and heat is produced. The human body’s ability to maintain a constant core temperature of 36,8°C using the mechanism of thermoregulation ensures homeostasis. Activity 5: Body temperature strenous cold shower exercise at 18°C body temperature (°C) Diagram I A normal body temperature B 0 10 20 30 40 50 60 70 80 minutes Diagram II 1. Which part of the brain responds to the temperature changes that occur at A and B on the graph? (1) 2. What was the maximum temperature reached? (1) 3. For what period of time did the person engage in strenous exercise? (1) 225 4. Why should body temperature not be allowed to fluctuate too much? (2) 5. Which diagram (I or II) would represent the condition of the skin after 15 minutes? (1) 6. Explain your answer to question 5. (2) (8) 226 The human endocrine system and homeostasis: End of topic exercises Section A Question 1 1.1 Various options are given as possible answers to the following questions. Choose the correct answer and write only the letter (A–D) next to the question number (1.1.1–1.1.5). For example, 1.1.6 D.. 1.1.1 The outermost layer of the human skin is the A hypodermis B epidermis C adipose D dermis 1.1.2 Which of the following CORRECTLY represents the events involved in the secretion and action of ADH (antidiuretic hormone)? Water level in Amount of ADH Amount of water blood relative produced relative to reabsorbed by to normal normal kidneys A Increase Increase Decrease B Increase Decrease Increase C Decrease Increase Increase D Decrease Decrease Decrease 1.1.3 A worker spent about ten minutes in a walk-in freezer. Below are some of the changes that occurred in his body in response to the drop in external temperature. (i) Blood vessels in the skin constrict (ii) Brain reacts (iii) Skin temperature changes (iv) Temperature receptors in the skin detect changes Which ONE is the correct sequence in which the changes occurred? A (ii) → (i) → (iii) → (iv) B (iii) → (i) → (iv) → (ii) C (iv) → (ii) → (i) → (iii) D (iv) → (i) → (ii) → (iii) 227  1.1.4 The graph below shows the relationship between the production of growth hormone and age A general conclusion can be drawn from the results is that … A growth hormone is not secreted after the age of 50 years B the amount of growth hormone secreted decreases with age C the amount of growth hormone secreted increases with age D the amount of growth hormone secreted remains stable over time 1.1.5 Which ONE of the following hormones prepares the body to react to emergency situations? A Insulin B Aldosterone C Adrenalin D Growth hormone (5 × 2 = 10) 1.2 Give the correct term for each of the following descriptions. Write only the term next to the question number. 1.2.1 The maintenance of a constant internal environment. 1.2.2 The maintenance of a constant body temperature. 1.2.3 Animals that control body temperature from within. 1.2.4 Widening of blood capillaries in the skin. 1.2.5 Method by which most heat is lost through sweating. 228  1.2.6 Control centre for temperature regulation in the brain. 1.2.7 A hormone that stimulates milk production in human females. 1.2.8 A disease that results from the bodies inability to produce insulin. 1.2.9 An enlarged thyroid gland that is caused by a deficiency of iodine. 1.2.10 The secretions that are produced in small quantities by the endocrine glands. (10 × 1 = 10) 1.3 Indicate whether each of the descriptions in Column I applies to A ONLY, B ONLY, BOTH A AND B or NONE of the items in Column II. Write A only, B only, both A and B or none next to the question number. Column I Column II A: medulla oblongata 1.3.1 Carbon dioxide levels B: cerebellum A: 10°C 1.3.2 Vasodilation B: 40°C 1.3.3 Increase the loss of heat in A: Sweating mammals B: Shivering A: internal environment 1.3.4 Cells and tissue fluid B: external environment 1.3.5 A gland that has a duct to carry its A: sweat glands secretion to where it is needed B: pancreas (5 × 2 = 10) 1.4 Complete the table below (10 × 1) = (10) Stimulus/variable Receptor/s Control centre effector osmoregulation 1.4.1 osmoreceptors hypothalamus 1.4.9 salts e.g. Na+ cells in salt concentration 1.4.6 kidneys (high and low) glomeruli carbon dioxide CO2 (high and 1.4.4 1.4.7 lungs concentration low) thyroxin 1.4.2 pituitary gland thyroid gland body cells blood glucose glucose 1.4.5 pancreas 1.4.10 (high and low) thermo- thermoregulation 1.4.3 1.4.8 skin receptors 229 1.5 Read the following and answer the questions that follow: South Africa’s very own mountaineer and adventurer Sibusiso Vilane, became a South African hero after summiting the Earth’s highest mountain, Mount Everest, in 2003. He started training in mountaineering by climbing the peaks of the Drakensberg mountains. Ex-President, Thabo Mbeki, said the following on the summit day “In this, he has shown the heights we can scale in life if we put our shoulder to the wheel. Sibusiso you have done us proud”. He continues to climb great peaks throughout the world and is actively involved in humanitarian work. Mountaineers like Sibusiso Vilane put their bodies under severe physiological strain when climbing up to heights of 8000 metres. The higher the altitude the greater the stress on the human body. The availability of oxygen decreases and hypoxia (oxygen starvation) can set in. It literally takes your breath away. Average temperatures are well below freezing. Climbers can become disorientated, develop acute mountain sickness (AMS) and get frostbite on their extremities. Many climbers have died on Everest. Athletes in extreme sports like this will try and acclimatize to these adverse conditions by preparing the body for the physiological challenges. The sherpas who assist climbers live at high altitudes all year around and their bodies have adapted well to the low oxygen levels and to the adverse cold. This leaves us with words from one of Sibusiso Vilane’s motivational speeches: “Every person has their own ‘Everest’ to climb”. 1.5.1 With low oxygen levels what other atmospheric gas levels could become a problem in the system of climbers like Sibusiso? (1) 1.5.2 How are the levels of the gas that you have given as your answer in 1.5.1 detected? (1) 1.5.3 In a flow diagram show how their bodies will try and compensate and adapt to the levels of the gas in their system. (3) 1.5.4 The climbers will lose a lot of heat through radiation to the environment. What conscious measures would you suggest they take to reduce this loss? (1) 1.5.5 Suggest a way in which climbers could prepare their bodies for the challenges of high altitudes, i.e. how in their training could they acclimatize to the conditions on Everest? (1) 1.5.6 What is the term used when the oxygen availability in the blood decreases? (1) 230  1.5.7 During an expedition led by Sibusiso in 2016, to the highest peak in Africa, Mount Kilimanjaro, one of our other famous sportsmen, Gugu Zulu, a rally driver, tragically fell ill and died. It seemed that he developed the onset of flu while on the climb. The fact that Gugu was not 100% healthy would have worsened the impact of high altitude and cold. One of the symptoms of a flu is fever. Which homeostatic mechanism that has been covered in this section would have been affected and briefly explain how the normal levels had been altered by the flu? (2) (10) Section A: Section B Question 2 2.1 Study the table below that shows the volume of urine produced by six different people on a hot day and on a cold day and answer the questions that follow. Volume of urine produced in cm3 Person Hot day Cold Day 1 430 890 2 350 1060 3 270 930 4 560 1280 5 400 680 6 390 1 160 Average 1 000 2.1.1 Calculate the average volume of urine in cm3 produced on the hot day. Show all workings. (2) 2.1.2 What can you deduce from the difference between the average volume of urine produced on the hot day and the average volume of urine produced on the cold day? (2) 2.1.3 Explain why, on a hot day, less water is lost from the body as urine. (2) 231  2.1.4 The composition (make-up) of urine depends on several factors. Name two factors that would affect the composition of urine. (2) (8) 2.2 The diagram below represents a negative feedback mechanism. X and Y represent hormones secreted by the respective glands. pituitary gland Y X thyroid gland 2.2.1 What is the role of any negative feedback mechanism in the human body? (1) 2.2.2 Identify hormone X. (1) 2.2.3 Explain the consequences for a person if hormone Y remained abnormally high for extended periods of time. (3) (5) 2.3 Triathletes competing in the Iron Man series are sometimes on the road and in the water for a total of over 8 hours. They will get very hot and thirsty. They will sweat profusely and will drink fluids continually through the race. They often do not need to urinate. Which two mechanisms would these athletes use to regulate their body temperature and fluid loss during the race. (2) 2.4 The picture below shows how important the skin is in controlling the core body temperature in very warm conditions. 232  On a hot day, the hypothalamus is stimulated and sends impulses to the blood vessels More sweat is released. Evaporation of the sweat cools the skin. Blood vessels dilate (become wider). This is called vasodilation.  More blood flows to the skin. Sweat gland  More heat is lost becomes more from the skin. active.  More blood is sent to the sweat glands. Using the information provided draw your own picture, alongside the one given, showing how the skin controls body temperature in cooler conditions. (5) Question 3 3.1 Type 2 diabetes is often linked to body mass index (BMI). The higher your BMI the greater your chances of developing Type 2 diabetes. The table below shows the results of an investigation into the BMI of women and their risk of developing diabetes (statistics from the American Diabetes Ass.) BMI (mass÷height) Relative risk of developing (kg/m2) diabetes in females (%) < 20 7,5 20 – 25 18,0 26 – 30 37,5 31 – 35 57,0 > 35 74,5 3.1.1 Draw a histogram using the data in the table. (5) 3.1.2 Which hormone is deficient in people with diabetes? (1) 233  3.1.3 In which organ is this hormone produced? (1) 3.1.4 Name another hormone that regulates the amount of glucose found in the blood. (1) 3.1.5 Write up your own conclusion from the data given in the table and from your histogram. (2) (10) 3.2 Lerato carried out an investigation to determine the effect of exercise on skin temperature. She asked 100 learners in her school to participate in the investigation. The sample consisted of 100 girls of the same age. The investigation was done as follows:  The learners were divided into two groups of 50 each (Group A and B).  The skin temperature was measured for all the participants.  Group A was asked to run around the sports field for 10 minutes.  Group B was asked to remain seated on the benches next to the field for 10 minutes. After 10 minutes the skin temperature of all participants was measured, and the average was calculated for each group (A and B). 3.2.1 In this investigation, identify the: (a) independent variable (1) (b) dependent variable (1) 3.2.2 State two steps that Lerato took into consideration during the planning of the investigation. (2) 3.2.3 What is the expected results for the participants in group A? (1) 3.2.4 Name the one factor that Lerato kept constant during the investigation. (1) 3.2.5 Which of the two groups (A or B) will release more sweat? (1) 3.2.6 Explain why sweat production will increase in the group identified in question 3.2.5. (3) (10) 234 3.3 The graph below shows a comparative glucose test on a healthy and a diabetic person. (In the glucose tolerance test the people being tested would have not eaten for at least 8 hours before the test. When they arrive at the hospital/clinic/doctor’s rooms their blood is taken, and glucose levels are tested. They then drink a glucose solution and their blood is taken and tested every 30 minutes over at least a 3 hour period.) Result of a glucose tolerance test on a healthy person (Person A) and on a diabetic (Person B) Blood glucose levels (mg/dL) Normal Person A Person B 0 30 60 90 120 150 180 210 Time (minutes) 3.3.1 What values does the graph accept as normal blood glucose levels? (1) 3.3.2 What is the difference in blood glucose levels at time 0 between person A and person B? (2) 3.3.3 What do you think happened in person A to cause the glucose levels to drop after 60 minutes? (2) 3.3.4 Name three lifestyle precautions should person B be taking? (3) 3.3.5 Besides lifestyle changes what treatment is available for diabetics to regulate their glucose levels? (2) (10) Section B: Total marks: 235