AQA Biology Homeostasis PDF

16 Homeostasis 16.1 Principles of homeostasis Learning objectives rn the previous chapter we looked at how complex organisms control and coordinate their activities. In particular we considered the way in...

16 Homeostasis 16.1 Principles of homeostasis Learning objectives rn the previous chapter we looked at how complex organisms control and coordinate their activities. In particular we considered the way in -+ Describe the nature of which such organisms respond rapidly to environmental changes using homeostasis. their nervous system. A feature of an increase in complexity is the ability -+ Explain the importance of of organisms to control their internal environment. By maintaining a homeostasis. relatively constant internal environment for their cells, organisms can -+ Explain how control limit the external changes these cells experience. This maintenance of mechanisms work. a constant internal environment is called homeostasis. In this chapter we shall learn about homeostasis and the role of the other coordination -+ Explain how control system, hormon al coordination, in an organism's physiological control. mechanisms are coordinated. The internal en viron men t is made up of tissue fluids that bath e Specification reference: 3.6.4.1 each cell, supplying nu trie n ts and removing wastes. Ma in taining th e featu res of this flui d at th e optimum levels protects the cells from ch a nges in th e externa l en vironment, thereby giving th e organism a degree of indepen den ce. What is homeostasis? Homeostasis is the maintenance of an internal environment within restricted limits in organisms. lt involves trying to maintain the chemical make-up, volume and other features of blood and tissue fluid within restricted limits. Homeostasis ensures that the cells of the body are in an environment that meets their requirements and allows them to function normally despite external changes. This docs not mean that there are The importance of temperature no changes. On the contrary, there arc continuous fluctuations brought and pH in relation to enzyme about by variations in internal and external conditions, such as changes activity [Topic 1.8) and water in temperature, pH and water potemial. These changes, however, occur potential in relation to cells around an optimum point. Homeostasis is the ability to return to that optimum poin t and so maintain organisms in a balanced equilibrium. ~ opic 3.7) make useful ackground reading for omeostasis. The importance of homeostasis -------­ Hom eostasis is essen tia l fo r the p roper functio ni ng of o rganisms for the following reasons amongst others: The enzymes that control the biochemica l reactions w ithin cells, and oth er proteins, such as channel proteins, arc sensitive to changes in pH and temperature. Any change LO these factors reduces the rate of reaction of enzymes or may even prevent them working a lcogether, for example, by denaturing them. Even small nuctuations in temperature or pH can impair the ability of enzymes tO carry out their roles effectively. Maintaining a fairly constant internal environment means that reactions take place at a suitable rate. Hint Changes to the water potential of the blood and tissue fluids may A change in water potential cause cells to shrink and expand (even to bursting point) as a result may affect the concentration of of water leaving or entering by osmosis. In both instances the cells substrates and enzymes and cannot operate normally. The maintenance of a constant blood therefore the rate of reactions. See glucose concentration is essential in ensuring a conscanr water Topic 1.8. potential. A constant blood glucose concentration also ensures a reliable source of glucose for respiration by cells.  Organisms with the ability to maintain a constant internal environment arc more independent of changes in the external environmenl. They may have a wider geographical range and therefore have a greater chance of finding food, shelter, etc. Mammals, for e xample, with their ability to maintain a constant temperature, are found in most habitats, ranging from hot arid deserts to cold, fro1en polar regions. Control mechanisms The control of any self-regulating system involves a series of stages that feature: the optimum point, the point at which the system operates besl. This is monitored by a... receptor, wh ich detects any deviation from the optimum point (ie., a stimu lus) and informs the... coordinator. which coordinates information from receptors and sends instructions to an appropriate... A Figure 1 Homeostasis allows effector. often a muscle or gland, which brings about the changes animals such as these camels in the needed to return the system to the optimum point. This return to desert [top] and these penguins in the normality creates a... Antarctic [bottom] to survive in extreme environmen ts feedback m echanism, by which a receptor responds to a stimulus created by the change to the system brought about by the effector. Figure 2 illustrates the relationship between these stages using the everyday example of co ntrolling a central heating system. Input Receptor Coordinator Effector Outout Change to the system Change detected 1n a Operational information Brings about changes to System returned to living system is stored here and used the system in order to optimum point return it to the optimum to coordinate effectors + point + Room temperature Room thermostat Programmer checks Boiler fires up, pump Room temperature is drops from 20°C to signals that the that heating should be circulates water. raised to 20"C 18 c temperature 1s below on at this time. If so. 1t rad iators become the optimum point starts boiler and hot circulation pump t Feedback loop in this case =negative feedback as it turns system off Circulation of air in room takes air at 20°C from radiator to thermostat A Figure 2 Components of a typical control system Coordination of control mechanisms Most systems, including biological ones, use negative feedback Negative feedback is when the change produced by the control system leads to a change in the stimulus detected by the receptor and turns the system off. We shall meet an example of negative feedback when we look at the regulation of blood glucose in Topic 16.3. Positive feedback occurs when a deviation from an optimum causes changes that result in an even greater deviation from the normal. One example occurs in neurones where a stimulus leads to a small influx of sodium ions. This influx increases the permeability of the neurone membrane to sodium ions, more ions enter. causing a further increase 16.1 Principles of homeostasis in permeability and even more rapid entry of ions. In this way, a small Summary questions stimulus can bring about a large and rapid response. 1 Describe homeostasis. Control systems normally have many receptors and effectors. This allows them to have separate mechanisms taht each produce a 2 Explain wh\:J maintaining positive movement towards an optimum. This allows a greater a constant temperature is degree of control of the particular fa ctor being regulated. Having important in mammals. separate mechanisms that controls departures in different directions 3 Suggest why maintaining from the original state is a general feature of homeostasis. It is a constant blood glucose important to ensure that the information provided by receptors is concentration might be analysed by the coordinator before action is taken. For example, important in mammals. temperature receptors in the skin may signal that rhe skin itself is cold and that the body temperature should be raised. However, information from regions in the hypothalamus in the brain may indicate that blood temperature is already above normal. This situation might arise during strenuous exercise when blood temperature rises but sweating cools the skin. By a nalysing the information from all detecto rs, the brain ca n decide the best co urse of action - in this case not to raise the body temperature furth er. In the same way, the control centre must coordinate the action of the effectors so that they operate harmoniously. For example, sweating would be less effective in cooling the body if it were not accompanied by vasodilation. Comparing thermoregulation in ectotherms and endotherms 8 As with all extension boxes, the material here is taking shelter. Lizards will shelter in the shade to broaden understanding ofmaterial beyond the to prevent over·heating when the Sun's radiation specification. is at its peak. At night they retreat into burrows in order to reduce heat loss when the external Animals such as birds and mammals derive most of their temperature is low. heat from the metabolic activities that take place inside their bodies. They are therefore known as endotherms gainingwarmth from the ground. Lizards will press their [meaning inside heat). Some animals obtain a proportion bodies against areas of hot ground to warm themselves of their heat from sources outside their bodies, up. When the required temperature is reached, they raise namely the environment. They are therefore known as themselves off the ground on their legs. ectotherms [meaning outside heat). Regulation of body temperature in ectotherms Many ectotherms gain heat from the environment, so their body temperature fluctuates with that of the environment. They therefore control their body temperature by adapting their behaviour to changes in the external temperature. Reptiles, such as lizards, are ectotherms. They control their body temperature by: exposing themselves to the Sun. In order to gain heat lizards orientate themselves so that the maximum surface area of their body is exposed to the warming rays of the Sun. Figure 3 A lizard showing thermoregulatory behaviour by gaining heat bothjrom the sun and the worm rock Regulation of body temperature in endotherms skin surface "-.. Endotherms gain most of their heat from internal metabolic little heat capillaries almost activities. Their body temperature remains relatively - -radiated empty of blood constant despite fluctuations in the extern al temperature. \l~ I Like ectotherms, endothermic animals use behaviour to shunt maintain a constant body temperature. Unlike ectotherms, vessel dilated however, they also use a wide range of physiological mechanisms to regulate their temperature. venule - fat (adipose) Conserving and gaining heat in response to } tissue (good a cold environment insulator) artery vein Mammals and birds that live in cold climates have evolved I connecting vessel a number of adaptations in order to survive in these dilated environments. One of the most important is having a body A Figure 5 Vasoconstriction with a small surface area to volume ratio. It is from within the volume that heat is produced and from the surface shivering. The muscles of the body undergo involuntary area that heat is lost. Mammals and birds in cold climates rhythmic contractions that pro duce metabolic heat. therefore tend to be relatively large, for example, the polar raising of hair. The hair erector muscles in the skin bear and penguin. Compared with animals in warmer contract, raising the hairs on the body. This enables climat es they also have smaller extremities, such as ears, a thicker layer of still air, which is a good insulator, to and thick fu r, feathers, or fat layers to insulate the body. be trapped next to the skin, insulation and conserving To make more rapid body temperature changes, mammals heat in mammals with thick fu r. use one or more of the following mechanisms: increased metabolic rate. In cold conditions more of the hormones that increase metabolic rate are vasoconstriction. The diameter of the arterioles near produced. As a result metabolic activity, including the surface of the skin is made smaller. This reduces respiration, is increased and so more heat is generated. the volume of blood reaching the skin surface through the capillaries. Most of the blood entering the skin decrease in sweating. Sweating is reduced, or ceases passes beneath the insulating layer of fat and so altogether, in cold conditions. loses little heat to the environment (Figure 5). behavioural mechanisms. Sheltering from the wind, basking in the sun and huddling together all help animals to maintain their core body temperature. Losing heat in response to a warm environment Long-term adaptations to life in a warm climate include having a large surface area to volume ratio and lighter coloured fur to reflect heat. Rapid responses that enable heat to be lost when the environmental temperature is high include: vasodilation. The diameter of the arterioles near the surface of the skin becomes larger. This allows warm blood to pass close to the skin surface through the capillaries. The heat from this blood is then radiated away from the body (Figure 6). increased sweating. To evaporate water from the A Figure 4 The penguin and the polar bear both have large skin surface requires energy in the form of heat. compact bodies with a small surface area to volume ratio. In relatively hairless mammals, such as humans, This helps them to conserve heat in the cold environments sweating is a highly effective means of losing heat. ofthe South and North Poles where they live In mammals with fur, cooling is achieved by the 16.1 Principles of homeostasis evaporation of water from the mouth and tongue, behavioural mechanisms. Avoiding the heat of the during panting. The high latent heat of vaporisation of day by sheltering in burrows and seeking out shade water makes sweating an efficient way of losing heat. help to prevent the body temperature from rising. lowering of body hair. The hair erector muscles in The graphs shown in Figure 7 compare the rates of the skin relax and the elasticity of the skin causes metabolic heat generation and evaporative heat loss them to flatten against the body. This reduces the in a mammal and a reptile as the environmental thickness of the insulating layer and allows more temperature changes. heat to be lost to the environment when the internal temperature is higher than the external temperature. 1 Give a reason why the values for heat generation and heat loss are measured per gram of body mass. 2 a Describe the relationship between metabolic capillaries heat generation and evaporative heat loss in a full of blood reptile. b Explain how this relationship differs in a mammal. constricted venule 3 Reptiles frequ ently seek shade when the environmental temperature rises above 25°C. Use arteriole dilated the graphs to explain this type of behaviour. - blood vein connecting vessel 4 Suggest a reason for the change in the evaporative heat loss in the mammal at point A on the graph. constricted.&. Figure 6 Vosodilotion 2.5 metabolic heat L.5 generation I Jh 1g 1 1.0 0.5 Mammal 0.0 15 20 25 30 35 2.0 Mammal evaporative heat L.5 loss/Jh 1g 1 1.0 Reptile 0.5 -------~--------------------------- 0.0-'-~r-~--~----,.----~--~ 15 20 25 30 35 environmental temperaturel°C.&. Figure 7 We saw in Topic 16. l, that the homeostatic control of any system Learning objectives involves a series of stages featuring: -+ Explain what negative the optimum point, or desired level (norm), at which the system feedback is. operates -+ Explain how negative a receptor. which dececcs che scirnulus of any deviation from the feedback helps to control set point (norm) homeostatic processes. a coordinator. which coordinates information from various sources -+ Distinguish between negative an effector, which brings about the corrective measures needed to feedback and positive return the system to the optimum point (norm) feedback. a feedback m echanis m. by which a receptor detects a stimulus Specification reference: 3.6.4.1 created by the change to the system and the effector brings about the appropriate response. Let us now look in more detail a t the last stage in the list - the feedback mechanism. When an effector has corrected any deviation and returned the system to the optimum point, it is important that this information is fed back to the receptor. If the information is not fed back, the receptor will con ti nue to stimulate the effector. leading to an over-correction and causing a deviation in the opposite direction. There are two types of feedback - negative feedback and positive feedback. Negative feedback Negative feedback occurs when che stimulus causes the corrective measures to be turned off. In doing so chis tends to return the system to its original (optimum) level (and prevents any overshoot). There are separate negative feedback mechanisms to regulate departures from the norm in each direction. An example is in the control of blood glucose that is covered in more detail in Topic 16.3. If there is a fall in the concentration of glucose in the blood this stimulus is detected by receptors on the cell-surface membrane of the o. cells (coordinator) in the pancreas. These a cells secrete the hormone glucagon. Glucagon causes liver cells (effectors) to convert glycogen to glucose which is released into the blood raising the blood glucose concentration. As this blood with a raised glucose concentra tion circulates back to the pancreas there is reduced stimulation of ex cells which therefore secrete less glucagon. So the secretion of glucagon leads to a reduction in its own secretion (=negative feedback). These events are illustrated in Figure 1. In the same way, if the blood glucose concentration rises, rather than falls, insulin will be produced from the~ cells in the pancreas. Insulin increases the uptake of glucose by cells and its conversion to glycogen Synoptic link and fat. The fall in blood glucose concentration that results reduces The concept of stimulus___., insulin production once blood glucose concentrations return to their receptor ___., coordinator___., effector optimum (=negative feedback). - response is a recurring theme Having separate negative feedback mechanisms that control departures in biology. For example, we met it from the norm in either direction gives a greater degree of homeostatic throughout Chapter 14. control. This is because there are positive actions in both directions. 16.2 Feedback mechanisms fall in blood containing normal blood a cells in glucagon \,.. liver blood.,. glycogen - glucose ~ glucose ~ the pancreas glucose concentration concentration t blood at optimum glucose concen_tr_a1_1o_n_ __ _, reduces stimulation of a cells.A. Figure 1 Negative feedback in the control ofblood glucose levels For example, if glucagon raised the blood sugar concentration above Study tip the optimum, it would take some time for it to fall again if the only If you are writing about negative way of lowering it was through metabolic activity. However, by having feedback, for example, control a second hormone, insulin, that lowers blood sugar concent.ration, its of blood glucose, make certain secretion brings about a return to optimum blood sugar concentration that you focus on negative far more rapidly. feedback rather than just giving a description of how insulin and Summary questions glucagon work. 1 Explain why negative feedback is important in maintaining a system at a set point. Hint 2 Explain the advantage of having separate negative feedback Negative feedback in the context mechanisms to control deviations away from normal. of hormones means that the secretion of a hormone (e.g., glucagon) leads to a reduction in the secretion of that hormone. Positive feedback occurs when the feedback causes the corrective measures to remain turned on. In doing so it causes the system to deviate even more from the original (normal) level. Examples are less common, but one occurs in neurones when a stimulus causes a small influx of sodium ions. This influx increases the permeability of the neurone to sodium ions so more ions enter, causing a further increase in permeability and even more rapid entry of ions. This results in a very rapid build-up of an action potential that allows an equally rapid response to a stimulus. Positive feedback occurs more often when there is a breakdown of control systems. In certain diseases, for example typhoid fever, there is a breakdown of temperature regulation resulting in a rise in body temperature leading to hyperthermia. In the same way, when the body gets too cold (hypothermia) the temperature control system tends to break down, leading to positive feedback resulting in the body temperature dropping even lower. 1 Oxytocin is a hormone that causes contractions of the uterus at childbirth. The contractions produce a positive feedback loop that results in the release of more oxytocin. Explain the advantage of positive feedback rather than negative feedback in this situation. If the temperature of the blood increases, 2 Cutting the nerves connecting the thermoreceptors thermoreceptors in a region of the brain called the to the heat loss centre in the hypothalamus might hypothalamus send more nerve impulses to the heat cause the death of the individual. In terms of the loss centre, which is also in the hypothalamus. This information in Figure 2, explain precisely why this in turn sends impulses to the skin [effector organ). action might cause death. Vasodilation, sweating and lowering of body hairs all lead 3 Cutting the nerves connecting the heat loss centre to a reduction in blood temperature. If the fact that blood to the skin would be less likely to cause death than temperature has returned to normal is not fed back to if those between the thermoreceptors and the heat the hypothalamus, it will continue to stimulate the skin loss centre were cut. Suggest why. to lose body heat. Blood temperature will then fall below 4 Negative feedback in temperature regulation occurs normal and may continue to do so causing hypothermia as a result of blood passing from the skin to the brain. and the death of the organism. In doing so, this blood passes through the heart. List, What happens in practice is that the cooler blood in sequence, the major vessels joined to the heart returning from the skin passes through the that this blood would pass through on its journey. hypothalamus. As a result thermoreceptors send fewer impulses to the heat loss centre. This in turn stops sending impulses to the skin and so vasodilation, sweating, etc. cease, and blood temperature remains at its normal level rather than continuing to fall. The blood, having been cooled to its normal temperature, has resulted in turning off the effector (the skin) that was correcting the rise in temperature. This is therefore negative feedback and is illustrated in Figure 2. 1 State what would happen to the temperature of the blood if the feedback was positive rather than negative..A Figure 3 Control ofbody temperature involves negative feedback mechanisms lhermoreceptors heat loss vasodilatron cooler blood 1n the centre in the hypothalamus sweatmg temperature hypothalamus nerve impulses nerve impulses lowering hairs ~ blood at original temperature turns off corrective measures =negative feedback.A Figure 2 Negative feedback in the control ofbody temperature