Regulation of Body Temperature PDF

2 Regulation of body temperature We feel cold and shiver when we have a fever Fever Think about......

2 Regulation of body temperature We feel cold and shiver when we have a fever Fever Think about... A certain part of our brain works like a thermostat, which controls 1 Which part of our brain works the set point of our body temperature (normally 37 °C). When we like a thermostat which are infected by pathogens, they may produce a substance called controls the set point of body temperature? pyrogen*, which leads to a fever. Pyrogen increases the synthesis of a compound called prostaglandin* in our body, raising the set point. 2 How does shivering help keep Once this happens, 37 °C means below normal to the ‘thermostat’ our body warm? and we feel cold. The ‘thermostat’ initiates mechanisms to conserve (Answers on p. 68) and produce more heat. One of the mechanisms is shivering. prostaglandin 前列腺素 pyrogen 致熱原 Acknowledgements and Important Notice: All questions from the HKDSE, HKCEE and HKALE are reproduced by permission of the HKEAA. Unauthorized use of the aforementioned questions in this electronic version is prohibited. V Human Physiology: Regulation and Control 2.1 Importance of regulation of body temperature The term ‘body temperature’ In Bk 2, Ch 18, we learnt that keeping body temperature stable is refers to the core body essential for survival. This is because metabolic reactions in the body are temperature, i.e. the temperature of blood and catalysed by enzymes. Enzymes only work efficiently within a narrow internal organs. Core body range of temperatures around 37 °C. A stable body temperature helps temperature is normally keep enzyme activities and hence metabolic rate steady. kept stable whereas the temperature of other body parts (e.g. skin) can vary To keep a stable body temperature, the amount of heat gain and heat a lot. loss must be balanced. We gain heat mainly from metabolism taking place inside our body, in particular from respiration in the liver and skeletal muscles. Our body also gains heat from the environment or loses heat to it through different ways. Fig 2.1 shows some of these ways. Evaporation* Refer to Background physics on radiation Water absorbs heat from the (from the sun) surroundings when it changes to p. 47 for details about conduction, vapour convection and radiation. evaporation (sweat) Example: evaporation The body loses heat when sweat Conduction* evaporates from the skin and (water in lungs) Transfer of heat between two water evaporates from the lungs. objects that are in direct contact air flow Example: The body loses heat when the skin touches a cold drink, and gains heat when the skin touches conduction convection the hot ground. (direct contact) Convection* Transfer of heat due to movement of air or water Radiation* radiation Example: (from body) Transfer of heat (as radiation) The body loses heat by the between two objects that are not upward flow of warm air and in contact the downward flow of cold air. Increased air flow (wind) promotes Example: the heat loss. The body gains heat from the sun by radiation. On the other hand, the body loses heat to nearby cooler objects not in contact by Key: direction of transfer radiation. of thermal energy conduction Fig 2.1 Heat exchange between our body and the environment conduction 傳導 convection 對流 evaporation 蒸發 radiation 輻射 46  2 Regulation of body temperature Heat transfer There are three processes of heat transfer: cold end 1 Conduction of y Conduction is a process of heat transfer in which sfer nerg tran rmal e thermal energy is transferred from the hot part of the hot an object to the cold part, or from a hot object to end a cold object that it is in contact with. This can be explained by the particle model (Fig 2.2). The particles in the hot part vibrate faster. They hit slower neighbouring particles, making them vibrate faster. This area therefore becomes hotter. Fig 2.2 Heat transfer in a solid by conduction 2 Convection Convection is a process of heat transfer that takes place in liquids and gases only. Thermal energy is hot water rises cold water transferred by the movement of the liquid or the thermal replaces hot gas itself. Fig 2.3 shows an example: water expands energy is water, producing transferred a convection and becomes less dense when it is heated. The upwards current hot water rises and thermal energy is transferred upwards. Its place is then taken up by the cold water, producing a convection current*. Fig 2.3 Convection inside water 3 Radiation space (vacuum) Radiation is a process of heat transfer which does not require a medium. For example, the space between the Sun and the Earth is a vacuum. Sun Thermal energy from the Sun is transferred to the thermal energy is Earth Earth by radiation. transferred by radiation Fig 2.4 Energy is transferred by radiation in space What is the importance of keeping body temperature stable? A stable body temperature allows enzymes in the body to function properly so that the metabolic rate can be kept steady. convection current 對流 47 V Human Physiology: Regulation and Control 2.2 The human skin The skin covers the surface of our body. It is the main site at which heat exchange between the body and the environment occurs. It has various structures that are important in the regulation of body temperature. Fig 2.5 shows the structure of skin. It consists of two main layers: the outer, thin epidermis and the inner, thick dermis. touch receptor hair sensory thermoreceptor sebaceous gland* receptors pain receptor pressure receptor erector muscle* epidermis* (outer thin layer) hair follicle* superficial capillary dermis* sweat duct* (inner thick layer) sweat gland* blood vessel subcutaneous fat* Fig 2.5 Structure of skin epidermis hair erector muscle sebaceous gland dermis hair follicle blood vessel sweat gland subcutaneous fat Fig 2.6 Human skin (L.S.) (×30) dermis 真皮 epidermis 表皮 erector muscle 豎毛肌 hair follicle 毛囊 sebaceous gland 皮脂腺 subcutaneous fat 皮下脂肪 sweat duct 汗管 sweat gland 汗腺 48  2 Regulation of body temperature 1 Epidermis The epidermis is the outer layer of the skin. It mainly serves to protect the body. The epidermis can be subdivided into three layers as shown in Fig 2.7. The outermost layer consists of dead cells. These cells are constantly shed and replaced by new cells from underneath. This layer is tough and waterproof. It serves to: - reduce water loss from the body by evaporation - protect the body from mechanical injury - prevent the entry of pathogens The middle layer consists of living cells produced from the innermost layer. The cytoplasm of these cells is gradually replaced by keratin*, which is a fibrous protein insoluble in water. These Fig 2.7 Skin epidermis (L.S.) cells finally die and become the outermost layer. (×100) The innermost layer consists of living cells. These cells: - carry out mitotic cell division to produce new cells - contain a dark pigment called melanin*, which absorbs ultraviolet light from the sun, preventing it from harming the body - produce vitamin D when exposed to sunlight Sun tanning Melanin gives skin its colour. It is made by certain cells in the epidermis by a process called melanogenesis*. UV radiation in sunlight stimulates this process and therefore causes the skin to become darker. Overexposure to sunlight can cause sunburn and speed up skin ageing. It may even lead to skin Fig 2.8 We should apply sunscreen to cancer. Therefore, we should protect our skin from strong avoid sunbathing for long and we sunlight should apply sunscreen to protect our skin. keratin 角蛋白 melanin 黑素 melanogenesis 黑素原生成 49 V Human Physiology: Regulation and Control 2 Dermis The dermis is much thicker than the epidermis. It is made up of connective tissues with a number of structures embedded in it. i) Sensory receptors Several types of sensory receptors are present in the skin. They detect stimuli such as temperature change, touch, pressure and pain. ii) Hair, hair follicles, erector muscles and sebaceous glands Hair follicles are ingrowths of the epidermis. At the base of each hair follicle, new hair cells are produced to form a hair. Each hair follicle is connected to an erector muscle. When the erector muscle contracts, the hair is pulled upright. When the erector muscle relaxes, the hair lies flat (Fig 2.9). These actions help regulate body temperature. This will be discussed in detail in Section 2.3. erector muscle relaxes erector muscle contracts and hair lies flat to pull hair upright Fig 2.9 Action of erector muscles on hairs Each hair follicle is usually associated with a sebaceous gland. This gland secretes an oily substance called sebum* to the skin surface. sweat Sebum keeps the skin and hair supple and waterproof. It is also duct an antiseptic, which can suppress the growth of microorganisms. iii) Sweat glands sweat Each sweat gland is a coiled tube. It secretes sweat, which passes along gland the sweat duct to the skin surface through the sweat pore (Fig 2.10). The evaporation of sweat helps remove heat from the body which is important for the regulation of body temperature. Fig 2.10 Section of human skin showing a Sweat contains mainly water, salt, urea and other metabolic wastes. sweat gland (×25) Therefore, sweating is also a form of excretion. sebum 皮脂 50  2 Regulation of body temperature iv) Blood vessels There are many blood vessels in the dermis (Fig 2.11). They supply oxygen and nutrients to the living cells of the skin and remove wastes from them. They also take part in the regulation of body temperature. This will be discussed in detail in Section 2.3. blood vessel red blood cell dermis blood vessels (hole) (×400) (×30) Fig 2.11 Section of the skin showing blood vessels in the dermis (L.S.) 3 Subcutaneous fat Beneath the dermis is a layer of subcutaneous fat. This fatty tissue acts as an insulating layer to reduce heat loss from the body. It also acts as an energy reserve. What are the different structures of the skin and their functions? sensory receptor sebaceous gland detects stimuli secretes sebum that keeps the skin and hairs supple hair follicle and waterproof, and produces hairs suppresses the growth of harmful microorganisms epidermis erector muscle reduces water loss hair is pulled upright when from the body it contracts; hair lies down protects the body from when it relaxes mechanical injury prevents the entry of sweat gland pathogens secretes sweat to remove metabolic wastes and help dermis regulate body temperature blood vessel subcutaneous fat supplies oxygen and acts as an insulating nutrients to cells layer to reduce heat loss helps regulate body acts as an energy reserve temperature 51 V Human Physiology: Regulation and Control 2.1 Identifying features of the mammalian skin that are related to temperature regulation (go to p. 163) Directions: Questions 1 to 3 refers to the photomicrograph below, which shows a section of the human skin. P Q R S (×30) Level 1 1 Which structure contains cells that are actively dividing? A P B Q C R D S p. 49 Level 2 2 If structure Q was damaged in a fire, which of the following might occur in the damaged part? (1) bleeding (2) excessive loss of water (3) bacterial infection A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) p. 49 3 Which structures are involved in the regulation of body temperature? A P and Q only B P and S only C Q and R only D R and S only p. 50 52  2 Regulation of body temperature Watch this to prepare for 2.3 Mechanism of body your class and answer the questions. temperature regulation Video & questions A Control centre of body temperature In order to keep body temperature stable, our body has different mechanisms to balance the amount of heat gain and heat loss. These mechanisms are under the control of the thermoregulatory centre in the hypothalamus of the brain. The thermoregulatory centre has two parts: the heat gain centre and the heat loss centre. These two centres work through a negative feedback mechanism to regulate body temperature. (Fig 2.12): negative feedback normal body temperature negative feedback (set point = 37 ºC) blood or skin temperature blood or skin temperature becomes lower than normal thermoregulatory centre* becomes higher than normal e st at im (detected by thermoreceptors) in hypothalamus (detected by thermoreceptors) ul ul im at st e responses of effectors to: initiate heat gain heat loss initiate responses of effectors to: reduce heat loss, and centre* centre* increase heat loss, and increase heat gain reduce heat gain Fig 2.12 Negative feedback of the control of body temperature Thermoreceptors* are present in the hypothalamus and the skin. Those in the hypothalamus detect the temperature of the blood passing through it (i.e. internal temperature) while those in the skin detect the temperature of the skin (i.e. external temperature). When the thermoreceptors detect a fall in body temperature (i.e. below the set point of 37 °C), they send nerve impulses to the heat gain centre. The heat gain centre is stimulated. It then sends nerve impulses to various effectors to produce responses that reduce heat loss and increase heat gain until normal body temperature is restored. When the thermoreceptors detect a rise in body temperature, they send nerve impulses to the heat loss centre. The heat loss centre is stimulated. It then sends nerve impulses to various effectors to produce responses that increase heat loss and reduce heat gain until normal body temperature is restored. heat gain centre 產熱中樞 heat loss centre 散熱中樞 thermoreceptor 温度感受器 thermoregulatory centre 體温調節中樞 53 V Human Physiology: Regulation and Control DSE B Regulation of body temperature by skin 12(II)Q1b, 15(II)Q1b, 18(II)Q1b In Section 2.2, we have learnt about the structure of the skin. Let us see how effectors in the skin (including sweat glands, erector muscles and arterioles) respond under cold and hot conditions in order to keep body temperature stable. 1 Under cold conditions Under cold conditions, more heat is lost from the body because the temperature gradient between the body and the environment is steep. This results in a decrease in body temperature. When the body temperature becomes lower than normal, the heat gain centre of the Animation E1, 2.1 thermoregulatory centre in the hypothalamus is stimulated. The heat gain centre then sends nerve impulses to different parts of the skin to reduce heat loss from the body (Fig 2.13). Heat loss is reduced. ❶ Sweating decreases. ❸ Erector muscles contract to pull hairs upright so that a thicker layer of still ➋ ❷ Arterioles constrict air is trapped for better (i.e. vasoconstriction). Less insulation of heat. blood flows to the capillaries near the skin surface. ❹ The layer of subcutaneous fat becomes thicker (long-term response). Fig 2.13 Response of the skin under cold conditions ❶ Sweating decreases Sweat glands secrete less sweat under cold conditions. As heat is lost from the body to the surroundings when sweat evaporates, secreting less sweat helps reduce heat loss. Sweat is not secreted under cold conditions. Sweat is secreted most of the time to remove metabolic wastes. Sweating decreases under cold conditions. 54  2 Regulation of body temperature ❷ Vasoconstriction of arterioles in the skin In the skin, the arteriole and the venule are connected by a shunt vessel. Under cold conditions, the arteriole constricts (vasoconstriction). This causes less blood to flow to the capillaries near the skin surface and more blood to flow through the shunt vessel, which is deeper in the skin (Fig 2.14). As a result, less heat is lost from the body by conduction, convection and radiation. 4 Heat loss by conduction, convection and radiation is reduced. 3 Less blood flows to the capillaries near the skin surface. arteriole venule 1 Arterioles constrict (i.e. vasoconstriction*). 2 More blood flows through the shunt vessel*, which is deeper in the skin. Fig 2.14 Vasoconstriction of the arteriole near the skin surface helps reduce heat loss Capillaries near the skin surface constrict under cold conditions. Arterioles that supply blood to the capillaries near the skin surface constrict under cold conditions. Capillaries do not constrict as they have no muscular walls. ❸ Erector muscles contract to pull hairs upright When the erector muscles Under cold conditions, erector muscles contract to pull the hairs upright. contract to pull the hairs This traps a thicker layer of still air close to the skin surface. Since air upright, the skin around the hairs is also raised slightly. is a good insulator of heat, heat loss to the surroundings by conduction This forms goose pimples*. and convection is reduced. However, this insulating effect is poor in humans because the hair on human skin is sparse*. Air cannot be trapped effectively between hairs. Fig 2.15 Goose pimples goose pimple 雞皮疙瘩 shunt vessel 分流血管 sparse 稀少的 vasoconstriction 血管收縮 55 V Human Physiology: Regulation and Control ❹ Layer of subcutaneous fat becomes thicker The layer of subcutaneous fat becomes thicker. Since fat is a good insulator of heat, a thicker layer of fat helps reduce heat loss from the body by conduction. This is a long-term response to a cold environment because subcutaneous fat takes time to develop. Measurement of body fat percentage Body fat percentage is an important measure of health. It can be predicted by measuring the thickness of skinfold*, which includes the subcutaneous fat under the skin. An examiner can use a caliper* (Fig 2.16) to measure the thickness of skinfold in several body parts (e.g. chest, abdomen and thigh). The data Fig 2.16 A skinfold caliper collected is then input into a formula to calculate the body fat percentage. Ways of reducing heat loss in other animals Animals living in cold environments usually have special features to reduce heat loss from the body. Below are two examples. a b Whales living in the polar Polar bears living in the Arctic regions have a very thick layer of have very thick and dense fur. The subcutaneous fat called blubber* fur traps a thick layer of air close to reduce heat loss. This blubber to the skin surface so that heat is more than a foot thick! loss is greatly reduced. blubber 鯨脂 caliper 皮摺脂肪夾 skinfold 皮摺 56  2 Regulation of body temperature 2 Under hot conditions Under hot conditions, less heat is lost from the body because the temperature gradient between the body and the environment is narrower. The body may even gain heat when the environmental temperature is higher than body temperature. Doing exercise also causes the body to gain heat because a lot of heat is generated when skeletal muscles contract strongly and frequently. If body temperature becomes higher than normal, the heat loss centre of the thermoregulatory centre in the hypothalamus is stimulated. The heat loss centre then sends nerve impulses to different parts of the skin to increase heat loss from the body (Fig 2.17). Heat loss is increased. ❶ Sweating increases. ❸ Erector muscles relax and hairs lie flat. Only ❷ Arterioles dilate (i.e. vasodilation). a thin layer of still air More blood flows to the capillaries is trapped between near the skin surface. hairs. Heat insulation is reduced. ❹ The layer of subcutaneous fat becomes thinner (long-term response). Fig 2.17 Response of the skin under hot conditions ❶ Sweating increases If air temperature is higher Under hot conditions, sweat glands secrete more sweat. The than body temperature, heat evaporation of more sweat from the skin absorbs more heat from the cannot be lost from the body by conduction, convection body surface. As a result, more heat is lost from the body. and radiation. Evaporation is the only way of heat loss Note that sweating alone does not cause heat loss. Heat is lost from from the body. the body only when sweat evaporates. If the humidity of air is high or there is no wind, the rate of evaporation of sweat will become very low. Under such circumstances, body heat cannot be lost to the surroundings effectively. 57 V Human Physiology: Regulation and Control ❷ Vasodilation of arterioles in the skin Under hot conditions, the arteriole in the skin dilates (vasodilation). This causes more blood to flow to the capillaries near the skin surface and less blood to flow through the shunt vessel (Fig 2.18). As a result, more heat is lost from the body by conduction, convection and radiation. 4 Heat loss by conduction, convection and radiation is increased. 3 More blood flows to the capillaries near the skin surface. 1 Arteriole dilates (i.e. vasodilation*). 2 Less blood flows through the shunt vessel. Fig 2.18 Vasodilation of the arteriole near the skin surface helps increase heat loss ❸ Erector muscles relax and hairs lie flat Under hot conditions, erector muscles relax so that the hairs lie flat. As a result, only a thin layer of still air is trapped close to the skin surface and more heat is lost to the surroundings by conduction and convection. ❹ Layer of subcutaneous fat becomes thinner The layer of subcutaneous fat becomes thinner so that the insulating effect is reduced. This is a long-term response to a hot environment. Frostbite In extreme cold, blood vessels near the skin surface of some body parts may constrict to reduce blood flow to such an extent that cells are damaged in these areas. This results in a condition called frostbite*. Frostbite commonly occurs in the fingers, toes, nose and ears because they are usually exposed to the environment and more heat is lost from them. Symptoms of frostbite include a prickling feeling, numbness and redness in the affected skin (Fig 2.19). Fig 2.19 Frostbitten fingers with red skin frostbite 凍傷 vasodilation 血管舒張 58  2 Regulation of body temperature Ways of increasing heat loss in other animals Different animals may have different ways of increasing heat loss to the surroundings under hot conditions. Below are two examples. a b blood vessel Jackrabbits* live in deserts. They Dogs do not sweat because they are exposed to extreme heat do not have sweat glands (except during the daytime. They have on their feet). To help cool their large ears with an extensive bodies, they pant* to promote network of blood vessels. This the evaporation of water from promotes heat loss from their the mouth and tongue, and by bodies. exchanging warm air in their lungs with cooler external air. C Regulation of body temperature involving skeletal muscles Besides the skin, skeletal muscles also play a part in the regulation of body temperature. When thermoreceptors in the body detect a fall in body temperature, the hypothalamus sends nerve impulses to the skeletal muscles to cause them to contract rapidly and repeatedly. This contraction of muscles is called shivering*, and is involuntary and uncoordinated. During shivering, the rate of respiration in muscle cells is increased in order to release more energy for muscle contraction. This generates more heat to keep the body warm. Fig 2.20 Our body shivers to generate more heat under cold conditions Jackrabbit 黑尾長耳大野兔 pant 氣喘 shivering 顫抖 59 V Human Physiology: Regulation and Control D Regulation of body temperature by hormones Body temperature is also regulated by hormones. Thyroxine* is one of these hormones. It is secreted by the thyroid gland, which is located at the front of the neck, below the larynx (Fig 2.21). Under cold conditions, the thyroid gland secretes more thyroxine. Thyroxine is transported in blood to all parts of the body. It increases the metabolic rate of body cells so that larynx more heat is generated to keep the body warm. A persistent increased thyroid gland* thyroxine secretion is a long-term response to a cold environment. Fig 2.21 Position of the thyroid gland Control of thyroxine secretion The secretion of thyroxine is controlled by thyroid-stimulating hormone (TSH) (or thyrotrophin*) secreted by the pituitary gland. The secretion of TSH is in turn controlled by thyrotrophin-releasing hormone (TRH) secreted by the hypothalamus. A fall in blood temperature causes the hypothalamus to secrete TRH, which stimulates the pituitary gland to secrete TSH. TSH then stimulates the thyroid gland to secrete thyroxine. A high level of thyroxine inhibits the secretion of TRH and TSH to avoid over-production of thyroxine. This works by a negative feedback mechanism. hypothalamus pituitary gland thyroid gland body cells TRH TSH thyroxine thyroxine inhibits the secretions of TRH and TSH (negative feedback) Adrenaline Besides thyroxine, adrenaline* is another hormone that is produced in response to cold. Adrenaline is secreted by the adrenal glands*, which are located on top of the kidneys. Like thyroxine, adrenaline also increases the metabolic rate of the body, thus increasing heat production. The action of adrenaline is widespread. This will be discussed in detail in Ch 3 of this book. adrenal gland 腎上腺 adrenaline 腎上腺素 thyroid gland 甲狀腺 thyrotrophin 促甲狀腺素 thyroxine 甲狀腺素 60  2 Regulation of body temperature Goitre Iodine is required to produce thyroxine. A lack of iodine in the diet reduces thyroxine secretion. Due to the decreased level of thyroxine in blood, the pituitary gland secretes more thyroid-stimulating swollen hormone (TSH) in an attempt to stimulate the thyroid gland to neck restore thyroxine production to normal. Prolonged stimulation by excess TSH causes the thyroid gland to enlarge, resulting in a swelling of the neck (Fig 2.22). This condition is called goitre*. Fig 2.22 A person with goitre E Regulation of body temperature by behavioural means All the actions in response to a fall or rise in body temperature mentioned above are involuntary. Humans also attempt to regulate body temperature by behavioural means which are voluntary (i.e. under conscious control of the cerebrum). Some examples are given below. Under cold conditions Under hot conditions ❶ Wear more clothes ❸ Curl up the body ❶ Wear fewer clothes ❸ Fan the body To trap more layers of To reduce the surface To trap fewer layers of air To increase heat air close to the skin area for heat loss by close to the skin surface loss from the body surface so that less heat convection so that more heat is lost by convection and is lost by conduction by conduction, convection evaporation of sweat and convection and evaporation of sweat ❷ Take in hot food or ❷ Take in cold food or drinks drinks To gain heat from the To lose heat to the hot food or drinks by cold food or drinks conduction by conduction ❹ Stay indoors ❹ Stay in shady places To avoid rapid heat To shield the body loss by convection and from direct sunlight so evaporation of sweat that less heat is gained under windy condition by radiation goitre 甲狀腺腫 61 V Human Physiology: Regulation and Control Behaviour of other animals in extreme cold In order to survive in extremely cold environments, some animals display special behaviour to conserve heat. Below are two examples. a b Emperor penguins living in the Some squirrels hibernate* in Antarctic sometimes huddle* winter. During hibernation, their together in severe winter weather. metabolic rates are kept low to This helps them reduce heat loss, reduce energy consumption. Their and shelters them from strong body temperature is lowered to wind. reduce heat loss. Level 1 1 We may shiver in cold conditions. What is the nature and significance of this shivering response? Nature of response Significance A voluntary reduce heat loss B voluntary increase heat production C involuntary reduce heat loss D involuntary increase heat production p. 59 Level 2 2 Which of the following will occur in the skin if there is a sudden drop in environmental temperature from 20 °C to 5 °C? (1) Sweating decreases. (2) Arterioles in the skin constrict. (3) The layer of subcutaneous fat becomes thicker. A (2) only B (1) and (2) only C (1) and (3) only D (1), (2) and (3) p. 54–56 hibernate 冬眠 huddle 擠在一起 62  2 Regulation of body temperature How is body temperature regulated in humans? The flow chart below summarizes the process of body temperature regulation in humans: To increase heat loss and reduce heat gain Physiological sweating increases ➞ increases heat loss by evaporation vasodilation of arterioles in the skin initiate ➞ allows more blood to flow to capillaries near the skin stimulate responses surface ➞ increases heat loss by conduction, convection and heat loss centre radiation thermoreceptors in of thermoregulatory hypothalamus and erector muscles in the skin relax and hairs lie flat centre in skin ➞ traps a thinner layer of air close to the skin surface hypothalamus ➞ increases heat loss by conduction and convection release of thyroxine decreases (long-term) ➞ lowers metabolic rate of body cells ➞ reduces heat production Structural rise in blood or skin layer of subcutaneous fat becomes thinner (long-term) temperature detected by ➞ reduces insulating effect ➞ increases heat loss by conduction Behavioural e.g. wear fewer clothes, fan the body, etc. body temperature drops normal body temperature negative feedback body temperature rises To reduce heat loss and increase heat gain Physiological sweating decreases ➞ reduces heat loss by evaporation drop in blood or skin vasoconstriction of arterioles in the skin temperature detected by ➞ allows less blood to flow to capillaries near the skin surface ➞ reduces heat loss by conduction, convection and radiation erector muscles in the skin contract to pull hairs upright ➞ traps a thicker layer of air close to the skin surface initiate ➞ reduces heat loss by conduction and convection stimulate responses shivering ➞ increases heat production by skeletal muscles heat gain centre thermoreceptors in release of thyroxine increases (long-term) of thermoregulatory hypothalamus and ➞ increases metabolic rate of body cells centre in skin ➞ increases heat production hypothalamus Structural layer of subcutaneous fat becomes thicker (long-term) ➞ increases insulating effect ➞ reduces heat loss by conduction Behavioural e.g. wear more clothes, take in hot food, etc. 63 V Human Physiology: Regulation and Control 2.4 Heat- and cold-related illnesses A Heat exhaustion and heatstroke It is likely that you have heard of people suffering from heatstroke* in the summer, usually when they have been working or doing activities outdoors. Hong Kong’s summer is very hot and humid. When we are exposed to such an environment for a long time, our body may fail to lose heat effectively through convection, conduction, radiation and evaporation of The body even gains sweat. This situation becomes worse when there is no wind. heat if the environmental temperature is higher than When this happens, our body temperature rises and heat exhaustion* body temperature. may occur. Symptoms of heat exhaustion include dizziness, headache, nausea and shortness of breath. If the body temperature rises to 41 °C or above, heatstroke occurs and the sufferer may become unconscious. Heatstroke can cause death if not properly treated. To prevent heat exhaustion or heatstroke, we should take the following measures in a very hot environment: In a very hot environment Do’s Don’ts Wear light-coloured, loose Don’t do vigorous exercise and clothing to reduce heat absorption prolonged activities such as hiking and facilitate sweat evaporation Drink plenty of water to prevent dehydration Don’t drink alcoholic drinks and caffeinated beverages (e.g. coffee and tea) Choose an indoor venue with Why should we avoid good ventilation ? drinking alcoholic drinks and caffeinated beverages in a hot Stop activities immediately if environment? feeling unwell heat exhaustion 熱衰竭 heatstroke 中暑 64  2 Regulation of body temperature B Hypothermia When the environment is cold, body heat may be lost faster than it is produced. This results in a fall in body temperature. If body temperature falls below 35 °C, hypothermia* occurs. Hypothermia can make a person sleepy and confused. It can also cause an irregular heartbeat, leading to heart problems and death. Below are some measures to prevent hypothermia. In a cold environment Do’s Don’ts Put on adequate clothing Don’t stay in cold or windy environments for long Take in hot food and drinks that are high in calories, like rice and Don’t drink alcoholic drinks hot milk because alcohol promotes heat loss by causing vasodilation in the Keep the home environment skin warm but well-ventilated Prevention of hypothermia in marathon runners Look at Fig 2.23. The marathon runner has wrapped herself in a silvery blanket right after finishing a race. Why did she do this? Once a runner has stopped running, his/her body produces less heat and starts to cool down. The evaporation of sweat from the skin and the wet clothes quickly remove heat from the body. Runners may suffer from hypothermia if they do not change their clothes shortly after running. To avoid the rapid loss of body heat, runners often wrap themselves in a special blanket called a space blanket after running. This blanket is made of material that was developed by NASA. It can help prevent heat loss from the body through conduction, convection and radiation. The space blanket is also commonly used in an emergency to keep a victim’s body warm (Fig 2.24). Fig 2.23 A marathon runner wrapped in Fig 2.24 An injured person covered by a space a space blanket blanket hypothermia 體温過低 65 V Human Physiology: Regulation and Control Learning through examples Skill builder Skill practice Mr Chan is a construction worker. He worked outdoors on a very hot sunny day. The table below shows some weather information from that day. Time (hour) Air temperature (°C) Relative humidity (%) Wind speed (km/h) 10:00 34 70 15 12:00 38 75 15 14:00 38 85 12 16:00 34 60 22 Mr Chan started working at 10:00 and finished at 16:00. a i Mr Chan’s face turned red and his body became wet after he had been working for an hour. Explain why. (5 marks) ii State the significance of the responses in i. (2 marks) b At what time was Mr Chan at the highest risk of heatstroke? Explain your answer. (6 marks) c Mr Chan drank plenty of water while he was working. What is the importance of doing this? (1 mark) Suggested answers a i While working, a lot of heat was produced in his body. 1 His body also gained heat from the sun by radiation. 1 The arterioles in the skin dilated, 1 Remember that vasodilation occurs in allowing more blood to flow to the capillaries near the skin surface. 1 the arterioles, not the Therefore, his face turned red. capillaries. Meanwhile, the sweat glands secreted more sweat. 1 This made his body wet. ii The responses increased heat loss from the body 1 so as to maintain a stable body temperature. 1 b At 14:00. 1 The air temperature was higher than body temperature. 1 Identifying The body could only lose heat through evaporation of sweat. 1 the effect of environmental However, the relative humidity was at its highest at this time 1 factors on body and the wind speed was at its lowest. 1 heat gain/loss The evaporation of sweat became very slow, 1 Refer to p. 67. leading to a rise in body temperature. Heatstroke was therefore more likely to occur. c To replenish the water loss due to sweating. 1 66  2 Regulation of body temperature Learning through examples Skill builder Skill practice Identifying the effect of environmental factors on body heat gain/loss The amount of heat gain/loss of our body is closely related to environmental factors such as air temperature, relative humidity and wind speed. In general, air temperature ➝ temperature gradient between the body and the environment ➝ heat loss ➔ ➔ by conduction, convection and radiation ➔ wind speed ➝ heat loss by convection and evaporation ➔ ➔ relative humidity ➝ heat loss by evaporation ➔ ➔ Take the weather information on p. 66 as an example. Time (hour) Air temperature (°C) Relative humidity (%) Wind speed (km/h) 10:00 34 70 15 ❶ 12:00 38 75 15 ❷ 14:00 38 ❸ 85 12 16:00 34 60 22 ❶ Heat loss from the body at 10:00 is greater than at 12:00 because: the air temperature is lower, making the temperature gradient between the body and the environment steeper. Thus, more heat is lost by conduction, convection and radiation. the relative humidity is lower. Thus, more heat is lost by evaporation of sweat. ❷ Heat loss from the body at 16:00 is greater than at 10:00 because: the relative humidity is lower. Thus, more heat is lost by evaporation of sweat. the wind speed is higher. Thus, more heat is lost by convection and evaporation of sweat. ❸ When the air temperature is higher than body temperature, the body cannot lose heat by conduction, convection and radiation. It can only lose heat by evaporation of sweat. Learning through examples Skill builder Skill practice A woman practises hot yoga* in a well-ventilated room which is kept at 40 °C. The relative humidity in the room is set at 40%. Why would it be dangerous if the ventilation fan in the room was switched off and the relative humidity was set at a higher level? (5 marks) Q11 (p. 72), Q18 (p. 75) hot yoga 高温瑜伽 67 V Human Physiology: Regulation and Control What are heat exhaustion, heatstroke and hypothermia? Heat exhaustion, heatstroke and hypothermia are illnesses related to the body’s inability to regulate body temperature. Heat exhaustion and heatstroke result from prolonged exposure to high temperatures while hypothermia results from prolonged exposure to low temperatures. Recall Think about... (p. 45) 1 The hypothalamus. 2 During shivering, the rate of respiration in muscle cells increases in order to release more energy for muscle contractions. This generates more heat to keep the body warm. Suggested answers to ? p. 64 Alcohol and caffeine are diuretic. They increase our urine output, thus speeding up water loss from the body. 68  2 Regulation of body temperature Key terms 1 dermis 真皮 11 pituitary gland 垂體 2 epidermis 表皮 12 shivering 顫抖 3 erector muscle 豎毛肌 13 skin 皮膚 4 hair follicle 毛囊 14 subcutaneous fat 皮下脂肪 5 heat exhaustion 熱衰竭 15 sweat gland 汗腺 6 heat gain centre 產熱中樞 16 thermoregulatory centre 體温調節中樞 7 heat loss centre 散熱中樞 17 thyroid gland 甲狀腺 8 heatstroke 中暑 18 thyroxine 甲狀腺素 9 hypothalamus 下丘腦 19 vasoconstriction 血管收縮 10 hypothermia 體温過低 20 vasodilation 血管舒張 Concept map Regulation of body temperature in humans controlled by thermoregulatory centre in hypothalamus works by negative feedback mechanism involving skin skeletal muscles thyroid gland cerebrum under cold structures involved secretes conditions blood vessels shiver thyroxine brings sweat glands leads to increases about erector muscles production metabolic and hairs of more heat rate subcutaneous behavioural fats change 69 V Human Physiology: Regulation and Control Exercise Section 2.3 Level 1 Section 2.2 MC 4 DSE IS 2019 IIA Q10 Level 1 The figure below shows a vertical section of the Directions: Questions 1 and 2 refer to the human skin. photomicrograph below, which shows a section of the human skin: 3 P R 1 Q 2 4 (×10) Which of the labelled structures are effectors for body temperature regulation? MC 1 Which structure reduces heat loss from the A P and Q only body? B P and R only C Q and R only A 1 B 2 D P, Q and R p. 54 C 3 D 4 p. 51 MC MC 5 DSE IS 2014 IIA Q7 2 Which structure secretes an oily substance that prevents bacterial growth? When Amy walks from a very hot outdoor A 1 B 2 environment into a very cold room, which of C 3 D 4 p. 50 the following responses will take place in her body? 3 Complete the following paragraph with suitable (1) The erector muscle relaxes. words. (6 marks) (2) Sweating stops. (3) The blood vessels supplying blood to the The skin is composed of two main layers. The skin surface constrict. outer layer is called the (a). It A (1) and (2) only mainly serves to (b) the body. B (1) and (3) only The inner layer is called the (c). C (2) and (3) only The sweat gland in it helps regulate body D (1), (2) and (3) p. 54, 55 temperature by secreting (d) , which passes along the (e) to the skin surface. In this inner layer, there are also (f) which can detect stimuli such as pain and temperature change. p. 49, 50 70  2 Regulation of body temperature MC 6 CE Bio 2011 I Q7 a–c 8 DSE IS 2012 IIA Q4 E1-1 E1-2 The Hong Kong Observatory issues ‘Very Hot In an experiment, a person stays in a room Weather Warning’ in very hot weather to alert kept at 10 °C for 1 hour. His core and skin the public. The following is an extract from a temperatures are shown below: pamphlet: Actions to be taken for Very Hot Weather Warning core temperature skin temperature = 37 °C = 28 °C 1. When engaged in outdoor work or activities, drink plenty of water. If you do not feel well, take a rest in Which of the following best explains a shady place as soon as the difference between his core and skin possible. temperatures? 2. Avoid prolonged exposure to sunlight. A The sweat glands stop producing sweat. B The erector muscles contract to pull the a With reference to one response of the body hairs up. when a person is engaged in outdoor work C The arterioles supplying blood to the skin in very hot weather, explain the importance constrict to supply less blood to the skin. of drinking plenty of water. (2 marks) D The metabolic rate of the body becomes b How can staying in a shady place help lower at a low environmental temperature. people working outdoors in regulating their p. 54, 55 body temperature? (2 marks) c People working outdoors are often exposed 9 DSE IS 2013 I Q2c to strong sunlight. State one health hazard The change in Betty’s blood glucose level due to their prolonged exposure to strong between 12:00 and 17:00 is shown in the graph sunlight. Suggest a way of reducing the risk below. She had a carbohydrate-rich lunch at of this hazard. (2 marks) 13:00 and started jogging at 15:00 for one p. 57, 64 hour. 140 blood glucose level Level 2 130 (mg dL–1) MC 120 7 DSE IS 2017 IIA Q6 110 Which of the following stated effects of the 100 90 thermoregulatory responses is incorrect? 12:00 13:00 14:00 15:00 16:00 17:00 Thermoregulatory Effect time of day response A sweating increases heat loss With reference to a structure in the skin, B vasoconstriction reduces heat loss explain how this structure increases heat loss C shivering increases heat gain during jogging. (2 marks) D contraction of the reduces heat gain p. 57, 58 erector muscle p. 54, 55, 59 71 V Human Physiology: Regulation and Control 10 Edexcel GCE (A) 2011 Air temperature (°C) Marathon runners can have difficulty with 24 28 32 36 thermoregulation over the course of a 26 mile 40 24 29 34 39 Relative humidity (%) race, particularly on a hot day. Two marathon 50 24 29 35 41 runners, A and B, had their core temperatures recorded during a race. 60 25 30 36 41 The graph below shows the core temperatures 70 26 31 37 42 recorded during the race. 80 26 32 37 44 42 90 27 32 38 45 runner A 41 100 27 33 39 46 core temperature (°C) 40 Note: Real feel temperatures are highlighted in 39 runner B grey. 38 a Describe the effect of relative humidity 37 on the real feel temperature. Explain this phenomenon with reference to the 36 effectiveness of heat loss from the body. 0 20 40 60 80 100 120 140 160 (3 marks) time since start of race (min) b How does the effect described in a change a Suggest an explanation for the change in at higher air temperatures? Explain core temperatures of both runners in the this phenomenon with reference to the first 30 minutes of the race. (2 marks) effectiveness of heat loss from the body. b Suggest an explanation for the constant Hint (p. 76) (3 marks) core temperatures of both runners between c Other than relative humidity and air 60 and 100 minutes of this race. (5 marks) temperature, suggest two environmental c During this race, runner A lost 3.02 kg of factors that may affect the real feel water and runner B lost 2.43 kg of water. temperature. Explain your answer. Using the information in the question and (4 marks) your own knowledge, suggest reasons for d People who are engaged in outdoor work the change in core temperature of runner A or activities are advised to drink plenty of after 120 minutes. (2 marks) water when the Hong Kong Observatory p. 57 issues the Very Hot Weather Warning. Based on your biological knowledge, 11 DSE Bio 2018 II Q1b suggest an explanation for this advice. Nowadays, many weather forecast apps list ‘real (1 mark) feel’ temperature in addition to air temperature. p. 57, 61, 64 The real feel temperature is the temperature which takes into account multiple factors influencing the effectiveness of heat loss from the human body. The higher the effectiveness, the lower is the real feel temperature. The table on the right shows the real feel temperatures at different air temperatures and relative humidities (other environmental conditions remain the same): 72  2 Regulation of body temperature 12 DSE IS 2016 I Q2b, c Level 3 MC Sophia wakes up early. She visits a theme park 13 DSE IS 2016 IIA Q9 after taking her breakfast. Contraction of the erector muscles in the skin a It is a hot sunny day. After Sophia rushes is one of the responses in body temperature for a bus, her face turns red and her regulation. Which of the following correctly forehead sweats. states the effect of this response and the i Figure 1 shows a vertical section nervous system controlling the response? of skin. Which labelled parts are responsible for bringing about these Effect Nervous system responses? Explain your answer. controlling this (3 marks) response A increases heat autonomic nervous production system B increases heat voluntary nervous C production system C reduces heat loss autonomic nervous A D system D reduces heat loss voluntary nervous system p. 53 B 14 A marathon runner ran a race on a very hot Figure 1 day. His body temperature was recorded during ii Which coordinator is involved in the race. The graph below shows the result. br