Health Sciences I: Heat Energy & Body Temperature Control MEDF1011 PDF

Health Sciences I Heat Energy & Body Temperature Control MEDF1011 Dr. Yuhuan ZHOU School of Biomedical Sciences, Faculty of Medicine [email protected] Learning Outcomes After completing this lecture, students should be able to: ✥ distinguish between heat and temperature ✥ explain the...

Health Sciences I Heat Energy & Body Temperature Control MEDF1011 Dr. Yuhuan ZHOU School of Biomedical Sciences, Faculty of Medicine [email protected] Learning Outcomes After completing this lecture, students should be able to: ✥ distinguish between heat and temperature ✥ explain the significance in the properties of water for having high specific heat capacity, latent heat of fusion and latent heat of vaporization ✥ describe the 4 processes of heat transfer between the human body and the environment, and explain how these can be changed ✥ distinguish between core and shell body temperature, and how they are measured ✥ describe the organization of body temperature control and how it illustrates the basic principle of feedback / homeostatic control. ✥ give examples of thermoregulatory responses that result in heat gain / conservation or heat loss. ✥ describe the proposed biochemical and physiological mechanisms of fever ✥ recognize the availability of various devices for measuring body temperature and discuss their use and limitations  Kinetic Theory of Matter Kinetic theory of matter Matter ✥ A matter consists of many small particles (atoms / ✥ It can be a type of gas / liquid / solid ions / molecules) in constant motion Properties of 3 types of matters Properties Separation Attractive force Definite shape & size Matter between particles Co Gas Far apart Weak No ool ol C Yes t Ho Liquid Relative far apart Strong (shape of the container) Ho t Solid Close Very strong Yes  Heat Energy Heat Low temperature High temperature ✥ A form of energy ✥ Caused by internal motion of For matter in gas state: the particles (kinetic energy) v higher kinetic energy of the ✥ The extent of motion is Energy transfer particles à higher pressure proportional to the (volume of the container temperature unchanged) § Higher temperature à Higher kinetic energy à Higher motion à Heat transfer à transfer of kinetic energy  Temperature & Temperature Scales Celsius scale Temperature Different temperature scales ✥ An objective measure of the degree Boiling point of 1. Celsius scale (o C) H2O of hotness or coldness 2. Fahrenheit scale (o F) § Non-SI unit, mainly used in USA 3. Kelvin scale (K) Measurement of temperature § SI Unit ✥ Using certain physical properties of matters: § Absolute 0 on the Kelvin scale § e.g. freezing / boiling point of water à to represents the predicted provide a temperature scale (i.e. coefficient temperature at which particles of of expansion) 37℃ matter have 0 kinetic energy Normal body temperature Conversation of different temperature scales Conversion of different scales Freezing point of ✥ oF = oC*1.8 + 32 H2O ✥ oK = oC + 273  Specific Heat Capacity of Water Specific heat capacity of different substances Specific heat capacity ✥ The amount of energy required to heat up 1 kg of a substance by 1 degree Oceans in Hong Kong (Celsius / Kevin) Heat capacity of water ✥ Comparing to other substances, water has a high specific heat capacity Useful implications of water (high heat capacity) ✥ Stabilizing temperature § e.g. Water makes up 60% of our body weight à less fluctuation of body temperature ✥ Storing heat energy and transporting it around § e.g. Blood circulating in our body  Change the State of A Matter Melting / fusion Vaporization Solid Liquid Liquid Gas Freezing / Condensation solidification Evaporation Changing the state of a matter ✥ A form of vaporization ✥ It happens when temperature increases or decreases, so the particles in the ✥ Occurs on the surface of a liquid when it matter gain or lose kinetic energy changes into gas phase at a temperature below the boiling point Latent Heat of Fusion & Vaporization Solid Liquid Latent heat of fusion ✥ The extra energy required to break the cohesive bonding between the molecules in a solid state à allow them to turn into liquid state ✥ No temperature change until a phase change is complete Liquid Gas Latent heat of vaporization ✥ The extra energy required to break the attraction force between the liquid molecules à allow them to turn into gas state Specific Latent Heat of Fusion or Vaporization Specific latent heat of fusion / vaporization ✥ The heat energy required to change 1 kg of a substance from solid to liquid / from liquid to gas without any temperature change Heat capacity of water is high Water has relatively high specific latent heat of fusion and vaporization 1 Significance ✥ As sweat production in body temperature regulation when body heat generation increases (latent heat of vaporization) 2 Significance ✥ As polar ice caps in slowing down global warming (latent heat of fusion) and dampening its fluctuations  Processes of Heat Transfer 2 1 Convection Conduction ✥ The process heat transfer occurs when bulk ✥ The process heat transfer occurs through an flow of a fluid (gas / liquid) carries heat with object / between objects in direct contact down the flow of matter in the fluid a temperature gradient without any flow of the material Convection 1 2 Conduction Convection 3 Radiation 3 Radiation ✥ The process heat transfer occurs through space without contact by electromagnetic waves called infra-red radiation Factors Affecting Heat Transfer by Radiation Surface temperature Shiny / light surfaces Dull / black surfaces ✥ As the surface temperature increases, emission of radiant heat increases Poor emission & Good emission & Radiant absorption absorption heat Can be reflected / blocked by shielding Polar ice caps ✥ Reflecting radiant heat from the sun back Foil Shielding into outer space and reducing global ✥ Reducing radiant & ✥ Radiant heat travels in a warming convective heat loss from straight line à can be the body blocked by shielding Working Principle of A Thermal Flask: Application of Heat Transfer Processes How is heat transfer minimized in a thermal flask? 2 1 3 4  Core & Shell Body Temperature Core temperature ✥ Vital organs in the thoracic and Body core & shell temperature abdominal cavities, and brain in the ✥ Varies due to the change of ambient cranium temperature (environmental temperature) Shell temperature ✥ The peripheral parts of the body § e.g. skin etc. In the cold environment ✥ The boundary of the core In the hot environment temperature shrinks to preserve ✥ The core temperature of 37°C heat in the vital organs extends almost to the ✥ The temperature of the limbs falls surface of the body § The most distal regions has the greatest temperature drop  Heat Exchange Between Our Body and the Environment: Heat Transfer Processes 4 processes of heat loss (in descending order of relative contribution): 2 Evaporation 3 ✥ Through lungs & skins Convection ✥ A unidirectional heat ✥ Air current flowing transfer only over body surface 1 Radiation ✥ Contributing to 60% of body heat loss ✥ Body surface temperature 4 Conduction ✥ Contacting objects Heat Exchange Between Our Body and the Environment: the 2 Steps ✥ The heat transfer between: § the central core of the body 1 § the peripheral shell (the skin) ✥ Occurs entirely through convection The heat exchange between the body and is regulated by the cutaneous and the environment can be viewed as a blood flow 2-step process: ✥ The heat transfer between: 2 § the body shell § the exterior ✥ Occurs through conduction, convection, radiation, and evaporation  Heat Transfer Between the Body Core & Shell Heat transfer between the core and the shell (e.g. the skin) of the body ✥ Occurs entirely through convection and is regulated by the cutaneous blood flow 1 When the cutaneous blood vessels dilate: ✥ e.g. in the hot environment ✥ more blood gets closer to the skin surface à carrying heat from the body core to the body shell 2 When the cutaneous blood vessels constrict: ✥ e.g. in the cold environment ✥ blood is diverted through arterio-venous anastomoses à less blood flows to the skin surface, and the heat remains trapped in the body core  Heat Transfer Between the Body Shell & the Exterior The transfer of heat from the shell to the exterior occurs through: Radiative heat loss / gain ✥ conduction, convection, radiation, and ✥ Depends on: evaporation § the surface temperature of our body Evaporative heat loss § the surrounding objects in the ✥ Regulated by the amount of sweating external environment Convective heat loss / gain ✥ Determined by the movement of air over the skin: § e.g. the wind / a fan § when air movement / The conductive heat loss / gain convection proceeds ✥ Depends on: slowly: § temperature gradient between § the layer of air trapped by the skin and the medium in contact body hairs next to the skin provides good § the thermal qualities insulation e.g. Specific heat capacity of the substance Thermal Balance of Our Body Heat generated in the body 1 v Evaporation v Radiation v Conduction 2 Heat gained from the environment v Convection Maintenance of a constant body temperature is possible: ✥ when the net heat lost from the body closely matches the amount of heat generated by the body and any heat gained from the environment  The Need for Thermoregulation Our body functions within a narrow temperature range ✥ Required by the numerous enzymatic / metabolic reactions § i.e. enzymes (normal protein functions) § Operate at optimal temperatures of around 37oC ✥ Failure in thermoregulation will affect the biochemical reactions and disrupt cellular activities Concept of Homeostasis Homeostasis ✥ Our body control in maintaining a stable internal environment ✥ Mainly maintained by: § the nervous system § the endocrine system Components of A Homeostatic Control System 3 components: 1 ✥ Sensor / receptor ✥ Integrating / control center ✥ Effector A homeostatic control system (or a feedback control loop) contains: 2 pathways: 2 ✥ Afferent (sensory) pathway ✥ Efferent (motor) pathway Feedback / Homeostatic Control of Body Temperature Structural & Functional Organization of the Nervous System Cerebral hemisphere, diencephalon, brainstem, cerebellum, spinal cord Analysis & integration of sensory & motor information Sensory Motor component component Visceral motor system Sensory ganglia & nerves Somatic (Sympathetic & motor system Parasympathet Sensory receptors Motor nerves ic division) (at surface & within the body) Autonomic ganglia & nerve Effector Internal & external Smooth muscle, environment Skeletal muscle cardiac muscle & glands Organization of Body Temperature Control  Thermoreceptors Central thermoreceptors ✥ Present in the hypothalamus Peripheral / cutaneous § Sensing the intracranial thermoreceptors temperature ✥ Present in the skin ✥ Present in the abdominal § Sensing the skin / viscera / around the spinal ambient cord & great veins (environmental) § Sensing the core temperature temperature Peripheral / cutaneous thermoreceptors ✥ Number of cold receptors > warm receptors in the skin § Providing cold-sensing information ✥ Peripheral thermoreceptors are mainly concern with detecting cool temperature for driving anticipatory thermoregulatory responses  Thermoregulatory Center Thermoregulatory center ✥ The thermoregulatory control center locates in the preoptic area of the hypothalamus (anterior hypothalamus) ✥ Based on the integration of sensory inputs, it coordinates the thermoregulatory responses via the posterior hypothalamus: 1 Heat gain (production / conservation) centre ✥ Locates in the posterior nucleus 2 Heat loss (dissipation) centre ✥ Locates in the anterior nucleus Physiological & Behavioral Thermoregulatory Responses 1 1 Behavioral responses 3 Physiological responses 2 ✥ Autonomic nervous system (sympathetic nervous system) § Control of skin blood vessel 4 diameter § Sweating § Piloerection / pilorelaxation § Non-shivering thermogenesis 2 3 ✥ Somatic nervous system § Reflex control of skeletal muscle contraction in shivering 4 ✥ Endocrine system § Epinephrine (adrenaline) § Thyroid hormone Thermoregulatory Responses  Hormones in Physiological Thermoregulation ✥ In human, significant effects of non-shivering thermogenesis are probably restricted to infants with more abundant brown fat, but less important in adults with little brown fat limited only to upper chest and neck Mechanisms of Fever Fever ✥ Develops with a normal thermoregulatory mechanism § Operating a higher set point in response to pathologic conditions (e.g. infection) ✥ Not related to exposure to hyperthermic conditions (hot environment) Exogenous pyrogens § Increased heat gain ✥ e.g. bacterial endotoxin - lipopolysaccharides in the cell wall of gram-negative bacteria 1 ✥ Other products from microorganisms, foreign antigens, etc. Pyrogen ✥ The chemical substances causing fever Endogenous pyrogens ✥ e.g. Pyrogenic cytokines produced by our immune 2 system such as interleukin-1 & 6 (IL-1, IL-6) , and tumor necrosis factor (TNF)-alpha etc. Mechanisms of Fever Mechanisms of fever Mechanism of fever ✥ The pyrogens promote prostaglandin E (PGE2) production in the hypothalamus § Raising the set-point of the hypothalamic temperature-regulating center to a higher level § à Body heat conservation and production continue until the body temperature becomes higher § à Resulting in fever The high body temperature: ✥ Assumed to suppress the growth of some pathogens ✥ But could also cause harm to the host Relief of fever by paracetamol / acetaminophen ✥ The antipyretic effect ✥ Mechanism: inhibition of prostaglandin synthesis  The Course of An Episode of Fever Time course of a typical fever ✥ Onset of fever - pyrogens Heat producing & Heat dissipation § Causing an increase in the hypothalamic temperature set- conservation point § Creating an error signal to activate heat-producing and heat-conservation mechanisms of the body: e.g. skin vasoconstriction & shivering Pyrogens increase Fever cessation hypothalamic decreases Time course of a typical fever temperature set-point hypothalamic temperature set-point ✥ Hypothalamic temperature set-point returns to normal when fever “breaks” § The high body temperature is lowered back to normal through heat dissipation: § e.g. skin vasodilation & sweating Normal Variations in Body Temperature Factors causing normal variations of body temperature ✥ Circadian rhythm Menstrual cycle in women ✥ Seasonal ✥ Body temperature rises in the luteal phase ✥ Menstrual cycle in women § Due to progesterone increases the set- § Basal body temperature rises in luteal phase point of thermoregulatory control ✥ Exercise / emotion Measure Body Temperature: Core or Shell Temperature? Shell / peripheral temperature ✥ The surface temperature of the body Features of shell temperature ✥ Affected by the surrounding temperature ✥ Mostly below the core temperature § At the body extremities, shell temperature can be considerably lower than core temperature ✥ Assessment body core temperature from the surface of the body, need to choose: § A representative site § An appropriate device Sites of Measuring Temperature Intracranial temperature ✥ Ear / tympanic membrane (anterior tympanic artery) Core temperature ✥ Oral cavity (sublingual artery) § ~ 0.5oC lower than rectum ✥ Rectum (inside body cavity) ✥ Vagina (inside body cavity) Shell temperature ✥ Armpit (axillary artery) § Closest to core temperature ✥ Over the forehead (skin temperature) ✥ Over the temporal artery Temperature Measurement Device Clinical thermometer ✥ Mercury-in-glass / alcohol-in-glass thermometer ✥ Mostly replaced by other devices due to the danger of breakage and toxicity of the mercury A thermistor (electronic thermometer) ✥ Giving a digital readout ✥ The probe can be modelled for placing in some parts of the body (e.g. rectum) and connecting to a recording device for continuous measurement Temperature Measurement Device Infrared tympanic membrane thermometer ✥ An accurate & quick measurement ✥ Detects infrared radiation emitted from tympanic membrane supplied by anterior tympanic artery ✥ Best represents intracranial temperature § Close to the hypothalamic thermoregulatory center Liquid crystal thermometer ✥ Skin temperature taken on the forehead ✥ Limited use since skin temperature is lower Temperature Measurement Device Infrared thermometer/ body temperature scanner ✥ Non-contact (patients with highly infectious ✥ Gives a digital display of temperature diseases) ✥ Gives pseudo-colors in representing the isothermal ✥ Quick and simple to use regions ✥ But the skin temperature is lower and affected by environment conditions Thank you for attending! Have a good day! Acknowledgement Slides courtesy: Prof. Simon AU