detecting temperature change chapter 10; p309. regulating heat exchange heat exchange – heat...
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Detecting temperature change
Chapter 10; p309
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Regulating heat exchange
• Heat exchange – heat transfer between the internal and external environment.
• Factors in regulating heat exchange are:– Effective temperature difference between internal
and external environment– Area of exposed surface– Efficiency of insulation– Physiological changes involving evaporation and
circulatory changes.
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Detecting temperature change
• In the hypothalamus of endotherms, there are temperature sensitive cells which act as misalignment detectors.
• Misalignment detectors – trigger homeostatic responses if blood temperature deviates from the optimal range, or set point.
• Skin receptors act as disturbance receptors – detecting external temperature changes and triggering responses.
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Maintaining within the optimal range
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The hypothalamus and temperature regulation
• The hypothalamus has a large number of temperature-sensitive receptors (75% heat, 25% cold.
• It is the temperature control centre as it receives information about temperature changes from other parts of the body.
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Producing heat through metabolism
• An increase in metabolism increases body temperature.
• The hypothalamus, anterior pituitary and thyroid play a central role in temperature regulation.
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The hypothalamus and metabolism
1. The hypothalamus produces thyrotropin releasing hormone (TRH) and is transported to the anterior pituitary.
2. TRH stimulates the secretion of thyroid-stimulating hormone (TSH) which is transported to the thyroid.
3. TSH stimulates the secretion of thyroxine which increases the metabolic rates of cells in the body.
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Hormone pathway for temperature regulation
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Heat Balance
Heat is gained or lost in 4 ways:• Conduction: is the transfer of heat energy
from a hotter object to a cooler object with which it is in contact.
• Convection: is the transfer of heat by means of air or water. Air and water movements help distribute heat energy and speed up loss of heat by conduction and evaporation.
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Heat Balance
• Evaporation: is the change in state of water from liquid to vapour – heat is required for this to occur.
• Radiation: is the transfer of heat from a hot body by means of infrared waves.
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Heat loss
• When the surrounding environmental temperature is lower than body temperature, animals tend to lose heat by radiation, evaporation, conduction and direct loss in body fluids.
• If the surrounding temperature is higher than the body temperature, heat can be lost by evaporation by sweating, panting, licking fur.
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Heat gain• Heat is generated by metabolism in cells.• Endothermic animals have insulation and
physiological mechanisms that allow them to retain heat.
• Adopting behavioural patterns can enable them to absorb heat from the environment through radiation and conduction – common behaviour in ectotherms.
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Heat gain and lossLizards being ectothermic lose heat overnight and need to gain heat by lying in the sun.
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Summary of heat exchange processes
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Behavioural regulation and heat exchange
• Limiting heat loss:– e.g. curling into a ball to reduce the surface area available for heat loss, huddling together, changing posture
• Avoiding heat gain:– Regulating temperature by moving out of the sun or
underground during hot weather– Nocturnal behaviour - restricting activity to nights
when the temperature is lower
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Penguins huddle together to reduce exposed surface area and create wind breaks.
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Basking in the sunThe Black Snake Fluctuations in body temperature are a direct result of exposure to the sun.
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Structual Adaptations: Insulation
• Endotherms have fur, feathers, fat layers for insulation which reduce heat exchange with their environment.• Feathers and fur trap a layer of warm air next to the skin and the external environment.
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Physiological regulation
• Regulation of the temperature gradient between the body and the environment involves:– Circulatory changes - e.g. selective constriction or
dilation of blood vessels in the skin and in heat-generating organs, such as muscles.
– Piloerection – hair stands on end and acts an insulating layer.
– Countercurrent arrangements of blood vessels that cause body heat to be retained within tissues
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Physiological responses to heat gain and loss
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Countercurrent exchange
• Countercurrent exchange involves two fluids (often blood) which pass closely by one another in opposite directions.
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Countercurrent heat exchange through blood
Explanation:• Blood coming from the heart (through arteries) is
warmer as it is nearer the body’s core. • Blood returning from the extremities (e.g. Penguin
flippers) is cold because it is more exposed to the environment.
• Because the warmer blood in the arteries is in close proximity to the cooler veins a diffusion gradient is maintained along the length of the tubes.
• This means some heat is transferred from the warmer blood in arteries to the cooler blood in veins, heating the cooler blood returning to the heart.
• This means the core temperature does not reduce as much and less energy is used to maintain constant internal temperature.
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Hibernation, metabolic rate and body temperatureThe ground squirrel.Note: Metabolic rate and temperature drop significantly.
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INTERNAL OR EXTERNAL SOURCES OF HEAT
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Ectotherms
• Ectotherms – have limited ability to generate their own internal heat.
• Use less energy than endotherms• Periods of inactivity hibernation• Some change their behaviour to maintain stable
body temperature. E.g. Reptiles basking in the sun.
• Poikilotherms – have fluctuating body temperatures. (e.g. Reptiles)
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Heterotherms
• Heterotherm - A type of ectotherm which has some ability to produce its own heat and increase its body temperature.
• E.g. Sharks and tuna which are able to maintain internal temperatures 14⁰ degrees above the surrounding water.
• The advantage of being a heterotherm is that they can be more active when it’s cold.
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Endotherms
• Endotherms – have the ability to produce their own internal heat.
• Generally have fur, feathers or fat layers for insulation.
• Physiological and behavioural mechanisms to regulate loss of metabolic heat.
• Homeotherms – have constant internal temperatures.
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The Polar BearThe polar bear can live in extreme cold with thick fur and insulating fat.
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Endotherms (continued..)
• The ratio of surface area to volume poses a challenge for endotherms, as greater surface area increases heat loss to the environment.
• Endothermic animals generally have a relatively constant core temperature which needs to be maintained within narrow limits.
• Humans 36⁰-38⁰C.
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Platypus (Monotremes)Monotremes have a core temperature of about 32⁰.The figure shows how Monotremes core temperature remains relatively stable during the day despite swimming in freezing water.
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