Detecting temperature change

Chapter 10; p309

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.

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.

Maintaining within the optimal range

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.

Producing heat through metabolism

• An increase in metabolism increases body temperature.

• The hypothalamus, anterior pituitary and thyroid play a central role in temperature regulation.

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.

Hormone pathway for temperature regulation

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.

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.

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.

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.

Heat gain and lossLizards being ectothermic lose heat overnight and need to gain heat by lying in the sun.

Summary of heat exchange processes

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

Penguins huddle together to reduce exposed surface area and create wind breaks.

Basking in the sunThe Black Snake Fluctuations in body temperature are a direct result of exposure to the sun.

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.

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

Physiological responses to heat gain and loss

Countercurrent exchange

• Countercurrent exchange involves two fluids (often blood) which pass closely by one another in opposite directions.

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.

Hibernation, metabolic rate and body temperatureThe ground squirrel.Note: Metabolic rate and temperature drop significantly.

INTERNAL OR EXTERNAL SOURCES OF HEAT

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)

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.

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.

The Polar BearThe polar bear can live in extreme cold with thick fur and insulating fat.

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.

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.