Four methods of heat transfer in and out of the body are conduction, convection, radiation and evaporation. Conduction is the transfer of heat between objects that are in direct contact with each other. For instance, if a person sits on the cold ground, heat moves from the body to the cold ground. Convection is the transfer of heat by the movement of air or liquid moving past the body. This explains why a breeze across the skin may cool one down, whereas trapping air inside clothing keeps the body warm. Radiation is the transfer of heat energy via electromagnetic waves through a vacuum. Conduction, convection, and radiation causes both heat loss and heat gain to the body, evaporation is a mechanism of heat loss only, in which a liquid is converted to a gas. Perspiration evaporating off the skin is an example of this heat loss mechanism (Hewitt, P 2002, Conceptual Physics, Prentice Hall, New Jersey).
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Ectotherms, Endotherms and Homeotherms:
(Nature Education, 2012).
Most animals are not able to regulate their body temperatures precisely. Non-vertebrates, fish, amphibians and reptiles, are around the same temperature as their environment. This is because they receive their heat from the sun and so are called ectotherms, meaning outside heat, or poikilotherms, meaning variable temperature. Because ectotherms use environmental energy and behaviour adaptations to regulate body temperature, and the temperature does not stay the same but varies, the ambient temperatures can be much higher or lower than that of endotherms (Kunz, Thomas 2008).
A endotherm is any organism that can control its internal temperature above that of its external temperature, which for humans is approximately 37°C. Mammals and birds can also be described as homeotherms, which means their temperature remains constant despite varying temperatures of the environment.
Why Thermoregulation is important:
The temperature of an organism is important because in order for its metabolic processes to work at its optimal rate its temperature cannot become too high or the enzymes denotate, and not too low or it will slow down reaction. A rise of just 2 °C will cause disruption to the internal functioning of a human and should the temperature rise between 43 °C and 45 °C, death may occur. The temperature needs to fall below 23 °C to cause death, which is a much greater tolerance than the high temperature (Jones, Mary and Geoff 1997, page 331).
A disadvantage of being an endotherm is that large amounts of food is required to provide for fuel for the generation of heat within the body. This is a far greater amount of food than a ectothermic animal does. Eating food is an outward sign of homeostasis and when food is not as abundant, such as in winter, and they are not able to generate enough heat some animals go into hibernation which is a behavioural response.
Although ectotherms, by definition, are not able to generate enough heat within their bodies to keep their temperature constant, they can use behavioural means to adjust their temperature. For example, a lizard may bask in the sun to help raise its body temperature to enable it to become active. When it becomes too hot, like in the middle of the day, it may stay in the shade so it does not over heat. Humans, which are endotherms, also use behavioural means to stay at a good temperature. When the body starts to overheat they can take of a layer of clothes, stay out of the shade, put on a fan, or drink cold water. When it is too cold they can do the opposite, such as put on extra clothes, try to stay warm in the sun, or drink something warm.
(Your Nature Photos, 2011).
Endotherm maintains its heat by a process called a negative negative feedback loop. It involves the lower part of the brain called a hypothalamus constantly monitoring the inside and the outside temperature. When the inside temperature gets too high above the set point the does a number of processes which helps to bring it back to the set point. The physiological adaptations which help control body temperature include increasing the body’s metabolic activity such as:
Piloerection which creates the hairs to stand up and trap air which them is warmed and acts as an insulator.
Increasing and reducing the body insulation according to the season. For example a dog grows a thicker coat of fur in the winter and loses it in the summer.
Increasing the rate of evaporation of water from the body by sweating for humans, panting for dogs, and birds flutter a membrane in their throat. Sweating involves the water taking away the heat when it is evaporated, which makes the body cooler.
Vasoconstrict- reducing the temperature of the body surface and extremities while maintaining a warm internal temperature, humans appear very pale when it is cold as a result of this.
Vasodilatation- dilates the capillaries, moving more blood near the surface of the body so that heat can be lost, making humans look red.
Finally counter-current exchange means the blood vessels leading to and from the extremities of the body are placed close together so the chilled blood returning to the heart obtains heat from the arteries leading form the heat (Brotherton, Judith and Mudie, Kate 1992, page 284).