Thermoregulation - Anatomy & Physiology
Thermoregulation is the ability of an endothermic organism to maintain a relatively constant body temperature, despite fluctuations in temperature of the external environment. This is a vital part of homeostasis.
Homeotherm/Endotherm: An organism with an internal temperature which remains relatively constant despite external temperature fluctuations.
Poikilotherm/Ectotherm: An organism having a body temperature which fluctuates with the external environment - eg. fish, reptile.
Mechanism of Action
The thermoregulatory mechanism consists of:
1. Sensory Component: Neurones that possess nerve endings with thermoreceptors, continuously monitor body temperature and transmit it to the integrating centre. These neurons may be within the skin or internal organs.
2. Integrating Centre: The hypothalamus is the control centre in the brain which compares the temperature information with an internal reference or set point.
3. Motor Component: Neurons which send signals to alter the internal temperature, altering heat production or heat loss. These command signals may travel via the somatic motor system and affect heat production in skeletal muscle or they may travel via the sympathetic nerve fibres and change blood flow to the skin, sweat gland activity or activity of the smooth muscles which regulate fur/plumage thickness.
Adaptation to Heat
Sweating causes heat loss via evaporation. Vasodilation is the reduction of the resistance of cutaneous arterioles. This allows more heat from the circulation to be transferred to the air. Hairs on the skin can also lie flat. The arrector pili muscle attached to the proximal end of each hair follicle, relaxes to minimise heat being trapped by the hairs.
Adaptation to Cold
Sweating stops, the cutaneous arteries constrict (vasoconstriction). This minimises heat loss from the blood to the body surface. The hairs on the skin are raised involuntarily by the arrector pili muscles attached to each hair follicle. This layer acts as an insulator, trapping heat. Heat production can be increased by shivering, caused by messages from the brain to muscles. This causes increased heat production as the muscle cells respire. Respiration can also be increased, as respiration is an exothermic reaction.
Specialised Temperature Regulation
Some animals can maintain their body temperature by behavioural adjustments, e.g. lizards bask in the sun during the day, and when they overheat they hide under rocks to allow their body temperature to fall. Some animals living in particularly cold areas are regionally heterothermic, and are able to allow their less insulated extremities to cool to temperatures much lower than their core temperatures, minimising heat loss through exposed body parts such as hooves, legs, feet, nose. Hibernation occurs in some species in order to allow survival during times of limited food resources and low temperatures. Estivation occurs in the summer, allowing some animals to survive periods of high temperature and minimal water (eg. turtles burrow into pond mud). Daily torpor occurs in small endotherms, e.g. bats and hummingbirds, who temporarily reduce their high metabolic rates in order to conserve energy.
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