thermoregulatiom in endotherms Flashcards
detecting temperature change
In any homeostatic system receptors are needed to detect a change in the internal environment. The peripheral temperature receptors are in the skin and detect changes in the surface temperature.
Temperature receptors in the hypothalamus detect the temperature of the blood deep in the body. The temperature of the skin is much more likely to be affected by external conditions than the temperature of the hypothalamus. The combination of the two gives the body great sensitivity and allows it to respond not only to actual changes in the temperature of the blood but to pre-empt possible problems that might result from changes in the external environment.
The temperature receptors in the hypothalamus act as the thermostat of the body, controlling the responses that maintain the core temperature in a dynamic equilibrium to within about 1 °C of 37°C.
principles of thermoregulation in endotherms
Endotherms use their internal exothermic metabolic activities to keep them warm, and energy-requiring physiological responses to help them cool down. They also have passive ways of heating up and cooling down, to reduce the energy demands on their bodies.
Like ectotherms, endotherms have a range of behavioural responses to temperature changes that include basking in the Sun, pressing themselves to warm surfaces, wallowing in water and mud to cool down, and digging burrows to keep warm or cool. Some animals even become dormant through the coldest weather (hibernation) or through the hottest weather (aestivation is a period of prolonged or deep sleep similar to hibernation but occurs in summer or during dry seasons to avoid heat stress rather than cold).
Humans have additional behavioural adaptations to help control body temperature - clothes are worn to stay warm, houses are built, and then heated up or cooled down to maintain the ideal temperature.
In spite of these behavioural responses, endotherms mainly rely on physiological adaptations to maintain a stable core body temperature, regardless of the environmental conditions or the amount of exercise being done. These adaptations include the peripheral temperature receptors, the thermoregulatory centres of the hypothalamus, the skin,
and muscles.
cooling down-vasodilation
The arterioles near the surface of the skin dilate when the temperature rises. The vessels that provide a direct connection between the arterioles and the venules (the arteriovenous shunt vessels) constrict. This forces blood through the capillary networks close to the surface of the skin. The skin flushes, and cools as a result of increased radiation. If the skin is pressed against cool surfaces, then the cooling results from conduction.
cooling down-increased sweating
As the core temperature starts to increase, rates of sweating also increase. Sweat spreads out across the surface of the skin. In some mammals, including humans and horses, there are sweat glands all over the body. As the sweat evaporates from the surface of the skin, heat is lost, cooling the blood below the surface. In some animals, the sweat glands are restricted to the less hairy areas of the body such as the paws. These animals often open their mouths and pant when they get hot, again losing heat as the water evaporates. In human beings, around 1 dm of sweat is lost by evaporation on a normal day. If the conditions are very hot and dry or the person is exercising very hard, up to 12 dm of sweat a day can be lost.
Kangaroos and cats often lick their front legs to keep cool in high temperatures.
cooling down-reducing the insulating effect of hair or feathers
As the body temperature begins to increase, the erector pili muscles (the hair erector muscles) in the skin relax - as a result, the hair or feathers of the animal lie flat to the skin. This avoids trapping an insulating layer of air. It has little effect in humans.
Endotherms that live in hot climates often have anatomical adaptations as well as the behavioural and physiological adaptations already described. These minimise the effect of high temperatures and maximise the ability of the animal to cool down through the surface area of the body. They include a relatively large surface area: volume (SA: V) ratio to maximise cooling (e.g., include large ears and wrinkly skin), and pale fur or feathers to reflect radiation.
warming up-vasoconstriction
The arterioles near the surface of the skin constrict. The arteriovenous shunt vessels dilate, so very little blood flows through the capillary networks close to the surface of the skin. The skin looks pale, and very little radiation takes place. The warm blood is kept well below the surface.
warming up-decreased sweating
As the core temperature falls, rates of sweating decrease and sweat production will stop entirely. This greatly reduces cooling by the evaporation of water from the surface of the skin, although some evaporation from the lungs still continues.
warming up-raising the body hair or feathers
As the body temperature falls, the erector pili muscles in the skin contract, pulling the hair or feathers of the animal erect. This traps an insulating layer of air and so reduces cooling through the skin. The effect can be quite dramatic and it is a very effective way to reduce heat loss to the environment in many animals. In humans this has little effect although you can observe the hairs being pulled upright.
warming up-shivering
As the core temperature falls the body may begin to shiver. This is the rapid, involuntary contracting and relaxing of the large voluntary muscles in the body. The metabolic heat from the exothermic reactions warm up the body instead of moving it and is an effective way of raising the core temperature.
Endotherms living in cold climates often have additional anatomical adaptations to help them keep warm. Many have adaptations that minimise their SA: V ratio to reduce cooling (e.g., small ears). Another common adaptation is a thick layer of insulating fat underneath the skin, for example, blubber in whales and seals. Some animals hibernate - they build up fat stores, build a well-insulated shelter, and lower their metabolic rate so they pass the worst of the cold weather in a deep sleep-like state.
Polar bears demonstrate many of the ways in which endotherms can survive in extremely cold conditions. They have small ears and fur on their feet to insulate them from the ice. The fur and skin of polar bears work together. The hairs are hollow so trap a permanent layer of insulating air. The skin underneath is black, so it absorbs warming radiation. They have a thick layer of fat under the skin. Polar bears are so well insulated that their external surfaces are similar in temperature to the snow and ice on which they live. Females dig dens in the snow and remain in them, warm and insulated, for months while they give birth to their cubs, only emerging when the cubs are large enough to survive the cold. Polar bears are so well adapted to life in temperatures down to -50°C in the Arctic that they can overheat at temperatures over 10°C.
controlling thermoregulation-the heat loss centre
This is activated when the temperature of the blood flowing through the hypothalamus increases. It sends impulses through autonomic motor neurones to effectors in the skin and muscles, triggering responses that act to lower the core temperature.
controlling thermoregulation-the heat gain centre
This is activated when the temperature of the blood flowing through the hypothalamus decreases. It sends impulses through the autonomic nervous system to effectors in the skin and the muscles, triggering responses that act to raise the core temperature.
The interaction of the sensory receptors, the autonomic nervous system, and the effectors in a sophisticated feedback system enables endotherms to maintain a very stable core body temperature regardless of environmental conditions or activity levels.
