Temperature Regulation One of the most important examples of homeostasis is thermoregulation. Not all animals can do this physiologically. Animals that maintain a fairly constant body temp are called endotherms. (all birds and mammals are endotherms but also some reptiles, fish and numerous insects) Those that have a variable body temp are called ectotherms (most reptiles, fish, amphibians and invertebrates are ectotherms)

Ectotherms Not cold blooded!! They do take on the temperature of their environment though Why do you think reptiles need to lie in the sun? They have a very low metabolic rate, so they can’t make much heat by themselves They basically need the sun to warm up enough to be able to move Advantages of this Don’t need to eat much Can tolerate a larger fluctuation in internal body temperature Disadvantages? Can’t tolerate large changes in external temperatures Are basically only really active in warmer months Can’t sustain long periods of vigorous activity

Endotherms Not warm blooded!!! Are able to use their internal mechanisms to keep their body temperature at a stable level (nerves and hormone controlled) What are the advantages of this? Able to live in a large variety of external temperatures Able to make heat via metabolism Able to sustain intense activity for longer. Eg: flying Disadvantages? Costs a lot of energy and they must therefore have a plentiful food supply

How heat energy is lost or gained Conduction is the transfer of heat energy from the hotter to the cooler of two objects in contact with each other. Radiation is the transfer of heat energy from a body to colder objects that are not in contact with it. Evaporation is the change of liquid to a vapour and it is accompanied by cooling.

How heat energy is lost or gained Convection is the movement of air resulting from local pockets of warm air being replaced by cooler air, and vice versa. Of course conduction, radiation and convection can work both ways. But a body cannot gain heat energy by evaporation- it can only lose it.

Convection Radiation Evaporation Conduction

It is well known that irrespective of fluctuations in the temp of the environment, the temp of the human body remains between 36-38 degrees Celsius. Many of the body’s organs and physiological processes contribute to thermoregulation.

Temperature Regulation In humans it is the core body temperature which is maintained by homeostasis.

Hypothalamus The temp regulation centre of the body is in the hypothalamus. It responds directly to changes in core temperature and to nerve impulses from receptors in the skin. Its response uses both the nervous and endocrine system. Why?

Receptors in the hypothalamus monitor the temp of the blood as it passes through the brain- the core temp. It is divided into two areas Heat detecting Cold detecting Receptors in the skin monitor the external temp. The thermoregulatory centre sends impulses to several different effectors to adjust body temp.

Components Receptors Thermoreceptors in the skin hypothalamus Coordinator The thermoregulatory centre in the hypothalamus (subdivided into heat gain centre and heat loss centre) Effectors Sweat glands Hair erector muscles Arterioles supplying skin capillaries Skeletal muscles Brown fat (in babies only) Glands (thyroid and adrenal)

Brown fat in babies Babies have a poor shivering mechanism and a thin layer of insulation. They have area of brown fat all over their bodies to help this Brown fat is extremely high in mitochondria so when stimulated by thyroxin and adrenaline what do you think it does? It is able to break down glucose for energy and generate heat Babies make about 5 x as much heat as adults. This process is called non-shivering thermogenesis (NST)

Responding to heat gain Vasodilation The blood vessels in the skin become wider. The effect of this is to increases the blood flow to the surface of the skin causing increased heat loss by radiation. Flattening of skin hair The skin’s hair erector muscles relax – this lowers the hairs on the skin surface. The hairs do not trap air when laid flat so they allow heat to be removed more easily by convection. Decreased metabolism Any reaction produces heat as a by product. Sweating Glands secrete sweat onto the surface of the skin. The sweat is composed mainly of water, which evaporates from the skin surface. As water has a high latent heat of evaporation the evaporating sweat removes heat from the surface of the skin. Behaviour adaptations Retreating into cooler environments, bathing etc.

Responding to heat loss Vasoconstriction The blood vessels in the skin contract which decreases the flow of blood to the skin. Less blood flow to the surface of the skin means less heat loss by radiation. Shivering The body’s skeletal muscles contract and relax involuntarily to generate heat. Increased metabolism Release of the hormones adrenaline, nor-adrenaline and thyroxine from the adrenal and thyroid glands. Both these hormones increase the body’s metabolic rate again generating heat. Fluffing of hair The hair erector muscles contract – this raises the hairs on the surface of the skin. The effect of this is to reduce airflow therefore reducing convection of heat from the skin.

Skin based effectors

How does size affect heat loss / gain? Large animals have a reduced SA:Vol ratio. This means that they actually lose less heat. Smaller animals have a high SA:VOL ratio but as a result lose more heat Also the larger and heavier you are, the lower your metabolism so you generate less heat Which animal would be more efficient at maintaining a stable body temperature in colder temperatures? A marsupial mouse or a wallaby? The wallaby.

Exam questions 1. All living organisms exist in changing external environments and many are able to control their internal environments. Explain how the body of a mammal may respond to a rise in the environmental temperature. (8 marks)

Exam questions 2. a) b) c) d) Size matters for marathon runners. Big athletes produce more heat and find it harder to keep cool. Shape matters too - a tall, thin runner has fewer problems keeping cool than a short, tubby runner of the same body mass. A 65 kg athlete running a marathon in 2 hours 10 minutes in reasonably dry conditions can avoid overheating at air temperatures up to 37 °C, but in humid conditions the same level of performance is possible only at temperatures below about 17 °C. Explain how athletes produce heat when they run. (2) Why does a tall, thin runner have fewer problems keeping cool than a short, tubby runner of the same body mass? (2) Explain why runners are more likely to overheat in humid conditions. (3) Describe how the body responds to a rise in core body temperature. (5) (Total 12 marks)