ThermoregulationHSC Biology

Module 9.2 Maintaining a Balance

Textbook ReferencesBIOLOGY IN CONTEXT: The Spectrum of Life – Peter Aubusson & Eileen Kennedy, Oxford Press,2001 Sections 5.5 (pp 241 -246) & 5.6 (pp 247 -251).

Heinemann BIOLOGY – Kate Moodie & Judith Brotherton, Heinemann, 2000 Section 5.1 pp 229 – 237.

Macmillan BIOLOGY -

Review: Limiting Factors

Limiting Factor: Any environmental resource that, because it is scarce or unfavourable, limits the ecological niche of an organism.

Temperature as a Limiting Factor

Two major effects:1. Influences rates of chemical reactions2. Changes biochemical structure eg. enzymes, and therefore their function

Corals – Temperature Sensitive Organisms

Corals species live within a relatively narrow temperature margin. (Approx 23 - 26oC on average - different species have acclimitized to different temperatures)

All reef corals are very sensitive to changes in water temperature.

Increases in temperature as apparently small as one degree above long term patterns causes coral bleaching.

Thermoregulation

• Definition: Thermoregulation – the process by which animals maintain internal temperature within a tolerable range.

• Rates of chemical reactions mediated by enzymes increase 2 - 3 fold for every 10 degree rise in temperature until temperature gets high enough to denature these proteins.

• Also membrane properties of cells are affected by temperature.

• Every animal species has an optimum temperature range.

Ectotherms and Endotherms

• Ectotherms gain heat from environment; do not have physiological mechanisms to regulate temp; have low metabolic rate; thermoregulate by behavioural means, seeking sun or shade

– Most invertebrates, fishes, amphibians, lizards, snakes, turtles

• Endotherms gain heat from metabolism; high metabolic rate

– Mammals, birds, some reptiles, some fish, some insects

Ectotherm vs Endotherm

Heat is exchanged with external environment by the

following physical processes:• Conduction- transfer of heat between objects in

direct contact with each other• Convection- heat is conducted away from an

object of high temp to low temp by fluid.

- Rate varies with different materials • Radiation- transfers heat between objects not in

direct contact

- sun energy• Evaporation- change of liquid to vapour

- cooling

4 Physical Processes of Heat Gain & Loss

Heat Exchange Between an Organism and its Environment

Australian Ectotherm Example: Blue-tongued Skink

• Blue-tongued skinks are diurnal and terrestrial in nature.

• Adult blue tongues average about 35 to 60 cm in total length. They have large, triangular heads with long, fat bodies. Their legs are small compared to their body size, and they have short, stocky tails.

• (How does this relate to temperature control?)

Blue-tongued Skink - Thermoregulation

• Blue-tongue’soptimum temperatureis 20 - 24oC, but like all reptiles they have no internal heating process, so they must regulate their temperature by using their external environment. • They accomplish this by moving from one temperature zone to another, including making use of underground burrows (cooler).• Ambient air temperature, digestion, pregnancy, skin-shed cycles and illness are factors that influence their thermoregulation.

Most Animals are Ectothermic, but Endothermy is Widespread

• Mammals and birds generally maintain body temperatures within a narrow range that is usually considerably warmer than the environment.– Body temperature is 36 - 38oC for most mammals and

39 - 42oC for most birds. – Because heat always flows from a warm object to

cooler surroundings, birds and mammals must counteract the constant heat loss.

– This accounts for the higher metabolic rates of endotherms over equivalent-sized ectotherms.

Mechanisms for Thermoregulation• Insulation

- Fur- Hair- Feathers- Fat- Blubber

• Evaporative cooling- sweating, panting, bathing

• Shivering• Non-shivering thermogenesis & brown fat• Circulation adaptations

- Countercurrent exchange- Vasodilatation (cooling)- Vasoconstriction (heat conservation)

• Behavioural responses

Hippos lack sweat glands

Adaptations (1) – Endothermy• This maintenance of warm body temperatures depends on

several key adaptations.– The most basic mechanism is the high metabolic rate of

endothermy itself.• Endotherms can produce large amounts of metabolic heat that

replaces the flow of heat to the environment.• They can vary heat production to match changing rates of heat

loss.– Heat production is increased by muscle activity during

moving or shivering.• In some mammals, non-shivering thermogenesis (NST) is

induced by certain hormones to increase their metabolic activity and produce heat instead of ATP.

• Some mammals also have a tissue called brown fat in the neck and between the shoulders that is specialized for rapid heat production.

– In cold environments, mammals and birds can increase their metabolic heat production by as much as 5 to 10 times minimal levels under warm conditions.

Adaptations (2) – Insulation

• Another major thermoregulatory adaptation that evolved in mammals and birds is insulation (hair, feathers, and fat layers).– This reduces the flow of heat and lowers the

energy cost of keeping warm.– The insulating power of a layer

of fur or feathers mainly depends

on how much still air the layer

of fur or feathers traps.

Adaptations (3) – Circulation

• Vasodilation and vasoconstriction

also regulate heat exchange and

may contribute to regional temperature

differences within the animal.

• • • • Countercurrent mechanisms

• are used by many endotherms• eg. whales & seals; in tails & flippers

that lack insulation.

Countercurrent Heat Exchange Mechanism

Goose leg

Dolphin flipper

Heat is transferred from one supply (arterial blood) to another (venous blood) without any actual mixing or contact.

Made up of blood vessels (arteries and veins) that run parallel to each other, but the blood flows in opposite directions.

Warm blood in the arteries from the body passes very close to cooled blood returning from the extremities in the veins.

Heat transferred from warmer blood to cooler blood, thus conserving

body heat that would have been lost in extremities.

Discovery of New Animal Heat-Exchanger

• Recently, zoologists have discovered that the tongue of grey whales, has counter-current heat exchangers to minimize heat loss from this very large organ.

• Because the tongue is muscular and used in feeding, it has very little fat for insulation and is a potential source for major heat loss.

• Study showed that the tongue of these whales has an extensive counter-current exchange system, used to conserve heat while feeding in cold arctic or antarctic waters.

Adaptations for Endothermy (4)

• Evaporative cooling often plays a key role in dissipating body heat.– If environmental temperature is above body

temperature, animals gain heat from the environment and by metabolic activity.

– Large ears of many hot climate animals act as heat radiators

– Evaporation is often the only way to keep body temperature from rising rapidly.• Additional mechanisms to enhance evaporative

cooling include panting, sweat glands, bathing, and using saliva as a water source.

Adaptations for Endothermy (5)

Sweat pores in cicada’s cuticle

Elephant ears are heat radiators

Bilby ears have a rich supply of blood vessels for heat exchange.

Temperature Regulation in Mammals

• Shivering

• Sweating

• Fever

Advantages of Endothermy

• Maintains stable body temp. – cooling and heating the body

• Enables high levels of aerobic metabolism

• Sustains vigorous activity for much longer than ectotherms eg.

• Long distance running• Flight

Disadvantages of Endothermy

Greater food consumption needed to meet metabolic needs

• Human metabolic rate at 200C & at rest 1,300 to 1,800 kcal per day.

• Crocodile metabolic rate at 200C & at rest 60 kcal per day at 200C.

Tuna Counter-Current Mechanism• Dark muscle on either side of the vertebral column (red) is maintained at higher temperature than rest of body due to countercurrent heat exchange.

• Cold, oxygen-rich arterial blood (from gills) passes into series of fine arteries that take the blood through the active muscles.

• Fine arteries lie side by side with veins draining these muscles [Diagram (b)]

• Cold blood picks up heat generated by active muscles and keeps it from

being lost to the surroundings.

(b) Electron micrograph of blood vessels

Fine artery

Vein

Brown fat- generates heat • important in neonates, small mammals in

cold environments, and animals that hibernate

• Located in neck and in inner scapula area

Non-shivering Thermogenesis• Larges amounts of heat produced by

oxidizing fatty acids in the mitochondria

Brown Fat & Non-shivering Thermogenesis

In humans it is the core body temperature which is maintained by homeostasis.Allows humans to cope with a wide range of environments.

Temperature Regulation in Humans

Receptors •1. Thermoreceptors in the skin•2. Thermoreceptors in the hypothalamus

Control Centre

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•Glands

Components of System

Skin-based Effectors: Responding to Heat Gain

Effector Response

Arterioles supplying skin capillaries

The muscles relax – this is known as vasodilation. The effect of this is to increases the blood flow to the surface of the skin causing increased heat loss by radiation.

Sweat glands

The 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.

Hair erector muscles

The 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.

Skin-Based Effectors: Responding to Heat Loss

Effector Response

Arterioles supplying skin capillaries

The muscle contracts – this is known as vasoconstriction. Blood flows through the shunt vessel as this is the path of least resistance. Less blood flows to the surface of the skin so there is less heat loss by radiation.

Sweat glands The sweat glands stop producing sweat therefore stopping the heat loss of evaporating sweat.

Hair erector muscles

The muscle 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.

Summary - Regulating Body Temp in Humans

• Endotherms (birds and mammals): grow a thicker fur coat in the winter and shedding it in the summer - and sometimes by varying the capacity for metabolic heat production seasonally.

• Ectotherms compensate for changes in body temperature through adjustments in physiology and temperature tolerance.

• For example, winter-acclimated catfish can only survive temperatures at high as 28oC, but summer-acclimated fish can survive temperatures to 36oC.

Acclimatization to New Environmental Temperatures

• Some ectotherms that experience subzero body temperatures protect themselves by producing “antifreeze” compounds (cryoprotectants) that prevent ice formation in the cells.– In cold climates, cryoprotectants in the body

fluids let overwintering ectotherms, such as some frogs and many arthropods and their eggs, withstand body temperatures considerably below zero.

– Cyroprotectants are also found in some Arctic and Antarctic fishes, where temperatures can drop below the freezing point of unprotected body fluids (about -0.7oC).

• Cells can often make rapid adjustments to temperature changes.– For example, marked increases in temperature

or other sources of stress induce cells grown in culture to produce stress-induced proteins, including heat-shock proteins, within minutes.

– These molecules help maintain the integrity of other proteins that would be denatured by severe heat.

– These proteins are also produced in bacteria, yeast, and plants cells, as well as other animals.

– These help prevent cell death when an organism is challenged by severe changes in the cellular environment.

• Torpor in Ground Squirrels– Body temperature: 37oC – Metabolic rate: 85 kcal per day.– During the eight months the squirrel is in

hibernation, its body temperature is only a few degrees above burrow temperature and its metabolic rate is very low.

Hibernation: long-term torpor as an adaptation to long-term winter cold and food shortage

Body Temperature and Metabolism During Hibernation