Homeostasis

What’s it all about?

Definition• Defined by Claude Bernard (French physiologist)• It is the ability to maintain a constant internal

environment despite a continually changing external environment.

• “Steady state” – dynamic response; not designed to pre-empt change but to cope or adapt to it

• Examples:• temperature 37◦ C• blood pH 7.4• blood pressure 120/80• heart beats 72-80/min

Control Mechanism

• 3 components:• Receptor – detects changes in the environment;

senses

• Control center (integrator) – processes information to direct an appropriate response (2 systems)

• Effector – delivers response (motor); muscle and glands

A homeostatic control system has three functional components

A receptor, a control center, and an effector

ResponseNo heat

produced

Roomtemperaturedecreases

Heaterturnedoff

Set point

Toohot

Setpoint

Control center:thermostat

Roomtemperatureincreases

Heaterturnedon

Toocold

ResponseHeat

produced

Setpoint

Feedback Control

• 2 types : negative versus positive

• Negative – triggers response that counteracts initial change (opposition); fever and the cooling center

• Positive – triggers response that amplifies the original change; labor response

Thermoregulation• Keep body temperature in a range to support life and

metabolic activities; is the process by which animals maintain an internal temperature within a tolerable range

• What is metabolism? (reactions that involve changes in energy)

• Role of heat - ^ rate of reaction in physical systems

• Biological systems have heat limitations imposed via the fragility of the molecules (protein denaturation)

Temperature limitations

• Species are adapted to a temperature range which is optimum to support their life processes.

• Generic range – between 0 and 50◦

• Extremes:• Freezing – preserves chemistry; slows down rate

of metabolic activity• Heating – increases rate of metabolism within life

range (ectotherms)

Methods of Heat Transfer

• Conduction – direct transfer of heat between environment and body surface; metal cage, hot rock, cold pool

• Diffusion of heat from high to low temperature

• Water is the better conductor and insulator

Radiation is the emission of electromagnetic waves by all objects warmer than absolute zero. Radiation can transfer heat between objects that are not in direct contact, as when a lizard absorbs heat radiating from the sun.

Evaporation is the removal of heat from the surface of aliquid that is losing some of its molecules as gas. Evaporation of water from a lizard’s moist surfaces that are exposed to the environment has a strong cooling effect.

Convection is the transfer of heat by themovement of air or liquid past a surface,

as when a breeze contributes to heat loss from a lizard’s dry skin, or blood moves heat from the body core to the extremities.

Conduction is the direct transfer of thermal motion (heat) between molecules of objects in direct contact with each other, as when a lizard sits on a hot rock.

Methods of Heat Transfer

Convection

• Movement of heat by current of air or water over surface of the body

• Examples: fan, wing flapping, wind chill factor

Radiation

• Emission of electromagnetic waves (heat) produced by all objects warmer than absolute zero (not require contact)

• Example: polar bear fur (fiber optic); clear fur directs heat to darker skin below

Evaporation

• Loss of heat via evaporation of liquid such as water to a gas (540 cal/gm)

• Examples: tepid bath, perspiration, panting, saliva spreading

• ***saturation limits

Organism Classification

• Definition based upon source of heat absorption (body metabolism versus external environment)

• Forms:• Ectotherms – obtain heat from external

surroundings

• Endotherms – obtain heat as a by-product of living metabolism (60% as by-product of respiration)

Classification

• Ectotherms– Include most invertebrates, fishes,

amphibians, and non-bird reptiles

• Endotherms– Include birds and mammals

***warm versus cold blood (misnomer)

Ectotherms

• Amphibians and reptiles other than birds are ectothermic, meaning that

– They gain their heat mostly from external sources

– They have lower metabolic rates

• ***What about plants?

Characteristics of Ectotherms

• Majority do not have an advanced mechanism for thermoregulation

• TB ∞ TA (ambient versus body temperature)

• Demonstrates van Hoff’s Principle – metabolic activity doubles for every 10◦ C increase in TA

• Metabolism is directly related to temperature• ****linear graph

Endotherms

• Birds and mammals are mainly endothermic, meaning that

– Their bodies are warmed mostly by heat generated by metabolism

– They typically have higher metabolic rates than ectotherms

Characteristics of Endotherms

• Maintains internal body temperature independent of ambient temperature

• Body temperatures typically higher than habitat; efficient at heat conservation

• Mammals – 36-39◦ C (96.8-102.2◦F)• Birds – 40-43◦C (104-109.4◦F)

Metabolic Effects

• Ectotherms – metabolic rate varies with ambient temperature (life range)

• Endotherms – metabolic rate remains fairly high and consistent despite ambient changes

Ectotherms v.Endotherms

River otter (endotherm)

Largemouth bass (ectotherm)

Ambient (environmental) temperature (°C)

Bod

y te

mpe

ratu

re (°

C)

40

30

20

10

10 20 30 400

Advantages of Endothermy

Endothermy is more energetically expensive than ectothermy (food requirement)

– effectively buffers animals’ internal temperatures against external fluctuations

– enables the animals to maintain a high level of aerobic metabolism

– able to live in a wide variety of habitats– metabolism independent of seasonal changes**– metabolism independent of day/night changes**

An animal’s metabolic rate is the amount of energy an animal uses in per unit of time

This rate can be measured in a variety of ways:

This photograph shows a ghost crab in arespirometer. Temperature is held constant in thechamber, with air of known O2 concentration flow-ing through. The crab’s metabolic rate is calculatedfrom the difference between the amount of O2entering and the amount of O2 leaving therespirometer. This crab is on a treadmill, runningat a constant speed as measurements are made.

(a)

(b) Similarly, the metabolic rate of a manfitted with a breathing apparatus isbeing monitored while he works outon a stationary bike.

Factors that effect metabolic rate

Body size/mass (metabolic rate per gram)

– Is inversely related to body size among similar animals

– Example tree shrew versus elephant (surface/volume ratio)

Thermoregulation Methods

• Thermoregulation involves physiological and behavioral adjustments that balance heat gain and loss

• Four basic methods:• Structural• Physiological – circulatory paths; thermogenesis• Behavioral phenomena• Evaporative cooling

Structural Methods

• Insulation, which is a major thermoregulatory adaptation in mammals and birds– Reduces the flow of heat between an animal

and its environment– May include feathers, fur, blubber,scales,skin– Oils and waxes may serve to insulate

Human Skin Anatomy

Epidermis

Dermis

Hypodermis

Physiological Methods

• Many endotherms and some ectotherms– Can alter the amount of blood flowing between the

body core and the skin• In vasodilation

– Blood flow in the skin increases, facilitating heat loss• In vasoconstriction

– Blood flow in the skin decreases, lowering heat loss

• ***regional vasoconstriction/dilation

Many marine mammals and birdsHave arrangements of blood vessels called countercurrent heat exchangers that are important for reducing heat loss

In the flippers of a dolphin, each artery issurrounded by several veins in acountercurrent arrangement, allowingefficient heat exchange between arterialand venous blood.

Canadagoose

Artery Vein

35°C

Blood flow

Vein

Artery

30º

20º

10º

33°

27º

18º

9º

Pacificbottlenosedolphin

2

1

3

2

3

Arteries carrying warm blood down thelegs of a goose or the flippers of a dolphinare in close contact with veins conveyingcool blood in the opposite direction, backtoward the trunk of the body. Thisarrangement facilitates heat transferfrom arteries to veins (blackarrows) along the entire lengthof the blood vessels.

1

Near the end of the leg or flipper, wherearterial blood has been cooled to far below the animal’s core temperature, the artery can still transfer heat to the even colderblood of an adjacent vein. The venous bloodcontinues to absorb heat as it passes warmer and warmer arterial blood traveling in the opposite direction.

2

As the venous blood approaches the center of the body, it is almost as warm as the body core, minimizing the heat lost as a result of supplying blood to body partsimmersed in cold water.

3

Countercurrent Heat ExchangeBluefin tuna. Unlike most fishes, the bluefin tuna maintainstemperatures in its main swimming muscles that are much higherthan the surrounding water (colors indicate swimming muscles cutin transverse section). These temperatures were recorded for a tunain 19°C water.

21º25º 23º

27º

29º31º

Body cavity

SkinArteryVein

Capillarynetwork withinmuscle

Dorsal aortaArtery andvein underthe skin

Heart

Bloodvesselsin gills

b) Great white shark. Like the bluefin tuna, the great white sharkhas a countercurrent heat exchanger in its swimming muscles thatreduces the loss of metabolic heat. All bony fishes and sharks loseheat to the surrounding water when their blood passes through thegills. However, endothermic sharks have a small dorsal aorta, and as a result, relatively little cold blood from the gills goes directly to the core of the body. Instead, most of the blood leaving the gillsis conveyed via large arteries just under the skin, keeping cool bloodaway from the body core. As shown in the enlargement, smallarteries carrying cool blood inward from the large arteries under theskin are paralleled by small veins carrying warm blood outward fromthe inner body. This countercurrent flow retains heat in the muscles.

Some endothermic insects have countercurrent heat exchangers that help maintain a high temperature in the thorax

Thermogenesis

• Heat production can occur via 2 methods:

• Shivering thermogenesis – piloerection; wing flapping

• Non-shivering thermogenesis – hormonal; white versus brown fat metabolism

Many species of flying insectsUse shivering to warm up before taking flight

PREFLIGHT PREFLIGHTWARMUP

FLIGHT

Thorax

Abdomen

Tem

pera

ture

(°C

)

Time from onset of warmup (min)

40

35

30

25

0 2 4

Behavioral Methods

• Huddling• Burrowing• Migration (land/water)• Hibernation (food versus temperature)• Nocturnal life style (predatory)• Surface area (sprawl)• Basking

Evaporative Cooling MethodsHeat of vaporization

• Perspiration• Transpiration• Panting• Saliva spreading• Bathing• Urination

Evaporative CoolingBathing moistens the skin (heat of vaporization)

Long term temperature control

• In a process known as acclimatization– Many animals can adjust to a new range of

environmental temperatures over a period of days or weeks

• Acclimatization may involve cellular adjustments (molecular optimums)– Or in the case of birds and mammals,

adjustments of insulation and metabolic heat production (fat bodies; degree of saturation –cell membranes)

Torpor

– Is an adaptation that enables animals to save energy while avoiding difficult and dangerous conditions

– Is a physiological state in which activity is low and metabolism decreases (decrease in heart and respiratory rate, decrease in body temperature) ***conserves food and energy

Torpor (decreased metabolic state)

• Estivation, or summer torpor– Enables animals to survive long periods of

high temperatures and scarce water supplies• Daily torpor

– Is exhibited by many small mammals and birds and seems to be adapted to their feeding patterns (bats, shrews, hummingbirds)

Hibernation is a form of long-term torpor

That is an adaptation to winter cold and food scarcity during which the animal’s body temperature declines

Additional metabolism that would benecessary to stay active in winter

Actualmetabolism

Anatomy of Heat Control

• In humans, a specific part of the brain, the hypothalamus

– Contains a group of nerve cells that function as a thermostat

– Contains heating and cooling centers

Thermostat inhypothalamusactivates coolingmechanisms.

Sweat glands secrete sweat that evaporates, cooling the body.

Blood vesselsin skin dilate:capillaries fillwith warm blood;heat radiates fromskin surface.

Body temperaturedecreases;thermostat

shuts off coolingmechanisms.

Increased bodytemperature (suchas when exercising

or in hotsurroundings)

Homeostasis:Internal body temperatureof approximately 36–38°C

Body temperatureincreases;thermostat

shuts off warmingmechanisms.

Decreased bodytemperature

(such as whenin cold

surroundings)Blood vessels in skinconstrict, diverting bloodfrom skin to deeper tissuesand reducing heat lossfrom skin surface.

Skeletal muscles rapidlycontract, causing shivering,which generates heat.

Thermostat inhypothalamusactivateswarmingmechanisms.

Hypothalmic Regulation