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Introduction to Animal Physiology

Homeostasis

Physiology• The study of the functions of living organisms

– whole organisms– organ systems– organs– tissues– cells

Physiology• groups of cells with similar characteristics or

specializations form tissues• different tissues combine to form organs

– discrete structures with specific functions• organs which function together form organ

systems

Physiology• tissues occur in four basic types

– epithelial tissues form linings or coverings• perform functions appropriate to organ

– connective tissues exist in a matrix• support and reinforce other tissues

– muscle tissues contract• provide movement or propulsion

– nervous tissues transmit and process information

tissues of the stomach wall

Figure 41.2

Table 41.1

Homeostasis• most organ systems contribute to homeostasis

– maintenance of a constant internal environment in spite of constant change• provides for material needs of cells• removes wastes from cells• regulates physical environment of cells• communicates among cells

homeostasis in a cellular suitcaseFigure 41.1

Homeostasis• homeostatic regulatory components

– controlled systems - effectors– regulatory systems

• acquire information• process information• integrate information• send commands

Homeostasis• homeostatic regulatory variables

– setpoint• optimal chemical or physical condition

– feedback information• actual current condition

– error signal• discrepancy between setpoint and feedback value

Homeostasis• homeostatic regulatory inputs

– negative feedback• reduces or reverses activity of effector• returns condition to set point

– positive feedback• amplifies activity of effector• self-limiting activities

– feedforward information• changes setpoint

the “responsible driver” exampleFigure 41.4

Homeostasis: thermoregulation• living cells cannot survive temperatures above

or below fairly narrow limits– thermosensitivities of organisms vary– thermosensitivities of effectors vary

• Q10 quantifies temperature sensitivity– ratio of physiological rate at one temperature

to the rate at 10˚C lower temperatureQ10 = RT / RT-10

biological range of Q10

values

Figure 41.5

Homeostasis: thermoregulation• acclimatization can alter an animal’s

temperature response– changes that allow optimal activity under

different climatic conditions [e.g. seasonal temperature variation]• metabolic compensation

–maintains metabolic rate in different seasons

–accomplished with alternate enzyme systems (e.g.)

acclimatization may

include metabolic

compensationFigure 41.6

Homeostasis: thermoregulation• animals are classified by how they respond to

environmental temperatures– homeotherm

• maintains a constant body temperature as ambient temperature changes

– poikilotherm• changes body temperature as ambient temperature changes

Homeostasis: thermoregulation• animals are classified by how they respond to

environmental temperaturesand

• their sources (sinks) of body heat– ectotherm

• external heat sources/sinks– endotherm

• active heat generation and cooling

ectotherms and

endotherms utilize

different sources of body heat

Figure 41.7

behavioral temperature regulation in an ectotherm

Figure 41.8

Homeostasis: thermoregulation• behavior is a common method of regulating

body temperature– ectotherms

• different microenvironments provide different temperatures

– endotherms• behavioral temperature regulation reduces metabolic costs

behavioral temperature regulation in endothermsFigure 41.9

Homeostasis: thermoregulation• heat exchange between body and environment

occurs through the skin– radiation - gain or loss– conduction - gain or loss– convection - gain or loss– evaporation - loss

Figure 41.10

Homeostasis: thermoregulation• heat exchange can be regulated by control of

blood flow to the skin– constriction/dilation of blood vessels

supplying the skin– change in heart rate

vegetarian marine iguanaFigure 41.11

an iguana regulates body temperature by altering heart rate in surf & sun

Figure 41.11

muscular contraction generates heat

brood warming by honey bees

Homeostasis: thermoregulation• some ectotherms use muscular contractions to

generate heat– insects flex wing muscles

• to achieve flight temperature• to warm brood above air temperature

– Indian python flexes muscles to warm brood above air temperature

– analogous to mammalian shivering

Homeostasis: thermoregulation• anatomical features allow some fish to retain

muscular heat– in “cold” fish

• blood is chilled in gills• cold blood is warmed by muscle mass• warmed blood returns to gills and is chilled

a cold fish

dumps muscular

heatFigure 41.12

Homeostasis: thermoregulation• anatomical features allow some fish to retain

muscular heat– in “hot” fish

• chilled blood from gills travels near skin• chilled blood enters muscle mass next to veins leaving muscle mass

• countercurrent heat exchange warms blood entering muscle mass

• countercurrent heat exchange removes heat from blood returning to the gills

a hot fish

retains muscular

heatFigure 41.12

Homeostasis: thermoregulation• thermal characteristics of endotherms

– thermoneutral zone• temperature window with no regulation

– basal metabolic rate • meets minimal metabolic needs

– lower critical temperature• below which metabolic rate increases

– upper critical temperature• above which active cooling occurs

basal metabolic rate vs. body massFigure 41.13

endotherms regulate

body temperature

metabolicallyFigure 41.14


– heat generation below the lower critical temperature• shivering heat production

–contractions of opposed muscles–releases heat from ATP hydrolysis


– heat generation below the lower critical temperature• nonshivering heat production

–occurs in brown fat tissue–due to thermogenin–uncouples respiratory electron transport from ATP synthesis

brown fat is

highly vascularized, has a high density of mitochondria,

and has smaller lipid dropletsFigure 41.15

reduced surface area

andincreased insulation conserve body heat

Figure 41.16


– anatomical features conserve heat below the lower critical temperature• reduced surface/volume ratio• increased thermal insulation• oil secretion resists wetting

increased surface area andreduced insulation release body heat

Figure 41.16


– heat loss above the upper critical temperature• increased surface area/volume ratio• increased blood flow to skin• evaporation

–sweat glands–panting

a thermostat controls the effectors

(furnace and air conditioner) in a house

metabolic rate and

body temperature respond to hypothalamic temperature

changesFigure 41.17

ambient temperature(feedforward information)

can alter the

setpoint for

metabolic heat productionFigure 41.18

Homeostasis: thermoregulation• mammalian thermal regulation

– the mammalian thermostat is the hypothalamus

– different effectors of thermal regulation have different set points

– environmental temperature can act as feed forward information to alter set points

– pyrogens increase the set point for metabolic heat production causing fever

Homeostasis: thermoregulation• torpor conserves metabolic resources

– torpor is regulated hypothermia– some birds engage in daily torpor during

inactive periods– in hibernating mammals, torpor may last

hours, days, or weeks

decreased metabolism, lower temperatureFigure 41.19

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