Homeostasis

Aim: Understand the principles of homeostasis and negative feedback using body temperature, blood

pH, blood glucose levels and water potential of blood as examples.

Be able to draw a general negative feedback diagram.

To describe and compare temperature control in ectothermic reptiles and endothermic mammals

Why is homeostasis important?

• Which conditions need to be maintained in the body?

• Why is this important?

Which conditions need to be maintained in the body?

• Temperature

• pH

• Water potential

Why is it important that conditions are maintained?

Temperature and pH Effect the efficiency of enzymes or denature

them

Water potential in blood and tissue fluidsChanging water potential causes cells to

shrink/expand due to osmosis affecting their function.

Water potential maintained by maintaining a constant glucose concentration.

Negative feedback system – Water bath

Input

Change to the system

Temperature drops

from 30°c to 29°c

Receptor

Measures level of a

factor

Thermostat signals

temperature has fallen below 30°c

Control unit

Operational information is

stored here and used to

coordinate effectors

Heating element is

switched on

Effector

Brings about changes to

the system in order to

return it to the set point

Heating element raises temperature

of water

Output

System returned

to set point

Heating element raise temperature of water to 20°c

Feedback loop

Mechanisms involved in heat gain

• Producing heat – metabolism of food during respiration

• Gain of heat from environment – conduction or convection

Mechanisms involved in heat loss

• Evaporation of water (sweating)

• Loss of heat to the environment – conduction or convection

Temperature control in ectothermic reptiles

• Exposing themselves to the sun

• Taking shelter

• Gaining warmth from the ground

• Generating metabolic heat

• Colour variations

Temperature control in endothermic mammals

Mechanisms involved in heat gain

Vasoconstriction – Blood vessels contract – Smaller area for heat loss, less blood reaches the surface

Shivering – contraction of body muscles releases heat

Raising of hair – traps still layer of air

Increased metabolic rate – brought about by increase in hormone levels

Decrease in sweating – reduced or halted

Behavioural mechanisms – Sheltering from wind ect

Temperature control in endothermic mammals

Mechanisms involved in heat loss

• Vasodilation – Blood vessels expand and therefore pass closer to the surface

• Increased sweating – heat energy required to evaporate the water

• Lowering of body hair – reduces insulating layer

• Behavioural mechanisms – Seeking shade

Negative feedback system

Stimulus

Change in body

temperature

Receptor

thermoreceptors

Control unit

Information is passed to the hypothalamus

in the brain

Effector

Skin responds to increase or decrease in temperature

Output

Return to normal body

temperature

Feedback loop

Receptors

• Hypothalamus • Monitors temperature of blood passing

through

• Thermoreceptors in skin • Detect changes in skin temperature • Messages sent to hypothalamus via

autonomic nervous system

Hypothalamus

2 centres

• Heat gain centre – Activated by fall in blood temperature

• Heat loss centre – Activated by rise in blood temperature