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 fluids Changing 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 Temperatur e drops from 30°c to 29 ° c Receptor Measures level of a factor Thermostat signals temperatur e 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 temperatu re 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 temperatur e Receptor thermorece ptors 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 temperatu re 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