1. All images from Nelson Biology 12 unless otherwise specified
Homeostasis
All images from Nelson Biology 12 unless otherwise specified

2. Homeostasis
The human body works best at a temperature of 37°C, with a 0.1% blood sugar level and a blood pH level of 7.35.
Homeostasis
Greek: homoios -- “similar” or “like,” and stasis -- “standing still”
Process in which your body adjusts to environmental conditions to maintain a stable internal environment
This system of balance requires active and constant monitoring about body conditions

4. Homeostatic control system
All homeostatic control systems have 3 functional components:
Monitor: signals coordinating center that an organ has begun to operate outside its normal limits
Coordinating center: relays information to appropriate regulator
Regulator: acts to restore normal balance
Example: increase in carbon dioxide and decrease in oxygen during exercise
Monitor: chemical receptors in brain stem are stimulated by increased CO2, chemical receptors in carotid arteries detect low oxygen levels
Coordinating center: receptors go to brain, which sends nerve signals to muscles to increase depth and rate of breathing
Regulator: increased breathing movements flush excess carbon dioxide from body and increases oxygen content
Because of the fluctuating nature of the control system, it is often called a dynamic equilibrium

6. Negative and Positive Feedback
The body maintains homeostasis through 2 feedback mechanisms:
Positive feedback
Negative feedback
Which do you think is more common?

7. Positive Feedback
The stimulus causes the reaction from the body to get stronger and stronger until the stimulus is gone
Positive feedback systems move the variable even further away from a steady state
In short: response increases initial stimulus
Useful for allowing a specific physiological event to be accomplished quickly
Not useful for maintaining homeostasis
For example: the Ferguson Reaction
Uterine contractions during birth release the hormone oxytocin, which stimulates more, stronger uterine contractions. This lasts until the baby is expelled from the uterus

8. Negative Feedback Similar to a thermostat
Body has a pre-determined set of internal conditions
When the body goes outside of those conditions, it will take action to return to its optimal range
In short: response reduces initial stimulus

9. Example of negative regulation: thermostat

10. Regulation of heat distribution
Thermoregulation
Regulation of heat distribution

11. Thermoregulation
Maintenance of body temperature within a range that enables cells to function efficiently
Two types of animals:
Ecotherms:
Invertebrates, and most fish, amphibians, and reptiles
Metabolic activity partially regulated by environment
Adapted behavioural adaptations, such as sunning on rocks or going into the shade
These animals depend on air temperature to regulate metabolic rates; therefore, activity

12. Thermoregulation Endotherms: Mammals, birds, “warm-blooded” creatures
Maintain constant body temperature regardless of surroundings
Increase rate of cellular respiration to generate heat
However, in humans, body temperature does vary slightly during the day, and from peripheral to core of body

13. Hypothalamus – internal thermostat
The hypothalamus is your body’s internal thermostat
Located in the middle of the brain
It controls the pituitary gland, which in turn controls all hormones in your body
Along with controlling temperature, the hypothalamus acts as a relay station between the nervous system and the endocrine (hormone) system

14. Response to Heat Stress
Thermoreceptors in skin detect a rise in body temperature
Nervous system in brain sends message to hypothalamus
Hypothalamus response:
Sweat glands to initiate sweating – evaporation of sweat cools body
Blood vessels to dilate – more blood goes to skin, which has been cooled by evaporation
When blood returns to core, cools internal organs

16. Response to Cold Stress
Thermoreceptors in the skin send a message to the hypothalamus
Hypothalamus response:
Constricts smooth muscle of arterioles – blood flow is limited, heat loss from skin is reduced
Constricts smooth muscle around hair follicles, causing hair to stand on end – erect hair traps warm air next to skin
Skeletal muscles to initiate rhythmic contractions (10 – 20x per minute) – shivering
If exposure to cold is prolonged:
Elevation of metabolism (hormonal response)
Deposition of “brown fat,” a special adipose tissue that can convert chemical energy into heat – especially important in newborns, because they are not able to shiver

18. Thermoregulation Summary

19. Mammalian diving reflex
If the body’s core temperature falls below the normal range (hypothermia), the individual can fall into a coma and die
Some people can survive sustained exposure to cold, because of the mammalian diving reflex
When a mammal is submerged in cold water, heart rate slows
Blood is diverted to brain and other vital organs
Heat is conserved

20. Freezing cells – facts and fiction
Suspended animation for humans is impossible – so far.
Your cells are mostly made of water.
As water freezes, it expands and forms ice crystals
These ice crystals completely destroy cells, blood vessels, organs, nerves, etc. – causing death
Scholander, P. F. et. al, Supercooling and osmoregulation in arctic fish. J. Cell. Comp. Physiol., 49: 5 – 24.
Found that the temperature of salt water off Baffin Island is below the freezing point of fish blood
Later found that these fish contained proteins that prevented ice crystals from forming
Storey, 1988: Canadian wood frog, Rana sylvatica
Wood frogs fill their cells with glucose, which reduces the amount of water in the cell
When the water freezes, there are not enough ice crystals made to damage cells