Homeostasis

1. 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. 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, 1957. 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