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

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

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

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

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

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

Example of negative regulation: thermostat

Regulation of heat distribution Thermoregulation Regulation of heat distribution

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

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

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

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

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

Thermoregulation Summary

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

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