Homeostasis??? DYNAMIC EQUILIBRIUM????

Homeostasis is often referred to as a DYNAMIC EQUILIBRIUM – a condition that remains stable within fluctuating limits Thus, although there are fluctuations in blood glucose levels, body temperature, blood pressure and pH, the homeostatic mechanism ensures that all the body systems function within an acceptable range to sustain life.

1.Negative Feedback Mechanisms that make adjustments to bring the body back within an acceptable range are referred to as NEGATIVE FEEDBACK E.g. pH changes When there is an increase in acids in the body the body will compensate to make things more neutral Most homeostatic mechanisms in animals operate on this principle https://www.youtube.com/watch?v=z0H8JbgeP9I

Negative Feedback stimulus (environmental change) sensor (e.g. free nerve ending in the skin) effector (a muscle or gland) response (system’s output) integrator (e.g. brain) In the negative feedback mechanism, the response of the system cancels or counteracts the effect of the original environmental change

Household Thermostat Thermometer: sensor Thermostat: control Centre Furnace: effector/regulator

2.Positive Feedback Less common in the body Reinforces change Moves the variable away from the norm E.g.: birth process A decrease in progesterone initiates small contractions of the uterus. This signals a release of oxytocin which causes stronger contractions and pushes baby closer to cervix Once baby is expelled positive feedback stops and negative feedback kick in to get the body back to normal

A. Response to Heat Stress Sensors in brain detect a rise in body temp. Nerve message is coordinated within the hypothalamus (region in brain) and a signal is sent to the sweat glands to initiate sweating Evaporation of perspiration from the skin causes cooling and a nerve impulse is sent to the blood vessels of the skin causing them to dilate (allowing more blood flow to skin) Overall, the skin has been cooled by the evaporation of sweat and the blood loses heat to the skin. When the blood from the skin returns to the core of the body, it cools the internal organs.

B.Response To Cold Stress When the external temperature drops, receptors in the skin send a message to the hypothalamus The hypothalamus sends a message to the body to increase body temperature Nerves cause smooth muscles to contract and arterioles to constrict limiting blood flow

This reduces heat loss from the skin and retains heat in the core of the body Nerves messages are also carried to the smooth muscle that surrounds the hair follicles in your skin, causing your hair to stand on end. The small bump made by the contraction is called a “goosebump” The erect hair traps warm, still air next to the surface of your skin and helps reduce heat loss The hypothalamus also sends nerve messages to initiate shivering which is a rhythmic contraction of skeletal muscle

Hypothermia A condition in which the body core temperature falls below the normal range A drop of only a few degrees can lead to coma and even death Some people have survived sustained exposure to cold temperatures. Their heart rate slows down and the blood is diverted to the brain and other vital organs to conserve heat This is called the MAMMALIAN DIVING REFLEX

Thermoregulation Different species of animals are adapted to different temperature ranges, each animal has an optimum range Thermoregulation: is the maintenance of body temperature within a range that enables cells to function properly

Ectotherms Invertebrates, most fish, amphibians, and reptiles Depend on air temperature to regulate metabolic rates Species have developed behaioural adaptations to overcome these limitations (ex: reptiles sun themselves on rocks or hide in the shade to regulate temperature)

Endotherms Mammals including humans, and birds Able to maintain a constant body temperature regardless of their surroundings Adjust to decreases in environmental temperatures by increasing the rate of cellular respiration to generate heat

Homework 1-7,10,13 page 435