Homeostasis and Feedback Mechanisms PDF

HOMEOSTATIS AND FEEDBACK MECHANISMS Homeostasis  The relative constancy of the body’s internal environment.  Even though external conditions may change dramatically, internal conditions will stay within a narrow range Homeostasis Examples in the human body: blood pH...

HOMEOSTATIS AND FEEDBACK MECHANISMS Homeostasis  The relative constancy of the body’s internal environment.  Even though external conditions may change dramatically, internal conditions will stay within a narrow range Homeostasis Examples in the human body: blood pH = 7.4 body temp. = 37°C blood pressure = 120/80 blood [glucose] = 0.1% Homeostasis is maintained through feedback mechanisms. Negative Feedback  A negative feedback mechanism maintains stability by its ability to sense a change and bring about an effect that reverses that change.  Control centers in the brain monitor and control body conditions ◦ pH, temperature, blood pressure, glucose levels Negative Feedback  Sensors all over body detect unacceptable levels and signal the appropriate brain center ◦ temperature sensors in skin stimulate brain if skin gets colder than 37°C  The control center directs body to behave a way so that normal state is regained ◦ shivering  This is called an adaptive response.  Once normal state is regained, the sensor stops signaling the brain center so adaptive response stops. Negative Feedback  Results in a fluctuation between two levels. The concentration of blood glucose is almost never exactly 0.1%. It’s usually a little bit above or a little bit below. Over the course of a day, however, it will average out to be exactly 0.1%.  Negative Feedback n two levels. e.g. the concentration of glucose in 1%. It’s usually a little bit above or a little bit ugh, it would average out to be exactly 0.1%. House temperature Thermometer drops in thermostat detects low temperature House temperature Thermostat returns to normal sends signal to furnace Furnace switches on Positive Feedback Positive Feedback  A stimulus triggers the stimulus to be increased. It causes a greater adaptive response, which causes a greater stimulus. ◦ Oxytocin causes the uterus to contract during labour. It is made in the hypothalamus and stored in the posterior pituitary. ◦ Just before birth, the baby's head exerts pressure against the cervix. The pressure triggers sensory nerves in the cervix to send a nerve signal to the posterior pituitary to release oxytocin. Positive Feedback ◦ Oxytocin is released into the blood, travels to the uterus and causes stronger uterine contractions, which causes greater stimulation of the sensory nerves, which causes more oxytocin to be released, which causes stronger uterine contractions, etc. The cycle ends when the baby is pushed out of the uterus, stopping the stimulation of sensory nerves to the pituitary. Positive Feedback  Positive feedback loops are also involved in blood clotting, digestion of proteins in the stomach, and increasing body temperature to fight off infection (fever)