SystemsPositive and Negative Feedback
Systems A system is a group of parts that work
together to perform a function or produce a result
Components Input Process Output Feedback
Example System
Is this a system?-Group of parts?-Working together to perform a function?-YES!
What is the input?
What is the output?
Negative Feedback To get back to a specific target point
A Nonliving Example of Negative
Feedback: Control of
Room Temperatu
re
ResponseNo Heat
Produced
RoomTemperatureDecreases
Heaterturnedoff
Set point
Toohot
Setpoint
Control center:thermostat
RoomTemperature
IncreasesHeaterturnedon
Toocold
ResponseHeat
Produced
Setpoint
Negative Feedback: Maintenance of Glucose Homeostasis by Insulin and
Glucagon
Beta cells ofpancreas are stimulatedto release insulininto the blood.
Insulin
Liver takesup glucoseand stores itas glycogen.
Body cellstake up moreglucose.
Blood glucose leveldeclines to set point;stimulus for insulinrelease diminishes.
STIMULUS:Rising blood glucose
level (for instance, aftereating a carbohydrate-
rich meal)
Homeostasis:Blood glucose level
(about 90 mg/100 mL)
Blood glucose levelrises to set point;
stimulus for glucagonrelease diminishes.
STIMULUS:Dropping blood glucoselevel (for instance, after
skipping a meal)
Alpha cells of pancreasare stimulated to releaseglucagon into the blood.
Liver breaksdown glycogenand releasesglucose intoblood.
Glucagon
Positive Feedback Over amplification and moves away from
the target point
Positive Feedback (Fruit)
Ethylene Gas
An Example of Positive Feedback:the Induction of Labor
Estrogen Oxytocin
fromovaries
from fetusand mother'sposterior pituitary
Induces oxytocinreceptors on uterus
Stimulates uterusto contract
Stimulatesplacenta to make
Prostaglandins
Stimulate morecontractions
of uterus
Po
siti
ve f
eed
bac
k
Feedback Flow Chart
Homeostasis
Feedback
PositiveNegative
Stabilization
Temperature Regulation
Amplification
Fruit Ripening
Alteration
Diabetes mellitus
Engineering Practices Two main ideas when designing in
engineering Criteria – the rules or directions that must be
followed; the requirements that must be met Constraints – restrictions that keep something
from being the best it can be; problems that arise or issues that come up.
What were the criteria and constraints in our design challenge?
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