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?