Assignment #
Homeostasis and Feedback Loops
Biology
March 11, 2016
Mrs. McCarthy
I.
Homeostasis is the maintenance
of a constant internal environment
A.
Homeostasis works to maintain
the organism's internal
environment within tolerance
limits - the narrow range of
conditions where cells are able to
function.
B.
Homeostasis is a self-adjusting
mechanism involving feedback
where the response alters the
internal conditions based on the
stimulus.
C. The body responds to:
1. The changing conditions of
the external environment
2.
The changing conditions of
the internal environment
II.
How homeostasis is achieved
A.
Structural: physical features
specific for survival
B.
Functional: metabolism
adjusts to changes in conditions
(controlled by hormones from the
endocrine system)
C.
Behavioral: the actions help an
organism to survive in its
environment.
III.
Feedback mechanisms and loops
A. Feedback is when the response to a
stimulus has an effect of some kind on
the original stimulus
1.
Stimulus - any factor that
causes a receptor to trigger
impulses in a nerve pathway
B.
Negative feedback is when
the response stops the
stimulus
1. Negative feedback is the
most common type
1.
Examples:
a. Blood glucose concentrations rise
after a sugary meal (the stimulus), the
hormone insulin is released and it
speeds up the transport of glucose out
of the blood and into selected tissues
(the response), so blood glucose
concentrations decrease (thus
decreasing the original stimulus).
b.
Exercise creates metabolic
heat which raises the body
temperature (the stimulus),
cooling mechanisms such as
vasodilation (flushed skin) and
sweating begin (the response),
body temperature falls (thus
decreasing the original stimulus).
C.
Positive feedback is when the
response enhances the
original stimulus.
1. Positive feedback is not
common
1. Examples:
a. A ripening apple releases the volatile
plant hormone ethylene (the stimulus).
Ethylene accelerates the ripening of
unripe fruit in its vicinity so nearby
fruit also ripens, releasing more
ethylene (the response). All the fruit
quickly becomes ripe together. ("One
'bad' apple has ruined the whole lot."
The biological explanation - positive
feedback - for an old saying!)
ADD THIS IN:
The endocrine system controls
homeostasis through hormones.
Hormones are chemical messengers
that have an effect on organs far
from the gland.
1. Examples:
b. A baby begins to suckle her mother's
nipple and a few drops of milk are
released (the stimulus). This
encourages the baby and releases a
hormone in the mother which further
stimulates the release of milk (the
response). The hungry baby continues
to suckle, stimulating more milk
release until she stops. (Positive
feedback, it would not have helped the
baby if suckling decreased milk flow,
as in negative feedback!)
c.
An example of positive feedback can be
found in childbirth. The hormone
oxytocin stimulates and enhances
labor
contractions. As the baby moves toward
the birth canal, pressure receptors within the
cervix (muscular outlet of uterus) send
messages to the brain to produce
oxytocin. Oxytocin travels to the uterus
through the bloodstream, stimulating the
muscles in the uterine wall to contract
stronger. The contractions intensify and
increase until the baby is outside the birth
canal. When the stimulus to the pressure
receptors ends, oxytocin production stops
and labor contractions cease.
Journal Entry #
March 11, 2016
Explain why homeostasis is important to
maintain life. Give at least two examples
using body systems we studied.
Circulatory System
Nervous System
Excretory System
Muscular and Skeletal Systems
Endocrine System – hormones involved in
feedback