FEEDBACK
MECHANISMS
M AINTAINING
E QUILIBRIUM

Living versus non-living

Living organisms have the ability to
sense and respond to changes in
environment

The cells of living organisms function best
in a constant, balanced internal
environment

Organisms must maintain a “steady state”
called homeostasis to survive
M AINTAINING
E QUILIBRIUM

Cells, tissues, organs, and organ systems
must work together to adjust to
changing conditions

Cold temperatures: humans respond by
shivering to increase body temperature
by muscle movement

Hot temperatures: humans may
respond by sweating to cool off
W HAT
IS
F EEDBACK ?
Feedback is the process in which part of the
output of a system is returned to its input in
order to regulate its further output.
N EGATIVE F EEDBACK

Have you ever been scolded for not cleaning your room?

Cleaning your room is how you respond to being scolded.

A negative feedback system involves the detection of an
undesirable condition and the response to make that
condition ideal.
Negative Feedback
+
_
N EGATIVE F EEDBACK
Basic structure:
INPUT
DETECTION
RESPONSE
OUTPUT
SHUT OFF
N EGATIVE F EEDBACK

Negative feedback occurs when the output
of a system acts to oppose changes to the
input of the system

A thermostat is an example of a negative
feedback system
Room temp
increases
Room temp is
below the
setpoint
Set point
is reached
THERMOSTAT
Set point
is reached
Room temp is
above the
setpoint
Room temp
decreases
A thermostat is a device for regulating the temperature of a system so that the
system's temperature is maintained near a desired setpoint temperature.
N EGATIVE F EEDBACK
IN B IOLOGY

Negative feedback also
regulates many systems
in organisms

The endocrine system
is one example

This diagram shows a
negative feedback loop
for stress hormones

Areas of negative
feedback are indicated
with a minus sign, (-)
KEEPING THE
BODY IN
BALANCE!
E NDOCRINE SYSTEM

The endocrine system is
composed of glands that
produces chemical
messengers called
hormones

Hormones are produced in
one part of the body and
travel to target organs
through the bloodstream
E NDOCRINE S YSTEM

The brain continuously sends signals to the endocrine glands
to secrete and release hormones

The glands, in turn, send feedback to the nervous system

The hypothalamus in the brain is
the master switch that sends
signals to the pituitary gland
which can release up to eight
hormones into the bloodstream

The hormone travels to its target
organ and usually results in the
release of another hormone into
the bloodstream
E NDOCRINE S YSTEM

The hypothalamus then detects the rising
hormone levels from the target organ
and decreases the release of hormones
from the pituitary which results in a
decrease in hormone release from the
target organ

The process of maintaining normal body
function through negative feedback
mechanisms is called homeostasis
G LUCOSE AND I NSULIN

Glucose intake occurs during
digestion of food that is
needed for energy
expenditure to perform
routine physical activities.

The pancreas is the key organ
that regulates the glucose
levels in body by secreting
two hormones, insulin and
glucagon.

The liver also helps to store
the excess glucose in form of
glycogen to be utilized later.
Pancreas
Liver
G LUCOSE AND I NSULIN N EGATIVE F EEDBACK L OOP
Increases
Glucose
Levels
Eating cake
(-)
Lowers Blood
Glucose levels
CYCLE 1
Insulin stimulates the
cells to take up
glucose from the
blood
Stimulates β cells of
pancreas to secrete insulin
Low Blood Glucose Levels
Stimulated Alpha Cells in
Pancreas
(-)
High blood glucose levels
and Cycle 1 continues
CYCLE 2
Glucagon is released
Glucagon stimulates liver cells to
release glucose into the blood
G LUCOSE AND I NSULIN N EGATIVE
F EEDBACK L OOP

Two primary Hormones
Insulin
Lowers Blood Glucose
Levels
Glucagon
Raises Blood Glucose
Levels
The opposite actions of these two hormones helps to maintain
normal blood sugar levels in the body hence maintain
homeostasis of the body.
TAKE A B REAK AND C HAT W ITH
YOUR N EIGHBOR !

Turn to the person behind or in front of you
and discuss the following topics:

What is a feedback and an example of a
negative AND positive system?

What is the endocrine system and why is it
important?

Define homeostasis and how normal body
function is maintained with feedback
mechanisms.
K IDNEYS

AND
WATER R EGULATION
The kidneys play a key role in maintaining
water regulation.
K IDNEY AND WATER R EGULATION

The nephron is the
most important
functional part of the
kidney.

It filters nutrients like
salts and amino acids
in the Bowman’s
capsule into ascending
loop and filters the
urine.
K IDNEY AND WATER R EGULATION
Anti-Diuretic Hormone, ADH (also called vasopressin), is secreted
by the pituitary gland and acts on the nephron to conserve water
and regulate the tonicity of body fluids.
AntiDiuretic
Hormone
ADH acts on Nephron to
reabsorb water and decrease
blood osmolality (saltiness)
ADH REGULATED WATER CONSERVATION IN KIDNEYS
Less water in the
blood
Stimulates osmoreceptors in
hypothalamus to send signals to
the pituitary gland
Pituitary glands secretes
high levels of ADH
ADH makes the tubules more permeable
and more water is reabsorbed back into the
bloodstream (urine is concentrated).
Excess water in
the blood
Stimulates osmoreceptors in
hypothalamus to send signals to
the pituitary gland
Pituitary glands secretes
low levels of ADH
Less ADH makes the tubules less
permeable and less water is reabsorbed
back into the bloodstream (urine is dilute).
ADH
REGULATED WATER
CONSERVATION IN KIDNEYS

Osmoregulators send negative feedback to
the hypothalamus about the concentration
of water in the bloodstream.

The hypothalamus then stimulates the
pituitary glands to secrete high or low
concentrations of anti-diuretic hormone.

ADH then makes the tubules more or less
permeable and hence, maintains water and
electrolyte homeostasis.
TEMPERATURE REGULATION OF BODY

Animals that are capable of maintaining
their body temperature within a given
range are called homeotherms.

Temperature is regulated by negative
feedback control.

Thermoreceptors located in hypothalamus
detect temperature fluctuations in the
body.
T EMPERATURE R EGULATION OF THE
B ODY

Increased temperature causes vasodilatation
(blood vessels near the surface of the skin
dilate).

The large surface area allows heat to be lost
from the blood and lowers the body
temperature.

Sweating also helps lower the temperature.

Decreased temperature causes
vasoconstriction (blood vessels constrict)
and minimal heat loss occurs which helps
maintain body temperature.

Hair on the body provides insulation and
helps maintain body temperature.
P OSITIVE F EEDBACK

A positive feedback loop occurs when the output
of a system acts to enhance the changes to the
input of the system.

One example of a biological positive feedback
loop is the onset of contractions in childbirth.

When a contraction occurs, the hormone
oxytocin is released into the body, which
stimulates further contractions.

This results in contractions increasing in
amplitude and frequency.
P OSITIVE F EEDBACK


Another example is blood clotting.

The loop is initiated when injured tissue
releases signal chemicals that activate
platelets in the blood.

An activated platelet releases chemicals
to activate more platelets, causing a
rapid cascade and the formation of a
blood clot.
Lactation involves positive feedback so that
the more the baby suckles, the more milk is
produced.
P OSITIVE F EEDBACK

In most cases, once the purpose of the feedback
loop is completed, counter-signals are released
that suppress or break the loop.

Childbirth contractions stop when the baby
is out of the mother's body.

Chemicals break down the blood clot.

Lactation stops when the baby no longer
nurses.
O THER K INDS OF
F EEDBACK
Feed-forward

Not technically feedback,
but similar.

The system responds in
anticipation of a change

Hunger is an example of
feed-forward. If you
typically eat lunch at
noon, your body will
begin to make digestive
enzymes in anticipation.