Homeostasis
Objectives
1) Discuss the importance of
homeostasis
2) Explore how negative feedback
systems work
3) Work through specific feedback
systems used to maintain blood
glucose and body temperature
It Is All About Balance…
• All living things must react
to their environment to
survive
– Homeostasis regulated
stable state; constantly
adjusting
• Unicellular direct contact
with world
• Multicellular ???
– Most internal cells do not
have direct contact
– ECF (extracellular fluid)
• Plasma
• Tissue fluid
Types of Adjustment
• What are the two ways an organism
can change?
1) Regulators adjust to resist
environment and maintain a
stable state
–
Warm-blooded animals (thermo
regulators)
2) Conformers adjust to match
environment to maintain their
stable state
– Cold-blooded animals (thermo
conformers)
• Either method, what things must be
regulated in all living things?
– Concentration of nutrients, O2,
CO2, waste, water, NaCl, pH, and
plasma
– Temperature
Types of Homeostasis
• Complex organisms require
multiple levels of homeostasis to
stay alive
• Cells maintain this own
homeostasis but must work
together in a complex organism
1) Local homeostatic controls
organs/tissue adjusting to
demands
–
Stomach maintains pH low
enough to work but not low
enough to hurt stomach
2) Systemic homeostatic
controls organs regulate
other organs
–
Hypothalamus (part of brain)
Feedback Systems: Key to Homeostasis
• Negative feedback keep homeostasis
regulated most of the time
1) Stimulus environmental change
2) Receptor senses stimulus (input)
3) Integrator compares information
from sensor to set point (normal
level)
4) Effector response from integrator
(output)
Example: It is cold in this room
1) Stimulus cold
2) Receptor temp. sensor in heater
(18oC)
3) Integrator computer compares
temp. sensor to set point temp. (25oC)
4) Effector heater turn on until room
is 25oC
Homeostasis In Living Things
• We are not perfect
machines; homeostasis
always fluctuates around
a set point
• What is the average
pulse rate of the human
heart?
– 60 Beats/min
• 60 beats is our set point;
but you rarely get 60
beats/min
– Fluctuate above and
below 60; average out to
set point
Feedback Loops: Blood Glucose
• Glucose is major source of
energy (respiration)
• Regulated by two hormones
and negative feedback
inhibition
• Glucagon promotes
breakdown of glycogen to
release glucose; happens
when hungry or during
exercise
• Insulin lowers glucose
levels in blood after eating
by
• Diabetes cannot regulate
glucose because defective
or missing insulin
Regulating Blood Glucose
1) You study all morning;
running low on energy
–
Blood glucose is too low!!
2) Hypothalamus tells pancreas
to release glucagon;
glycogen glucose
–
Blood glucose back to normal
3) LUNCH TIME!! You eat and
digestion breaks starch into
glucose
–
Blood glucose too high!!
4) Hypothalamus tells pancreas
to release insulin; liver,
muscles, fat tissue absorb
glucose
–
Blood glucose back to normal
5) Repeat all day!
Layers of the Skin
• What is the job of the skin?
–
–
–
–
Protect the body
Stop pathogens from entering
Prevent water loss
Detect changes in temp. and
pressure
– Detect pain
– Regulate heat loss
• Hair/Fur:
– Cold weather hair stand up to
help trap air around skin
– Hot weather hair lays flat to
limit air around ski
• Fat layer insulates body
against cold
Feedback Loop: Body Temp
• What is the homeostatic
point for body temp?
– 37oC
• In hot weather:
1) Skin sends info to
hypothalamus
2) Vasodilation arteries
expand to release heat by
radiation
3) Sweat glands salty
sweat evaporates from
skin which cools you
down
Feedback Loop: Body Temp (part 2)
• In cold weather:
1) Skin sends info to
hypythalamus
2) Vasoconstriction
arterioles shrink to
keep
3) Sweat glands stop
producing sweat
4) Shivering muscle
contractions burn ATP
to generate more heat