An Introduction to
Anatomy and Physiology
Chapter 1
Homeostasis
Slides 90-116
1.13_C_Homeostasis-Intro
(Slides 1-9)
INTRO
1.13_C_Homeostasis-Intro
Homeostatic Regulation
Ex: Household temperature
control
1. Variable that is regulated
2. Receptor: monitors the
variable (receives and
transmits “stimulus”)
3. Control Center:
integrates information and
compares it to a “set point”
(decides on appropriate
response) & transmits
command
4. Effector: receives &
executes response to
stimulus
Human thermostat
5
Pyrogens change the temperature control center
“set-point” to a new higher level.
• Fever = higher body temperature, often accompanies bacterial or viral
infections
• Blood flow shifted to core to conserve heat, shivering, increase in metabolic
rate
• Chills stop when higher temp reached
• Benefits of a higher temperature:
– Inhibits bacterial growth
– Speeds up metabolic reactions for repair
– Increases delivery of white blood cells to infection sites
Negative Feedback: Blood glucose regulation
Negative Feedback: Fluid/Electrolyte balance
Negative Feedback Loop Restores
Homeostasis (Temperature)
 In negative feedback systems the response of the effector
negates or opposes the stimulus (shuts off the original stimulus)
– Too hot > effector leads to less hot (sweat, increase skin blood flow,
decrease metabolism)
– Too cold > effector leads to less cold (shiver, behavioral changes,
increase metabolism)
 Responses are often controlled by nervous and endocrine
systems (e.g. brain regulates body temp) EXTRINSIC
CONTROL but sometimes controlled at the cell and tissue
level = AUTOREGULATION or INTRINSIC CONTROL
Negative Feedback: Regulation of
Blood Pressure
 External or internal stimulus increase BP

Baroreceptors (pressure sensitive receptors)





Detect higher BP
Send nerve impulses to brain for interpretation
Response sent via nerve impulse sent to heart and blood vessels
BP drops and homeostasis is restored
Drop in BP negates the original stimulus
Homeostatic Regulation
 Homeostasis- The existence of a stable internal
environment.
 Has 3 Parts:
Receptor: a sensor that is sensitive to a
particular environmental change or stimulus.
Control Center: receives and processes the
information supplied by the receptor.
Effector: a cell or organ that responds to the
commands of the control center and whose
activity opposes or enhances the original
stimulus.
Feedback System
 Cycle of events
 Body is monitored and re-monitored
 Each monitored variable is termed a controlled condition
 Three Basic components
 Receptor
 Control center
 Effector
Feedback Systems
 Receptor
Body structure that monitors changes in a
controlled condition
 Sends input to the control center

 Nerve ending of the skin in response to temperature
change
Feedback Systems
 Control Center
Brain
 Sets the range of values to be maintained
 Evaluates input received from receptors and
generates output command
 Nerve impulses, hormones

 Brains acts as a control center receiving nerve
impulses from skin temperature receptors
Feedback Systems
 Effector
Receives output from the control center
 Produces a response or effect that changes the
controlled condition

 Found in nearly every organ or tissue
 Body temperature drops the brain sends and impulse
to the skeletal muscles to contract
 Shivering to generate heat
Paramecium eating pigmented yeast
http://www.youtube.com/watch?v=l9ymaSzcsdY
"As the paramecium stuffs itself with a meal of pigmented yeast, vacuoles form.
As food particles are digested by acidic enzymes, the congo red will reveal the
pH drop-color will shift from a more alkaline red, to a more acidic blue."
Blood Clot Formation
http://www.mhhe.com/biosci/esp/2002_general/Esp/folder_structure/tr/m1/s7/trm1s7_3.htm
Body’s
Internal
Environment
18
Homeostatic Imbalances
 Normal equilibrium of body processes are
disrupted

Moderate imbalance
 Disorder or abnormality of structure and function
 Disease specific for an illness with recognizable
signs and symptoms
 Signs are objective changes such as a fever or
swelling
 Symptoms are subjective changes such as headache

Severe imbalance
 Death
Positive Feedback
Definition: enhances the original
stimulus causing a greater deviation from
the set point.
– Activates infrequent events that require
immediate action
– Most are not related to the maintenance of
homeostasis.
– Positive feedback often results in disease.
– Examples:
• Blood clot
• Uterine contractions in childbirth (stretch
releases oxytocin from posterior pituitary),
• Cancer
Positive Feedback Systems: Normal Childbirth
 Uterine contractions cause
vagina to open
 Stretch-sensitive receptors in
cervix send impulse to brain
 Oxytocin is released into the
blood
 Contractions enhanced and baby
pushes farther down the uterus
 Cycle continues to the birth of
the baby (no stretching)
Childbirth
Positive Feedback: Blood Loss
 Normal conditions, heart pumps blood
under pressure to body cells (oxygen and
nutrients)
 Severe blood loss
Blood pressure drops
 Cells receive less oxygen and function less
efficiently
 If blood loss continues

 Heart cells become weaker
 Heart doesn’t pump
 BP continues to fall
Positive Feedback -Disease
Two Types of Homeostasis
• Negative Feedback: • Positive Feedback:
when the effector(s) initial stimulus
activated by the
produces a response
control center
that exaggerates or
oppose or eliminate
enhances
its
effects.
stimulus.
• Labor Contractions
• Many examples;
most common
• Blood Clotting
feedback system.
Negative Feedback
Positive Feedback
• Inhibitor
• Stimulatory
• Oppose change by creating
response opposite in direction of
initial disturbance
change – temperature drop
response – heat production
initial disturbance – temp fall
below normal set point
• Amplifies/reinforces change which can
be harmful and disastrous
• Stabilize physiological variables
• Ex: sneezing, birth of baby, immune
response to infection, blood clot
• Maintain constant internal
environment
•Ex: goosebumps, sweating
• Cause instability and disrupt
homeostasis (continuous temp
increase)
HOMEOSTASIS TUTORIALS & QUIZ
1.17_C_HOMEOSTASIS TUTORIALS_QUIZ