What would kill you first if your
body’s homeostasis systems failed?
Think about which organ systems do
what as you rank them.
 A change in body temperature
 An increase in the concentration of carbon dioxide
 A change in the concentration of oxygen
 A change in concentration of electrolytes
 An increase in the concentration of waste
 A change in concentration of blood sugar
The kidneys sense that the oxygen levels in the blood
are low. The kidney sends a hormone signal to the bone
marrow to make more red blood cells. The number of red
blood cells increases, so more oxygen is carried in the blood.
A.
This is a positive feedback loop
B.
This is a negative feedback loop
C.
This is not a feedback loop
10.
An example of feedback is seen in blood clotting. Part
of the complex biochemical pathway of clotting is the
production of an enzyme that forms the matrix of the blood
clot, but also speeds up the production of still more thrombin.
That is, it has a self- catalytic , self-accelerating effect, so that
once the clotting process begins, it runs faster and faster until,
ideally, bleeding stops. Thus, this positive feedback loop is
part of a larger negative feedback loop, one that is activated
by bleeding and ultimately works to stop the bleeding.
A.
This is a positive feedback loop
B.
This is a negative feedback loop
C.
This is not a feedback loop
What type of feedback loop is this?
Feedback is seen in protein digestion, where the presence
of partially digested protein in the stomach triggers the
secretion of hydrochloric acid and pepsin, the enzyme that
digests protein. Pepsin digests proteins, casing more HCl
and pepsin to be released.
INTRODUCTION TO HOMEOSTASIS
Important variables within the body:
 blood sugar
 fluid balance
 body temperature
 oxygen levels
 blood pressure
 pH
These variables must stay within certain ranges.
Changes in the external environment can cause these
variables to change.
Homeostasis:
The process by which a constant internal environment is
maintained despite changes in the external
environment.
The hypothalamus
 Part of the brain
 Often serves as the co-ordinating centre:
 Receives messages from monitors
 Initiates a hormonal/nervous response
Dynamic equilibrium
 Homeostasis is also called dynamic equilibrium:
 Conditions do fluctuate, but within an acceptable range
 How is dynamic equilibrium maintained?
 Feedback systems
 Negative feedback
 Positive feedback
Negative feedback
 Negative feedback: Response triggered by changed
conditions serves to reverse the change
 E.g., Body temperature increases  Skin blood vessels
dilate  Body temperature decreases
Body temperature
increases
Normal
body
temperature
Body temperature
decreases
Hypothalamus
sends a message
to blood vessels
Skin blood vessels
dilate
Example: Blood Pressure Regulation
Positive feedback
 Positive feedback: The response triggered by changing
conditions serves to move the variable even further away
from its steady state
 E.g., uterine contractions are stimulated by oxytocin 
baby moves towards cervix  more oxytocin is released
Thermoregulation
Maintaining body temperature
Body temperature
Average human body temperature: 37°C
 Core body temperature is slightly higher
 Interindividual variation
Thermoregulation
 Thermoregulation: The maintenance of body
temperatures within a range that enables cells to
function effectively
 Recall: Enzymes have optimal temperatures
 High temperatures can denature proteins
Types of temperature stress
 Heat stress
 Cold stress
Response to Heat Stress
Response to Heat Stress
 Co-ordinating centre is the hypothalamus
 Responses:
 Skin blood vessels will dilate
 Sweat glands will produce perspiration
 Both responses serve to lower body temperature
 Return to normal range
Response to Cold Stress
Response to Cold Stress
 Co-ordinating centre is the hypothalamus
 Responses:
 Skin blood vessels will constrict
 Skeletal muscle will contract rapidly (shivering),
increasing metabolism
 Smooth muscle around hair follicles will contract,
producing goosebumps
 Responses serve to raise body temperature 
Return to normal range
Prolonged exposure to cold stress
 May cause an increase in the rate of metabolism
 Special fat cells called “brown fat”
 Particularly important for babies
Hypothermia: Occurs when core body temperature
drops below normal range.
 Usually results in coma, then death
Summary of thermoregulation
Stimulus
Physiological
response
Adjustment
Cold
•constriction of blood
vessels in skin
•hairs on body erect
•shivering
•heat is conserved
•dilation of blood
vessels in skin
•sweating
•heat is released
Heat
•heat is generated by
increasing metabolism
Homework
 pg.111 Answer All Questions