1. Biological Feedback Systems
How hormones regulate homeostasis

2. THE STIMULUS RESPONSE MODEL
Homeostatic factors that need to be maintained at optimal levels for maximum metabolic activity
Internal Environment
blood glucose
core body temp
gas conc.
ion conc.
pH levels
water levels
THE STIMULUS RESPONSE MODEL
chemo-receptor
thermo-receptor
chemo-receptor
chemo-receptor
chemo-receptor
osmo-receptor
Nervous System
STIMULUS
RECEPTOR
CELL-TO-CELL COMMUNICATION
EFFECTOR
RESPONSE (FEEDBACK)
Receptors that monitor these factors
receptors
receptors
levels ABOVE optimal
levels BELOW optimal
STIMULUS
Negative feedback
neurons
neurons
Effectors act to LOWER levels
Effectors act to RAISE levels
Endocrine System
hormones
muscles
muscles
hormones
DECREASE in homeostatic factor
INCREASE in homeostatic factor
RESPONSE

3. THE STIMULUS RESPONSE MODEL
Homeostatic factors that need to be maintained at optimal levels for maximum metabolic activity
Internal Environment
blood glucose
core body temp
gas conc.
ion conc.
pH levels
water levels
THE STIMULUS RESPONSE MODEL
chemo-receptor
thermo-receptor
chemo-receptor
chemo-receptor
chemo-receptor
osmo-receptor
Nervous System
POSITIVE FEEDBACK
When feedback reinforces the response
NEGATIVE FEEDBACK
When feedback reverses the response
Receptors that monitor these factors
receptors
receptors
levels ABOVE optimal
levels BELOW optimal
STIMULUS
Positive feedback
neurons
neurons
Effectors act to RAISE levels
Effectors act to LOWER levels
Endocrine System
hormones
muscles
muscles
hormones
INCREASE in homeostatic factor
DECREASE in homeostatic factor
RESPONSE

4. Regulating Blood Glucose Levels - an example of negative feedback
Glucose is important for energy production
C6H12O6 + 6O2 –> 6CO2 + 6H2O (cellular respiration)
Glucose is obtained from diet
Glucose travels around the body dissolved in the plasma of blood (hence: “blood sugar”)
Blood sugar levels controlled by two hormones:
INSULIN
GLUCAGON

5. Regulating Blood Glucose Levels - an example of negative feedback
INSULIN
controls the uptake of glucose from the blood into body cells
decreases blood sugar levels
GLUCAGON
controls the release of glucose into the blood
acts on the liver to convert stored glycogen into glucose
increases blood sugar levels
Both hormones are produced by the pancreas

6. Regulating Blood Glucose Levels - an example of negative feedback
Islet of Langerhans
region of pancreas that contains endocrine cells
cells have plasma membrane-bound glucose receptors
blood sugar level sensors
Alpha cells
produce glucagon
Beta cells
produce insulin

7. Regulating Blood Glucose Levels - an example of negative feedback
STIMULUS
STIMULUS
blood glucose levels ABOVE normal
blood glucose levels BELOW normal  
RECEPTOR
RECEPTOR
Islet of Langerhans cells
Islet of Langerhans cells
Negative feedback  
EFFECTORS
EFFECTORS
Alpha cells
Beta cells
Alpha cells
Beta cells
 glucagon
Negative feedback systems like this one act to maintain homeostatic factors within their optimal limits
 insulin
 glucagon
 insulin    
Liver cells
Body cells
Liver cells
Body cells
glucose from blood converted into glycogen
increased absorption of glucose from blood
glycogen converted into glucose, enters blood
decreased absorption of glucose from blood    
RESPONSE
RESPONSE
blood glucose levels DECREASE
blood glucose levels INCREASE

8. Childbirth - an example of positive feedback
Positive feedback is far less common than negative feedback
In childbirth, positive feedback is used to maintain contractions
Oxytocin is a hormone produced by the posterior pituitary gland (in the brain) during childbirth
Oxytocin stimulates uterine contractions

9. Childbirth - an example of positive feedback
Positive feedback increases oxytocin levels, ensuring continued contractions until the baby is born
childbirth starts, baby’s head pushes downward, causing the cervix to stretch
STIMULUS 
RECEPTOR
mechanoreceptors in the cervix 
posterior pituitary gland
Positive feedback
EFFECTORS
 oxytocin 
RESPONSE
uterine contraction; cervical stretch
oxytocin also stimulates the pituitary gland to produce more oxytocin

10. The nervous and endocrine systems at work!
Homeostasis
The nervous and endocrine systems at work!
Control of blood pressure = pg 190
Maintaining core temperature = pg 191
Maintaining water balance = pg