3. Homeostasis- Definition
• Homoios means like and stasis means standing still
• Homeostasis is defined as the maintenance of nearly constant
conditions in the internal environment.
• Walter Cannon coined the term ‘Homeostasis’
• Homoeostasis is defined as the self regulatory capacity to maintain
steady states in the body, essential for keeping the individual alive
despite of changes in the external environment.
4. • Claude Bernard coined the term milieu intérieu which means internal
environment.
• Internal environment is the fluid that surrounds the cells.
• Internal environment is the extracellular fluid (plasma + interstitial fluid).
• All the cells live in the same environment that is the ECF.
•
What is internal environment?
Why ECF is considered as internal environment for the cell ?
• ECF bathes the cells continuously.
• ECF has two components- plasma and interstitial fluid
• Plasma delivers nutrients, O2, growth factors, hormones etc. from the
parent organ to the recipient site.
5. • There will be movement of these substances from the plasma into the
interstitial fluid and from the interstitial fluid into the intracellular fluid
(ICF).
• Similarly, there will movement of waste products (CO2, urea, uric acid) from
the ICF into the interstitial fluid and then back to the plasma.
• Plasma then takes these substances to the organ concerned for their removal.
• Hence, any change in composition of ECF will cause change in intracellular
fluid composition.
6. Disturbance in external environment
Disturbance in ECF
Detected by sensors
Activation of homoeostatic
mechanisms to reverse the change
8. Components of homeostatic mechanisms
Disturbance in external
environment
Disturbance in internal
environment
Variation in
regulated variable
Detected by sensors
Effector response
Effector
Stimulate control
center
9. Components of homeostatic mechanisms
1. Regulated variable- kept within the certain range, sensed by a sensor
2. Sensor – measures the value of regulated variable
3. Control center – control the output of effector response
4. Effector – generates the effector response
5. Controlled variable – manipulated to achieve regulation of regulated
variable
10. Feedback mechanisms
• Feedback mechanisms occurs in response to variation in a variable to
bring the variable to its set point.
• There are two types of feedback mechanisms-
1. Negative feedback
2. Positive feedback
11. External environment
Internal environment
Disturbance in external
environment
Disturbance in internal
environment
Variation in
regulated variable
Detected by sensors
Detected by sensors
Variation in
regulated variable
Stimulate control
center
Effector
Effector
Effector response Effector response
Stimulate control
center
12. Negative feedback mechanism
• Negative feedback mechanism is a series of changes that return the
regulated variable toward its mean value or within its normal range
by a negative response to the initiating stimulus (change in regulated
variable).
• Error reduction
13. Variation in
MAP (regulated variable )
Detected by baroreceptors
(sensors)
Stimulate medulla (control center)
Impulse go to heart, blood vessels (effector)
Change in heart rate, arteriolar diameter,
venomotor tone (effector response)
External environment
Internal environment
Disturbance in external
environment
Disturbance in internal
environment
14. Homeostatically regulated variables
Regulated
variable
Normal
range
Senser Control
center
Effectors Effector response
Arterial PO2 75-100
mmHg
Chemoreceptors (carotid
bodies & aortic body)
Medulla Diaphragm &
respiratory
muscles
Change breathing
sequence & tidal
volume
Mean Arterial
pressure
93 mmHg Baroreceptors (aortic
arch, carotid sinus)
Medulla Heart &
blood vessels
Change in HR,
arteriolar diameter,
venomotor tone
Blood
glucose
concentration
70-110 mg/dl Fed state- ATP sensitive
K+ channel
Fasting state-
chemosensors
Pancreas,
Hypothalamus
Liver,
adipose
tissue,
skeletal
muscle
Alter glucose
metabolism
Blood
osmolarity
280-296
mosM/kg
Osmoreceptors in
hypothalamus
Hypothalamus Kidneys Alter water
reabsorption
15. Gain
• Gain determines the efficiency of negative feedback mechanism.
• Greater the value of gain, more efficient will be the mechanism.
• Gain = correction applied/ residual error
• For e.g. The MAP raised from its set point of 90 mmHg to 120mmHg
during exercise. However, baroreceptor reflex brings it down to
100mmHg. Find the gain of baroreceptor reflex.
• Solution- Correction applied = 120-100 mmHg = 20 mmHg
Residual error = 100-90 mmHg = 10 mmHg
Gain = correction applied/ residual error = 20/10 = 2
16. Find the Gain
• A cold exposure which is expected to bring the body temperature from
37˚C to 30˚C, actually brings it down to only 36.5˚C. Calculate the
Gain of the thermoregulatory mechanism.
Write you answer in comment section
17. Positive feedback mechanism
• Positive feedback mechanism is a series of changes that increase the
variation in regulated variable by positive response to the initiating
stimulus.
• Error amplification
• It is a vicious cycle which terminates when the initiating stimulus is
removed.
18. Why positive feedback mechanism is
considered a homeostatic mechanism ?
• The positive feedback itself is part of an overall negative feedback
process.
• For example, in the case of blood clotting, the positive feedback
clotting process is a negative feedback process for maintenance of
normal blood volume.
• Another example is parturition reflex which is performed to restore
the nongravid physiology.
19. Examples of Positive feedback mechanism
1. Enzyme cascade hypothesis of blood coagulation
2. Parturition reflex
3. Hodgkin’s cycle
4. Activation of digestive enzymes
5. LH surge
20. • Blood coagulation
Injury to blood vessel
Activation of clotting
factors
Activated clotting factors act on
other inactivated enzyme
• This will continue till the permanent clot is formed and hence the stimulus is
itself terminated.
21. Matured fetus cause cervix
distension
Afferent signals from uterus
goes to posterior pituitary
Posterior pituitary
secrete oxytocin
Oxytocin cause
myometrium contraction
• Parturition reflex
• This will continue till birth of child and hence the stimulus is terminated itself.
22. Feedforward mechanism
• Feedforward or anticipatory control mechanisms permit the body to
predict a change in the physiology of the organism and initiate a
response that can reduce the movement of a regulated variable out of
its normal range. Thus, feedforward mechanisms may help minimize
the effects of a perturbation and can help maintain homeostasis.
• For example, anticipatory increases in breathing frequency will reduce
the time course of the response to exercise-induced hypoxia
23. Feedback Vs Feedforward mechanism
Feedback mechanism Feedforward mechanism
Generate response after the variation is detected Generate response anticipatory to variation
Certain time lag between variation and response No time lag between variation and response
