Pdf physiology on the topic homeostasi.s

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1. LECTURE -2

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5. The Maintenance of a relatively stable internal environment is
termed homeostasis (homeo means “similar”; stasis means “to
stand or stay”)

6. French Physiologist Claude Bernard said that the
'stability of the internal environment (milieu interieur)
is the condition for the free and independent life'.
Milieu Interieur

7. Organism's ability to keep a nearly constant internal
environment was termed as homeostasis by Walter Bradford
Cannonan American Physiologist in 1929. If the body loses
homeostasis, illness or even death will occur.

9. Feedback Homeostasis
Regulations

10. What is Feedback?
It is flow of information in a closed loop.

11. Intrinsic Controls Extrinsic Controls
Homeostatic Control Mechanism

12. Homeostatic Control Mechanisms
Feedback
Mechanisms
Feedforward
Mechanisms

13. Feedback Mechanism
In this the effect or output of an action is returned(feedback) to
modify the next action. In this the change actually occurs.

14. Feed Forward Mechanism
This type of control system anticipates the disturbance and
prevent the change in controlled variable by generating a
corrective command. It means system is creating a condition in
anticipation of the change that will occur.

15. Feedback Mechanisms
Negative Feedback MechanismPositive Feedback Mechanism

16. Negative Feedback
When the effector opposes the stimulus(such as a dropping
temperature) and reverses the direction of change (causing the
temperature to rise).The controller acts opposite and the error
is minimised.

17. Components of Negative Feedback:

18. Most systems supporting homeostasis operate by
negative feedback.

19. Positive Feedback
When the effector reinforces the
stimulus (such as uterine contractions
during childbirth, which trigger the
release of the hormone oxytocin) and
amplifies the direction of change
(causing even greater contractions and
further release of oxytocin)

20. Positive Feedback
The controller acts in same direction and error is amplified.

21. Because positive feedback is stimulatory, there are only a few
situationsin which it is beneficial to the body (such as during
childbirth or in blood clotting).
More often, positive feedback is harmful(such as when a high
fever continues to rise)

22. In each case where positive feedback is useful, it is actually a
part of an OVERALL negative feedback process. For Example in
case of blood clotting the positive feedback clotting process is a
negative feedback process for maintenance of normal blood
volume.

23. Positive Feedback Mechanism
When the response to a stimulus increases the original stimulus,
it is known as positive feedback mechanism. It is also known as
VICIOUS CYCLE.

24. Negative Feedback Mechanism
When the response to a stimulus reducesthe original stimulus, it
is the negative feedback mechanism.
It is the Primary type of homeostatic control.

25. Negative Feedback Mechanism
(Examples)

26. Examples of Negative Feedback Mechanism
1-Thermoregulation
2-Insulin mediated control of blood glucose
3-Pituitary Hormone Release.
4-Increase in Carbon dioxide concentration.
5-Blood Pressure Regulation

27. 1. Thermoregulation:The core temperature range of a human
body is between 36.1 °C and 37.8°C, with average of 37°C and
this is considered to be the normal body temperature in an
adult.

28. 1. Thermoregulation:As the body temperature increases, the
sensory receptors detect and send the information to the
hypothalamus.

29. 1. Thermoregulation:The hypothalamus responds to the stimuli
by initiating vasodilatation and sweating (sweat glands in the
skin are stimulated via cholinergic sympathetic nerve) and this
decreases the body temperature.

30. 1. Thermoregulation:When core body temperature decreases,
there is vasoconstrictor response via sympathetic nerve
stimulation, shivering, along with adaptive change via
psychological response (wearing warm clothes and curling up in
the bed which reduces the surface area (skin) and this aids in
preventing heat loss.

31. 1. Thermoregulation:Increase in metabolic rate, thermogenesis
and shivering response helps to overcome hypothermic effects.

32. 2. Insulin mediated control of blood glucose.Blood glucose
concentration increases after meals (stimulus). This releases
insulin from pancreas, and it transports glucose from the blood
into tissues (the response).
Blood glucose concentrations then lowers down which decreases
the secretion of insulin into the blood.

34. 3. Pituitary hormone release: Release of releasing hormone from
hypothalamus and stimulating hormone from pituitary gland is by
negative feedback mechanism. The hormone then work on target
gland to produce desired effect.

35. 3. Pituitary hormone release:Adrenocortical releasing hormone
(ACTH), thyrotropin releasing hormone (TRH), etc. are released
by negative feedback mechanism.

36. Increased level of T3 , T4 hormonedecreases the secretion of
thyrotropin releasing hormone (TRH) from hypothalamus and
thyroid stimulating hormone (TSH) from pituitary directly and also
via TRH thereby decreasing the T3 , T4 hormone level and vice
versa.

37. 4. Increase in concentration of carbon dioxidein the blood,
stimulates the chemoreceptors; which further stimulates the
respiratory centre to increases the rate and depth of breathing.
The increased ventilation removes more carbon dioxide and CO2
level comes down.

38. 5. Blood Pressure Regulation (Baroreflex)
Baroreceptors sense the increase in BP as they are stretch
receptors.These send impulses to Medulla of brain which
decreases the BP.

40. Positive Feedback Mechanisms
Positive feedback mechanisms brings over further increase in
variable in response to initial increase in variable. It leads to
instability rather than stability and in some cases can even cause
death.

41. Positive Feedback Mechanism
(Examples)

42. Examples of Positive Feedback Mechanism
1-Blood Clotting
2-LH Surge prior to Ovulation
3-Ferguson Reflex(Parturition Reflex)

43. Examples of Positive Feedback Mechanism
4-Lactation(Suckling by baby)
5-Hodgkin’s Cycle(Generation of action potential)
6-Calcium release from S.R during Muscle Contraction

44. Examples of Positive Feedback Mechanism
4-Micturition
5-Head’s Paradoxical Reflex(Lung inflation in neonate)
6-Activation of digestive enzymes

45. 1. Clotting cascade:When the blood vessel gets damaged,
platelets adhere to the injured site and release chemicals which
further attract more platelets. The platelets continue to pile up
and initiate clotting cascade.

46. 1. Clotting cascade:The clotting factor which is activated
further acts as enzyme to activate the other clotting factors
until a clot is formed.

47. 2. Parturition reflex:At full term of pregnancy head of fetus gets
engaged and exerts pressure over cervix and sensory information
of mechanical stretch of the cervix is relayed to paraventricular
and supraoptic nuclei of hypothalamuswhich further increase the
secretion of oxytocin from the posterior pituitary.

49. 2. Parturition reflex:Oxytocin acts on myometriumwhich
stimulates uterine contractions, and this in turn further increases
pressure on the cervix until the fetus is delivered.

50. 3. Hodgkin's cycle: Opening of set of sodium channels in a
membrane (nerve, neuron, etc.) further activates opening of
more sodium channel leading to depolarization.

52. 4. LH Surge:

53. 5. Suckling Reflex:

54. 6. Activation of Digestive Enzymes:

55. Examples of Feed Forward Control

56. ❖Anticipatory Tachycardia
Increase in heart rate and respiratory rate even before the start
of exercise due to psychic stimulation

57. ❖Cephalic phase of Gastric Secretion
(Just thinking about food increases the gastric secretion).
❖Role of Cerebellum in motor coordination.
❖Insulin secretion in response to entry of food in intestine.

58. ❖Thermoregulation.
Thermoreceptors are located within the body(Central
Thermoreceptors) as well as in the skin( Peripheral
Thermoreceptors)

59. Thermoreceptors
Peripheral ThermoreceptorsCentral Thermoreceptors

60. ❖Central Thermoreceptors
These function as conventional feedback sensors. If CORE body
temperature falls to 36 Degrees then the control system will bring it
back to SET POINT i.e 37 Degrees.

61. Peripheral Thermoreceptors
These function as feedback forward sensors. Decrease in
environmental temperature is rapidly sensed by skin
thermoreceptors and compensatory themoreguatory responses are
initiated before there is any appreciable fall in CORE body temp.

63. Potentiation:
This is when one substance augments the response to another
substance, even though the first substance does not exert a
significant response on its own.

64. Potentiation:
For example release of the GI hormones from the intestine in
response to a meal. These hormones can potentiate the pancreatic
insulin response to absorbed glucose.

65. Potentiation:
It is feedforward potentiation, as GI hormones “announce”the
upcoming increase in blood glucose before glucose absorption
actually occurs in the small intestine. When glucose finally arrives
via bloodstream at pancreas, there is a potentiated insulin
response such that hyperglycemia is prevented.

66. Regulatory Factor (R):
It is a measure of accuracy of regulation system that leads to
some residual change in the controlled variable, but is to a lesser
extent. Thus, reaching to state of perfect harmony of
physiological balance; may not be attainable.
R = Change with regulation/
change without regulation.

67. Gain of Control System
The degree of effectiveness with which a control system
maintains constant condition is determined by the gain of
negative feedback. Control system is not 100% efficient; some
error always remain:
Gain = Correction/ Error

68. Gain of Control System
For example, let us assume that a large volume of blood is
transfused into a person whose baroreceptor pressure control
system is not functioning, and the arterial pressure rises from the
normal level of 100 mm Hg up to 175 mm Hg

69. Gain of Control System
Then, let us assume that the same volume of blood is injected
into the same person when the baroreceptor system is
functioning, and this time the pressure increases by only 25 mm
Hg

70. Gain of Control System
Thus, the feedback control system has caused a “correction” of
−50 mm Hg, from 175 mm Hg to 125 mm Hg. There remains an
increase in pressure of +25 mm Hg, called the “error,”which
means that the control system is not 100% effective in
preventing change

71. Gain of Control System
In this example, the correction is −50 mm Hg, and the error
persisting is +25 mm Hg. Therefore, the gain of the person’s
baroreceptor system for control of arterial pressure is −50
divided by +25, or −2.

72. Gain of Control System
Rapid loss of 1 L of blood leads to a decrease in mean blood
pressure from the set point of 100 to 75 mm Hg. Reflexes
restore blood pressure to 95 mm Hg. The correction, therefore, is
20 mm Hg and the remaining error is 5 mm Hg.

73. Gain of Control System

74. Gain of Control System

75. Gain of Control System
Higher the gain, the higher the ratio of correction to remaining
error and the better the control system is at restoring the system
to its set point.

76. Gain of Control System
Gains of some other physiological control systems are much
greater than that of the baroreceptor system. Gain of the system
controlling internal body temperature when a person is exposed
to moderately cold weather is about −33.

77. Examples
Calculation of gain:
1. The systolic blood pressure in a subject is 120 mm of Hg under
physiological condition. The BP becomes 100 mm with regulation and
60 mm without regulation. What is the gain of the regulatory system?
The error is 20 mm;
correction is 40 mm.
Thus,gain 40/20 = 2

78. Examples
Calculation of gain:
2. A cold exposure which is expected to bring the body temperature
down to 20°C actually brings it down only to 36.5°C. Thus, the
observed change is only 0.5°C. The expected change without
regulation is 17°C and correction is 16.5°C.
Thus, gain 16.5°C/0.5°C = 33.
The gain of the system is 33.

79. Examples
Calculation of gain:
3-A patient got his BP reduced by 10 mmHg when he stood
up and he regained only 8 mm of Hg. What was the gain ?
Q. Correction here is +8 mmHg and
error is -2 mmHg.
Ans. Gain is +8/-2 i.e -4

80. Internal Factors Influencing Homeostasis
Genetics: Genetic predisposition leads to development of certain
genetic disorders and diseases.

81. External Factors Influencing Homeostasis
Lifestyle modification such as balanced dietand regular physical
activities also aid in maintenance of homeostasis. Diet devoid of
iron may lead to anaemia while balance diet will restore iron
level.

82. External Factors Influencing Homeostasis
Regular physical activityimproves mental and physical wellbeing,
increases muscular mass and stability, and increases the ability of
the cardiovascular system to deliver oxygen to the tissues.

83. EXAM ORIENTED QUESTIONS (Essay Type)
1.Define homeostasis. Enlist the various body systems
involved in homeostasis. Discuss positive and negative
feedback mechanism.

84. Short Notes Type
1.Describe positive feedback mechanism citing two
examples.
2.Describe negative feedback mechanism citing two
examples.

85. MCQ Type
1-Which of the following has the highest feedback gain?
A) starting blood pressure = 100; low point in blood pressure = 70; final
blood pressure after feedback correction = 90
B) starting body temperature = 37.2°C; high point in body temperature =
38.9°C; final body temperature after feedback correction = 37.4°C

86. MCQ Type
C) starting blood glucose = 80 mg/dL; high point in blood glucose = 110
mg/dL; final blood glucose after feedback correction = 85 mg/dL
D) starting plasma osmolality = 280 mOsm/kg; low point in plasma
osmolality = 270 mOsm/kg; final osmolality after feedback correction = 278
mOsm/k

87. 23+ years Teaching Experience
1 Million Students & Teachers Mentored
Dr. Sachin Kapur
M.Phil, Phd.
AIR-1 (AIIMS)
AIR -1 (AIPMT)
AIR -1 (DPMT)

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