Download MBBS Physiology Presentations 42 Homeostasis Lecture Notes

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Internal environment ? milieu interieur
extracellular fluid which includes
interstitial fluid.

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Volume and composition of this fluid
should be maintained constant (within
narrow limits) in spite of changes in
external environment for the normal

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function of cell.

Failure of homeostasis leads to disease
Control systems of the body ?

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- genetic control in all the cells

- systems working within the organs

control function of individual organs

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- systems operating throughout the

entire body to control interrelation

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between the organs

Composition of ECF is same all over

the body because of constant mixing

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of interstitial fluid and blood and

circulation of blood
Some regulating systems and their

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functions ?
Resp. system - CO2 content
Liver and pancreas ? glucose
Kidney ? conc. of ions

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CVS ? blood flow and temp regulation
Nervous and endocrine systems

regulate the function of other systems

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Components of control system ?

1. sensor to detect disturbance,
2. controlling center,
3. effector

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Homeostatic Mechanisms-
1.Negative feedback mechanism
2.Positive feedback mechanism
3.Feed forward or anticipatory mechanism

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Imp 1.Negative feedback mechanism ?

- most common

- the effect of controlling system is in the

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opposite direction of change in the
parameter so that disturbance is
minimized.

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-ve feedback mechanism

e.g.

regulation of blood pressure by

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baroreceptors,

regulation of hormonal secretion,

regulation of PCO2 of ECF.

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Components of -ve feedback regulation

Disturbance

Controlled variable at a set point

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Sensor monitors currant value

Feedback signals

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Controller- compares

Set point

If error

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Output through effectors

Effector response

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Correction

-ve feedback mechanism

Characteristics ?

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-the disturbance is not corrected
immediately ? latency ? dead time so
oscillations are possible
-The disturbance is not corrected
completely ? residual change

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-Regulation factor = change with regulation

change without regulation
-ve feedback mechanism

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-Effectiveness of regulating system ?

Gain =

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correction applied / residual change

-ve feedback mechanism

E.G. if large vol of blood is transfused

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without baroreceptor system increase in
blood pressure is 100 mmof Hg to 175
mm of Hg and with baroreceptors it is
125 mm of Hg then,

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Regulation factor is 25 / 75 = 1/3

Gain is -50 / +25 = -2

Gain of body temp.regulating system is -33

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ImpPositive feedback mechanism ?

the effect of regulating system
magnifies the error and sets in vicious
cycle which stops only when the initial

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stimulus is removed.

2 lits of blood loss

+ve feedback

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mech.

Ineffective heart pump

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B.P.

blood flow to heart muscle

Weakness of heart muscle

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heart pump

Vicious cycle stops when blood volume

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is corrected
+ve feedback mech.

Examples of Useful +ve feedback regulation ?

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for rapid magnification of basic corrective
mechanisms

1.Hodgkin's cycle for Na+ transport during AP
2.Oxytocin secretion during parturition

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3.LH surge for ovulation
4.Enzymatic cascade for blood coagulation
5.Activation of digestive enzymes

3. Feed forward mechanism ? anticipatory

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mechanism ? conditioned reflex
By detecting rate of change rapid

anticipatory corrective measures which

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may not be accurate e.g. acceleration,

exposure to cold
Limitations ?

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- age

Undamped oscillations due to long dead
time and more gain of system ?

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overcorrection

e.g. Chyene Stoke's breathing, clonus
during deep reflexes

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Metabolic tissue

Intracellular water-40 %

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Total

body

water

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Extracellular water -20%

Bone tissue

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Fat

Body fluid compartments-

60% of body wt.(70kg.) ? 42 lits.

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ICF-2/3 -28lits

ECF ? 1/3 ? 14 lits

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2 lits in RBC

Transcellular -

Plasma -3lits -

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Interstitial fluid 1-2 lits

5% 1/5

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-11 lits. 3/4

7% of fluid in intravascular compartment
Body fluids

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?62% of body wt in males and 52 % in
females
?72% of lean body mass (adipose free)
body wt is 70kg in man
body water is 42 lits.

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28 lits ( 2/3) ICF ? 2 lits - intravascular
14 lits. (1/3) ECF ? 3 lits - intravascular

Body fluids

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Measurement of body fluid compartments-

?indicator dilution technique ?

?Criteria for ideal dye ?

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?Dye should get uniformly diluted only in
the compartment whose whose volume
is to be measured
Body fluids

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?Dye should be nontoxic,

?should not change fluid volume,

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? should not be metabolized, altered

?or excreted in significant amount

? should be able to estimate easily

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Body fluids

Two spaces are calculated indirectly ?

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- Intracellular fluid compartment

( TBW ? ECF)

- Extravascular fluid compartment

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(ECF - plasma vol.)
Body fluids

Example

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Inulin for ECF ? 100 mg inulin injected
After 30 min. inulin conc in blood ?

0.75 mg /dl
25 mg excreted in urine during 30

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mins.

ECF vol = 100 ? 25 / 0.75 = 10000 ml
= 10 lits.

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Body fluids

Substances used ?
TBW ? D2O, tritiated H2O, aminopyrine

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ECF ? Na thiosulphate, sucrose, mannitol,
Intravascular fluid ?
Plasma vol. - Evan's blue, T

131

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1824 ,

I2

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Hematocrit

Osmolarity of ECF = 290 mOsm / lit