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Homeostasis Physiology Lecture

Homeostasis is the first concept you will learn in medical

physiology it's more than a concept it's going to be the underlying theme

of your entire physiology course. Today we are going to

discuss this homeostasis in a conventional way. That is

how it was originally proposed by Claude Bernard that is "stability of

internal environment" What did he suggested as

internal environment? let's start with a single cell organism like bacteria

bacteria can survive in extremes

temperatures, like different types of bacteria adapt to different

environmental conditions. like some bacteria live under

you know boiling temperatures and some bacteria lives

in freezing conditions like less than zero degree centigrade.

different species of bacteria had adapted

to survive under different environmental conditions like

temperature oxygen pH and so many other things.

though they have ability to adapt to a different condition over period of time

a given bacteria, for example these bacteria cryophilic bacteria as we call

them, which lives under sub-zero temperature

if we put them at a higher temperature they may not thrive. A single bacterial species

has evolved to survive only under a particular environmental condition so if

we change it it's going to die!! But when we come to multicellular

organisms they create a micro environment

surrounding their cells we call it as a internal environment.

It's actually their body so this internal environment here as

these are the group of cells and this is a body of the organism and you

can see this is external to the body. so

there is a controlled environment surrounding their 'cells' which is called

as the internal environment and the actual surrounding of the entire

'organism' is called as a external environment. The multicellular

organisms try to control the internal environment's

variables like oxygen level temperature pH and so many other things.

This is called the stability of life conditions in the internal environment.

if you take a human body, it has trillions of cells, like the cells inside

are of different organs are going

to perform different functions like an rbc is going to transport oxygen

nerve cell is going to transfer information to other cells

muscle cell is going to produce force. so there are many cells which will

perform different functions but collectively they are all trying to

maintain the conditions in the internal environment as a stable

constant one. It's surrounded by the protective cells

and there are exchange cells which will transfer or exchange energy and

bio-molecules with the environment. this is the

conventional way of looking at it that is the in internal environment means

the extracellular fluid and that's where the cells and other things survive

and the homeostasis refers maintaining the conditions of the extracellular fluid

that is internal environment. so this is the conventional way of

looking at it, though today we understand that even single solar

organisms maintain their own internal environment

that is within the cells. it is also considered as homeostasis .

so now we understand that entire body

not just the extracellular fluid is considered as an internal environment

the cells produce it, they're part of it and they all enjoy the benefit of it that

they survive within this constant live conditions this was

originally proposed by Claude Bernard he is doing an experiment

over here he is a big fan of experimental medicine and he's

also called as a father of Modern Physiology. let's see what he said

about this concept maintenance of stable 'milieu interior'

'milieu interior is a french word' and in English it is internal environment

he states 'All the vital mechanisms

however varied they may be, here we are referring to all vital mechanisms like

respiration, excretion, locomotion and all the other functions

however varied they may be have one common goal

that is to maintain the uniformity of the conditions of the

internal environment the life conditions we are referring here as this

oxygen level ph temperature glucose levels and all other things. he also stated the

stability of this internal environment is the condition for free and

independent life. though Claude Bernard gave the concept

the word 'homeostasis' was coined by Walter Cannon

he further expanded the concept and popularized it

in his book 'wisdom of life' and he also coined the term

flight or fight response also. let's see what he has said

in his book. Some of the important ideas were, "constancy in an open system

requires mechanisms that act to maintain the system"

open system here refer to the fact that human body continuously

exchanges like substances and energy with the

external environment if something is called as a closed

system then there is nothing is exchanged with the environment. so human

body is an open system because continuously substances are

exchanged with the environment. so such system is called as a steady

state like a system has a stable internal environment

but there is continuous exchange with the environment happening all the time.

a stable state happens even in another condition called as an 'equilibrium'

where the internal environment of a system is constant

because there is "NO net exchange" happening between the system

and its environment. Equilibrium is not compatible with life because to sustain

life processes energy is required which must be gained

from the environment and other things like waste products are

to be lost to the environment so the condition maintained within the

body that is a constant internal environment despite the fact

that there is a 'continuous exchange' happens with the environment this is

called as a steady state. and steady state conditions require that

tendency towards change are automatically met with factors that

resist the change steady state indicates that a variable

in the system is kept constant though it is continuously being added or

removed from the system maintenance of steady state require

energy and a control system which matches the input and output of the

variable. For example the temperature of a human

body is maintained at 37 degrees by balancing continuous heat production

by the human body and heat loss to the surroundings.

Any maintained variable including the temperature cannot be exactly constant

but it is maintained within the narrow range so called

a relatively constant value so that's why it is used in the definition of

homeostasis "Homeostasis is the maintenance of steady

state that is a relatively constant internal physical and chemical

conditions in the body by coordinated physiological control

system" so he also adds homeostasis does not

occur by chance but is a result of organized

self-government and yes this is true and that is what is

the function of a homeostatic control system

let's see what it is homeostasis involve dynamic mechanisms that detect

and respond to deviations in physiological variables

from their set point values by initiating an effector response that

restore the variables to the original optimal physiological

range. so, these are the key words in this

definition we are going to understand this by

using an example of air conditioner the air conditioner in your room is

maintaining a variable that is room temperature

and let's say the room temperature is 30 degrees

and you set it up at 28 degrees you know through your remote so the

value you are setting is the set point and the actual level of the variable

is 30. once this is set the exact value is

detected by a sensor. which is going to

detect what is the actual value and it is going to compare with

the set point and if there is a difference then it is

going to generate a response that is the AC is going to generate a response

of sending cool air into the room so

thereby eventually it restores the room

temperature and when it restores it's not going to

be exactly maintained at 28 degree centigrade there may be a small

range for example if it goes below 27 it may switch off and then if it goes

above 29 then it may start working again. so there is a range in which the room

temperature is maintained though we are giving a set point of 28 there is

optimal range in which the room temperature is

maintained. and this is the explanation for all the key points in a homeostatic

control system let's discuss the components of

homeostasis control system as we discussed about there is a sensor

which is going to detect a changes in the variable and it

is going to send this information to feedback

controller which is a part of the central comparator

and it compares set point value with the sensor detected value

and if there is a difference then it is going to send the information

to the effector which is going to influence the regulator variable again

This is called as a feedback loop

feedback loop and let's there are two types one is a

positive and a negative feedback loop let's see each of the things with an

example let's say there is a regulator variable

at a particular level there is a disturbance which causes the

regulated variable to increase and the sensor detects the change sends

to the feedback controller and it detects the difference

and now it initiates effector response to put down

this regulator variable. so that it comes back to its normal level again

let's see this negative feedback with an example of plasma glucose level

maintenance here the regulated variable is plasma

glucose let's consider a disturbance that is

consumption of food which cause the glucose levels to

increase and this is sensed by the sensor which

is located in the beta cells within the pancreas and it's going to

sense the difference between

the set point value and the actual variable value

and if it is higher the actual value is higher it is going to release

insulin so this is the pancreatic beta cell which is going to

function as the sensor and the feedback controller and the effector

is the insulin. Insulin is going to act on various body tissues and

going to bring the glucose level down so here original change is increase but

actual response of this control system is to (decrease)

minimize the change so this is the basic concept of a negative feedback

system in a negative feedback system the stability is

maintained. In a positive feedback there is a regulated variable

and if a disturbance causes it to increase which is detected by a sensor

and feedback controller and ultimately the effector is going to again

cause more and more increase you know it is going to do produce change

in a same direction whatever the original change direction the effector

is also going to augment this change. let's see this with an example opening

of voltage-gated sodium channel. NaV stands for voltage-gated sodium channel

in action potential. Here the regulated variable is membrane potential

membrane potential is going to be maintained at -70 mV

but when there is a small depolarization disturbance

this is going to be sensed by sensor within the voltage-gated sodium channels

we call it as a voltage sensor and this is going to communicate with

the other components of this sodium voltage-gated sodium channel and

they are going to open this when the NaV opens

it's going to cause further depolarization

so if the original change is from -70 mV to -60 mV, then opening of more and

more sodium channels is going to depolarize

it further to let's say 0 mV or even +10 mV

so this is what happens during an action potential this is an example of positive

feedback so positive feedback is essential for

promoting a change in the system they are not essential for maintaining the

stability as such of the variable. But they promote

change in a condition. This is essential in a large

picture this is essential for the homeostasis or the survival of the

organism even the positive feedbacks are essential

though sometimes the positive feedbacks can work

in a vicious cycle and may produce deleterious effect.

um let's see one other mechanism that is feed forward

last two mechanisms are feedback loops now this is a

feed forward mechanisms and this is Glucagon like Peptide-1 induced

insulin release Here the regulated variable is

again glucose and first i am going to draw the

feedback loop involved, when there is a disturbance like when is food

consumption and glucose levels are sensed by the

the beta cells of pancreas

and ultimately it is going to release the insulin and going to decrease the

glucose level this is a feedback loop but sometimes even before the change

happens the disturbances which are causing

the change in the regulated variable are sensed by

other controllers like a feed forward controller

which is going to influence this regulated variable through either a

feedback controller or effector that here directly insulin is

released by the presence of food in the intestine. certain cells sense the

presence of food in the intestine and releases

GLP-1 which is going to stimulate the insulin release

though the regulated variable is glucose which has not changed

yet, even before the change happens the glucose

levels are going to be regulated by the insulin

levels so this is called as a feed forward mechanisms

here though the feedback mechanisms both positive and negative operates only when

the regulated variable changes but even before the regulated variable

change the disturbance itself is sensed by a

feed forward controller and it proactively maintains the system.

sometimes this is referred as anticipatory control system

thanks for watching our next lecture is

on Negative Feedback gain stay tuned