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Homeostasis

Dr. Maha SajaPhysiology department,

Level 2,Office 89,

Email: msaja@ksu.edu.sa

• Define the concept of the “internal environment” and state itsphysiologic importance.

• Differentiate between the external and internal environments.

• Define and discuss the concept of homeostasis and itsimportance to the living organism.

• Discuss the physiologic control mechanisms that enablemaintenance of the normal steady state of the body.

• Define a feedback mechanism and describe its components.

• Differentiate between positive and negative feedbackmechanisms and give examples for each in the body.

Objectives

Study source for this lecture:(Guyton & Hall Textbook of Medical Physiology, 13th ed, Chapter 1)

• All the cells in the body arecontinuously bathing in fluid.

• Because this fluid is outsidethe cell, it is calledextracellular fluid (ECF).

• It is from the ECF that cells getthe ions and nutrients neededto maintain life.

The Internal Environment“Milieu Intérieur”

(Guyton and Hall Textbook of Medical Physiology. 13th ed. Ch-1)

Because,ØAll body cells live in the same environment (i.e. ECF).ØThe composition of ECF is almost similar between the different species.

It was named the “internal environment” by the French physiologist ClaudeBernard.

Energy (ATP) Work

GlucoseOxygen

Chemicalreactions enzymes

pHTemperature

wasteCO2Urea

Ions

Fatty acidsAmino acids

In order for the cell to function properly,

ECF(Mr. Ford’s class-Introduction to anatomy and physiology-youtube)

The internal environment need tohave the right amount/level ofthese substances/variables.(not too much and not too little)

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External vs Internalenvironment

• ECF = theinternalenvironment.

• The skinseparates thisenvironmentfrom the outsideworld whichknown as theexternalenvironment.

“Internal environment”

In Multicellular organisms

“External environment”

External vs Internalenvironment

In Unicellular organisms

Internal environment = External environment

External vs Internalenvironment

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Homeostasis• The internal environment must be keptconstant in the face of an ever changingexternal environment.

• The internal environment of the body (ECF)isin a dynamic state of equilibrium

• The process by which the body keeps the internalenvironment constant despite changes in theexternal environment is known as“Homeostasis”.

• Homeostasis means:– Homeo- : sameness, similarity– -stasis: standing

• Essentially all the functions of the body organsand tissues aim at keeping the internalenvironment at a nearly constant state.

“Homeostasis”

(Guyton and Hall Textbook of Medical Physiology. 13th ed. Ch-1)

Internalenvironment

O2

CO2

ProtectionSkin

Immune system

Control systems

Nervoussystem

Endocrinesystems

Body constituents are normally regulated within a range rather than afixed value;

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Control Mechanisms• The body has thousands of control systems.

• They function to restore balance when it is lost.

• Control systems operate;– Within the organ itself– Throughout the body→ to control interrelations between organs.

Feedback Mechanisms

What is meant byfeedback?ØA loop system inwhich the systemresponds toperturbation eitherin the same direction(positive feedback)or in the oppositedirection (negativefeedback).

What are the components of afeedbackmechanism?

The Thermostat Analogy

Negative feedbackThe effector response of thesystem is in the oppositedirection to the stimulus thatinitiated the response.

E.g;• A high level of in CO2 in the

ECF will increase pulmonaryventilation, increasing theamount of CO2 expiredwhich will bring the level ofCO2 in ECF down.

• Most of the control systemsof the body act by negativefeedback.

Types of Feedback Mechanisms

Positive feedbackThe effector response is in thesame direction of the stimulusthat initiated the response.

E.g;• In nerve signaling, entry of a

small amount of Na+ intothe cell will open more Na+channels causing more Na+to enter the cell.

• Only few systems displaypositive feedbackmechanisms..WHY?

(Guyton and Hall Textbook of Medical Physiology. 13th ed. Ch-1)

Examples of Negative FeedbackMechanismsBody temperature control

Blood pressure control

Examples of Positive FeedbackMechanisms

Childbirth

Can you find otherexamples for a positivefeedback mechanism in

the body?

Disease is a state of disturbedhomeostasis