GENERAL PRINCIPLES

OF HOMEOSTASIS

Denny Agustiningsih

Dept. of Physiology

Concept of homeostasis was formulated by a French Physiologist Claude Bernard in 1865

He noticed that La fixité du milieu intérieur est la condition de la vie libre (it is the fixity of milieu intérieur which is the condition of free and independent life)

WHAT IS HOMEOSTASIS

From Greek words

Homeo= same Stasis= to stand

or stay Term proposed

by American Physiologist Walter B Cannon, 1933

WHAT IS HOMEOSTASIS

BASIC CELL FUNCTIONS Sensing and responding to changes in

surrounding environment Control exchange of materials between

cell and its surrounding environmentObtain nutrients and oxygen from surrounding

environmentEliminate carbon dioxide and other wastes to

surrounding environment Perform chemical reactions that provide

energy for the cell Synthesize needed cellular components

HOMEOSTASIS

HOMEOSTASIS1. DYNAMIC EQUILIBRIUM2. MULTIPLE STIMULI3. MAINTAINED BY NEGATIVE

FEEDBACK

HOMEOSTASIS

Homeostasis involves dynamic

mechanisms that detect and respond to

deviations in physiological variables from

their “set point” values by initiating

effector responses that restore the

variables to the optimal physiological

range.

MAINTENANCE OF HOMEOSTASIS Nervous system

Controls and coordinates bodily activities that require rapid responses

Detects and initiates reactions to changes in external environment

Endocrine systemSecreting glands of endocrine regulate

activities that require duration rather than speed

Controls concentration of nutrients and, by adjusting kidney function, controls internal environment’s volume and electrolyte composition

INTERNAL COMPONENTS OF HOMEOSTASIS

Concentration of oxygen and carbon dioxide

pH of the internal environment

Concentration of nutrients and waste products

Concentration of salt and other electrolytes

Volume and pressure of extracellular fluid

CONTROL OF HOMEOSTASIS

Homeostasis is continually being disrupted byExternal stimuli

heat, cold, lack of oxygen, pathogens, toxins

Internal stimuli Body temperature Blood pressure Concentration of water, glucose,

salts, oxygen, etc. Physical and psychological

distresses Disruptions can be mild to

severe If homeostasis is not maintained,

pathological condition even death may result

CONTROL OF HOMEOSTASIS

HOMEOSTATIC CONTROL SYSTEMS In order to maintain homeostasis,

control system must be able toDetect deviations from normal in the

internal environment that need to be held within narrow limits

Integrate this information with other relevant information

Make appropriate adjustments in order to restore factor to its desired value

CONTROL MECHANISMS IN HOMEOSTASIS

The organs and tissues which maintain homeostasis are called control systems.

Autoregulation (local control) when cells, tissues, organs or systems automatically

change in response to signals within themselves. Intrinsic control

controls that generate a response to change that built into the organ.

Extrinsic control changes stimulated by signals from outside of the cell,

organ, or system Mediated by

Nervous system: brain & spinal cord signals cause change; response is rapid but short term.

Endocrine system glands produce hormones that cause changes; response is slower, but lasts longer

HOMEOSTATIC CONTROL SYSTEMS Feedforward - term used for responses

made in anticipation of a change Feedback - refers to responses made

after change has been detectedTypes of feedback systems

Negative Positive

COMPONENTS OF FEEDBACK MECHANISMS Stimulus:

The change from ideal or resting conditions. Receptor:

The cells or tissue which detects the change due to the stimulus

Relay: The transmission of the message, via nerves or

hormones or both, to the effector. Effector:

The cells or tissue, usually a gland or muscles, which cause the response to happen.

Response: An action, at cell, tissue or whole organism level which

would not have occurred in the absence of the stimulus

Feedback: The consequence of the response on the stimulus. May be positive or negative.

COMPONENTS OF A BIOLOGICALCONTROL SYSTEM

NEGATIVE FEEDBACK

A REGULATORY MECHANISM IN WHICH A CHANGE IN A CONTROLLED

VARIABLE TRIGGERS A RESPONSE THAT OPPOSES THE CHANGE.

NEGATIVE FEEDBACK Most common homeostatic feedback

mechanism responses that resists any change from normal range.

Restoration of homeostasis because Negative feedback causes action that a value is falling

too low. Negative feedback causes action that a value is rising

too high. Values fluctuate within Set limits, Values change from hour to hour, sometimes

with regular diurnal cycles ( circadian rhythms). Elevated or reduced changes are corrected

before they become too extreme. Can continue forever Maintains homeostasis

EXAMPLE: body temperature - Set point = 37 C, 98.6 F

Normal limits 36.5-37.5 C, 97.7-99.5 F

Examples:.Body Temperature – Nervous System.Blood Calcium Levels – Endocrine System

When blood calcium level drops, parathyroid glandssense that and secrete hormones that cause releaseof calcium stores from bone. Blood calcium levelsreturn to normal, secretion stops.

HOMEOSTASIS OF BLOOD PRESSURE

Baroreceptors in walls of blood vessels detect an increase in BP

Brain receives input and signals blood vessels and heart

Blood vessels dilate, HR decreases

BP decreases

POSITIVE FEEDBACK

A REGULATORY MECHANISM IN WHICH THE RESPONSE TO A

STIMULUS, IN A CONTROL SYSTEM, CAUSES THE CONTROLLED VARIABLE

TO MOVE FARTHER FROM THESET POINT.

POSITIVE FEEDBACK Rare homeostatic feedback

mechanism The response enhances the

original stimulus Positive feedback promotes

changes that are developing, so they become more extreme

Positive feedback causes action that a value is falling causing it to fall farther.

Positive feedback causes action that a value is rising causing it to rise higher.

Positive feedback in disease - ex. Hypothalamus increases set point. Development of a fever.

Examples:.Hemorrhage.Childbirth.Lactation

POSITIVE FEEDBACK Unstable system It is used to trigger a sudden event or

phenomenon Can not continue forever, always has a

limit Does not result in homeostasis Beneficial only in special circumstances

Example in childbirth, stretching of uterus elicits production of hormones that stimulate contraction of uterine muscles; this leads to more stretching of uterus, more hormone secretion, more muscle contractions; loop is broken upon birth of a baby

POSITIVE FEEDBACK DURING CHILDBIRTH Stretch receptors in walls of uterus send

signals to the brain Brain induces release of hormone (oxytocin)

into bloodstream Uterine smooth muscle contracts more

forcefully More stretch, more hormone, more

contraction etc. Cycle ends with birth of the baby & decrease

in stretch

COMPARISON BETWEEN FEEDBACK LOOP