Introduction to Homeostasis September 2006 Clinical Science Team CardiffUniversitySONMS©CSANTeam

Introduction to Homeostasis

September 2006

Clinical Science Team

CardiffUniversitySONMS©CSANTeam

Learning Outcomes

•Define the term homeostasis

•Define the term internal environment

•Explain the principle of homeostatic set range

•Define the term stress as applied to physiological systems

•Define the term stressor

•Define the two control systems

•Describe and illustrate a typical physiological control loop

•Explain the principle of negative feedbackCardiffUniversitySONMS©CSANTeam

Homeostasis‘The regulatory mechanisms of the body can be understood in terms of a single shared function: that of maintaining constancy of the internal environment. A state of relative constancy of the internal environment is known as homeostasis, and it is maintained by effectors that are regulated by sensory information from the internal environment (Fox 2002, p.5)’


HomeostasisThe Body in Balance



External Environment


Threats from the External Environment


Internal Environment

Nutrients & Oxygen

etc

Waste Products


Homeostasis• A condition in which the body’s internal environment remains within set

physiological limits (homeo = same; stasis = standing still).


• Physiological set homeostatic points, or the set homeostatic range• refer to the normal range of values for given physiological factors• equate with normal function and health of both the cell and the individual, for example:•Plasma glucose – 4 -7 mmol/litreArterial plasma pH – 7.35 -7.45

Set Point or Set Homeostatic Range


STRESS


In physiological terms stress is defined as:

any stimulus that creates an imbalance (above or below the set homeostatic range), within the internal environment.

Stress


• The stimuli that produce imbalances in homeostasis are called stressors. These fall into three categories:

1. Physical

2. Psychological

3. Sociological

Stressors


Control SystemsThe body detects and responds to homeostatic imbalances via two complementary control systems:

The Nervous System

The Endocrine System

These two control systems work together to maintain homeostatic balance


Control Systems Respond to and Regulate Imbalances in Homeostasis


Negative Feedback LoopsFor constancy of the internal environment to be maintained, the body must have:

Sensors (receptors) that are able to detect deviations from a set homeostatic point or range.

An integrating centre that receives information from the sensor (particular region of the brain/spinal cord, or distinct cells within an endocrine gland). The integrating centre responds by influencing the action of effectors.

Effector cells or organs function to re-establish the normal homeostatic range.

An analogy of this control loop is seen in temperature control via a house thermostat:


Negative feedbackThe house thermostat….. Imagine, the thermostat in your house is set

to 20°C (set point), it’s a warm day and the temperature soon exceeds 20°C,

1) the thermostat (sensor) senses this change,2) its equivalent of an integrating centre

instructs the air conditioner (effector) to activate which lowers the temperature below the set point.

3) It reverses the temperature change.


Negative feedback



Antagonistic effectors• Most factors are controlled by several

effectors• These often have antagonistic (opposite)

effects• Control by antagonistic effectors can be

described as ‘push-pull’• Increasing activity of one effector is

accompanied by decreasing activity of an antagonistic effector

• This affords a finer degree of control


Negative feedback loopCardiffUniversitySONMS©CSANTeam

Negative feedback loops


Positive feedback• Works in the opposite direction to negative

feedback

• Positive feedback amplifies the effect of the change to the set point

• Think of the thermostat, if the mechanism was positive feedback, a rise in temperature would be amplified by the effector, thus the temperature would continue to increase


Positive feedback• An example of positive feedback occurs in

child birth

1. Contractions cause uterine muscle stretch

2. Signals sent to posterior pituitory

3. Oxytocin (a hormone) is released

4. Stimulates further contractions

+vefeedback

Unsuccessful Return to

homeostasis

Nervous Endocrine Immune Learned Behavioural Responses

Physical & Mental StressorsInternal & External

Microbiology

Malnutrition

INTERVENTIONSPharmacologyNutritionNursingMDT

Successful Return to homeostasis & health

Altered Physiological function - Ill Health

Adaptation

Monitoring & Regulating systems

Imbalance in homeostasis

Homeostasis and HealthCardiffUniversitySONMS©CSANTeam

ConclusionIn this session we have briefly explored the

following :

1. Homeostasis

2. The internal and external environment

3. Stress in physiological terms

4. Set point/ set range

5. Negative feedback loops

6. Positive feedback


Documents

Introduction to Homeostasis September 2006 Clinical Science Team CardiffUniversitySONMS©CSANTeam