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 feedback CardiffUniversitySONMS©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)’ CardiffUniversitySONMS©CSANTeam

Homeostasis The Body in Balance CardiffUniversitySONMS©CSANTeam

External Environment CardiffUniversitySONMS©CSANTeam

Threats from the External Environment CardiffUniversitySONMS©CSANTeam

Internal Environment Nutrients & Oxygen etc Waste Products CardiffUniversitySONMS©CSANTeam

Homeostasis A condition in which the body’s internal environment remains within set physiological limits (homeo = same; stasis = standing still). CardiffUniversitySONMS©CSANTeam

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/litre  Arterial plasma pH – Set Point or Set Homeostatic Range CardiffUniversitySONMS©CSANTeam

STRESS CardiffUniversitySONMS©CSANTeam

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 CardiffUniversitySONMS©CSANTeam

The stimuli that produce imbalances in homeostasis are called stressors. These fall into three categories: 1.Physical 2.Psychological 3.Sociological Stressors CardiffUniversitySONMS©CSANTeam

Control Systems The 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 CardiffUniversitySONMS©CSANTeam

Control Systems Respond to and Regulate Imbalances in Homeostasis CardiffUniversitySONMS©CSANTeam

Negative Feedback Loops For 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: CardiffUniversitySONMS©CSANTeam

Negative feedback The 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. CardiffUniversitySONMS©CSANTeam

Negative feedback CardiffUniversitySONMS©CSANTeam

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 CardiffUniversitySONMS©CSANTeam

Negative feedback loop CardiffUniversitySONMS©CSANTeam

Negative feedback loops CardiffUniversitySONMS©CSANTeam

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 CardiffUniversitySONMS©CSANTeam

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 +ve feedback

Unsuccessful Return to homeostasis NervousEndocrine ImmuneLearned Behavioural Responses Physical & Mental Stressors Internal & External Microbiology Malnutrition INTERVENTIONS Pharmacology Nutrition Nursing MDT Successful Return to homeostasis & health Altered Physiological function - Ill Health Adaptation Monitoring & Regulating systems Imbalance in homeostasis Homeostasis and Health CardiffUniversitySONMS©CSANTeam

Conclusion In 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 CardiffUniversitySONMS©CSANTeam