Homeostasis and Feedback Mechanisms

• Homeostasis and feedback are key topics
• The lecture content covers components of a feedback system, negative feedback, and positive feedback
• Body fluids, fluid compartments, osmosis, its effects, and the applications of osmosis to physiology are also considered

Learning Outcomes

• Define homeostasis and identify situations where it is applied
• Explain the relevance of feedback loop components
• Describe the differences between negative and positive feedback
• Give examples of homeostatic mechanisms and use the principles of feedback loops and homeostasis to identify causes/consequences of homeostatic mechanism failure

Homeostasis

• Maintenance is achieved for many elements like blood glucose, blood-pressure, ion balance and water balance
• Every organ system contributes to homeostasis
• The endocrine and nervous systems are particularly important as they allow communication throughout the body
• This balance enables the body to maintain a stable internal environment and adapt to changing circumstances

Mechanisms for Maintaining Homeostasis:

• Homeostasis restores the body to its normal state after a disturbance via feedback mechanisms/loops
• Feedback loops are control systems that maintain variables within a set range after a disturbance

Components to Maintain Homeostasis

• Communication between the receptor, control center, and effector is essential

Homeostasis and Components of a Feedback Loop

• When the body is in homeostasis, the levels of variables are within a physiological range
• Also while in homeostasis Input at Comparator has a dynamic balance.
• If the body is offset "output to target organs" is given
• Target organs (effector) are activated
• A sensor (or receptor) measures the actual value of a variable
• Communication between the receptor, control center, and effector is essential for normal operation of the system

Stimulus

• A change occurs in the internal or external environment
• This shifts the variable's actual value and disturbs homeostasis

Sensor

• Sensors obtain real-time information about a variable
• They monitor the environment and respond to the stimulus by sending information to the control center

Comparator

• A comparator determines a set point (physiological range)
• This is done to ensure that a variable is maintained
• Information travels from the control center to the effector along the efferent pathway
• The comparator compares the current value from the afferent pathway (input) to a desired value within the physiological range

Effector

• Effectors modify parameters to restore a variable back to its physiological range

Dynamic Feedback Loops

• If a difference exists between the set level and input at the comparator, an output to target organs will modulate the variable
• Actual value of the variable is measured by a sensor as output signal
• The input signal is delivered via the afferent pathway

Essential Feedback

• Feedback information, relayed to the comparator, has any form of positive or negative influence
• The value of the controlled variable may be influenced by external factors

Summary of feedback loops

• Feedback loops maintain homeostasis.
• A physiological range is accepted.
• A sensor, control center, and effector work together
• Communication between sensor, control center, and effector is essential for normal operation
• Communication between components is known as ‘feedback’

Negative Feedback

• Change occurs in directions opposite to the original stimulus
• This type of feedback reduces the output of a system and decreases target organ function in response to a stimulus
• Most homeostatic control mechanisms use this form of feedback
• Examples are acute restoration of blood pressure, and return of plasma osmolality to normal, and temperature regulation

Negative Feedback Processes

• If there is a difference between the set level and input at the comparator, an output to target organs will restore the variable

Decreased Variable

• The comparator output (efferent arm) increases the value of the variable
• If the variable is decreasing, a sensor will register a reduced signal

Increased Variable

• The comparator output (efferent arm) decreases the value of the variable
• If the variable has increased, a sensor registers an augmented signal

Negative Feedback Example

• Maintenance of blood pressure is an example
• If arterial blood pressure has decreased to 90/70 mmHg compared to a normal range of 120/80 mmHg, then baroreceptors register a reduced arterial blood-pressure
• A result is that there is reduced sinus nerve firing
• Cardiovascular centers increase the "output", increasing heart & myocardium activity and pushing blood pressure to a higher variable level.

Homeostatic Feedback Loops

Negative

• Change occurs in the opposite direction to the original stimulus
• This reduces the output of a system or decreases a function in response to a stimulus to restore the body's variables to steady state

Positive

• Change occurs in the same direction as the original stimulus
• This increases function in response to a stimulus
• Furthermore, stimulus is enhanced so reaction continues at an even faster rate

Positive feedback

• Enhances the stimulus so that the reaction continues at an even faster rate
• Change occurs in the same direction as the original stimulus
• Causes variable to deviate further from original range
• Usually controls infrequent events
• An outside factor stops the feedback cycle

Examples of positive feedback

• Labor: uterine contractions become more frequent and stronger after they have begun
• Micturition: urination increases after the flow of urine has started

Secretion of hormone oxytocin during childbirth

• Contractions of the wall of the uterus force the baby's head or body into the cervix
• This increases stretching of the cervix.
• Stretch-sensitive nerve cells in the cervix are activated
• Nerve impulses send an afferent signal to the hypothalamus
• The brain interprets the input and releases oxytocin
• Muscles in the wall of the uterus contract more forcefully which further stretches the cirvix

Summary Positive Feedback

• Change occurs in the same direction as the original stimulus
• Enhances the stimulus and increases the function of the effector in response to a stimulus, speeding up physiological changes

Why do we need homeostasis

• Successful compensation restores homeostasis.
• Failure to compensate or disruption of homeostasis results in pathology, illness, or death
• Changes can be too great/rapid to control by feedback
• Sensors fail to detect changes
• Messages may be sent or fail to reach their targets
• Serious injuries can overwhelm homeostatic mechanisms
• Viruses or bacteria can change the body's internal chemistry

Cell-to-cell communication

• Uses electrical and chemical signals

Long-distance communication

• Uses electrical and chemical signals using neurotransmitters and hormones

Intracellular signaling pathways

• Chemical signals activate a receptor, translate chemical signals to cellular signals and create a response