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Lecture

homeostasis.doc


Department
Anatomy and Physiology
Course Code
ANP 1105
Professor
Jacqueline Carnegie

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Homeostasis
homeostasis- the ability to maintain relatively stable internal conditions even though the
outside world changes continuously; indicates a dynamic state of equilibrium ( a balance in
which internal conditions vary but always within limits)
-the body is in homeostasis when its needs are adequately met and it is functioning
smoothly
variable- the factor or event being regulated
-adequate blood levels of vital nutrients must be continuously present
-heart activity and blood pressure must be constantly monitored and adjusted
-wastes must not be allowed to accumulate
-body temperature must be precisely controlled
-all homeostatic control mechanisms are processes involving at least 3 components that
work together
1) Receptor
-some type of sensor that monitors the environment and responds to changes called stimuli,
by sending input information to the 2nd component called the control centre along the
afferent pathway (afferent = approaches the control centre)
2) Control Centre
-determines the set point (the level or range at which a variable is to be maintained)
-it analyzes the input it receives and determines an appropriate response
-output information flows from the control centre to the 3rd component called the effector
along the efferent pathway (efferent = exits the control centre)
3) Effector
-provides the means for the control centre's response (output) to the stimulus
-the results of the response then feed back to influence the effect of the stimulus by either
reducing it (negative feedback) so that the whole control process shuts off or enhancing it
(positive feedback) so the process continues even faster
Negative Feedback
-associated with most homeostatic control mechanisms
-the output shuts off the original effect of the stimulus or reduces its intensity and causes
the variable to change in a direction opposite to that of the initial change, returning it to its
ideal value (basically, the output (i.e.: a hormone) reduces or shuts off stimulus)
-e.g.: hormonal negative feedback mechanism controls the blood volume using the
antidiuretic hormone (ADH)
-as blood volume drops, receptors in the body sense the change and the
hypothalamus (contains control centre) stimulates the release of ADH into the
blood
-this change prompts the kidneys to reabsorb more water and return it to the blood
stream
-the rising blood volume then ends the stimulus for ADH release

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Positive Feedback
-the result or response enhances the original stimulus so that the response is accelerated
-the change that results proceeds in the same direction as the initial change, causing the
variable to deviate further from its original value
-usually control infrequent events that do not require continuous adjustments
-they usually set off a series of events that may be self-perpetuating and have an amplifying
effect
-often referred to as cascades
-e.g.: the enhancement of labor contractions during birth (oxytocin intensifies labor
contractions that become more frequent and more powerful and these contractions cause
more oxytocin which cause more contractions; birth ends the stimulus and positive
feedback mechanisms are shut off) and blood clotting
Homeostatic Imbalance
-most diseases can be regarded as a result of homeostatic disturbance
-may occur when negative feedback mechanisms are overwhelmed and destructive positive
feedback mechanisms take over
Autonomic Nervous System
-the system of motor neurons that innervates smooth and cardiac muscle and glands
-means self (auto) and govern (nom)
-a subdivision of the peripheral nervous system
-also called the involuntary nervous system which reflects its subconscious control or the
general visceral motor system which indicates the location of most of its effectors
-shunts blood to more needy areas, speeds up or slows down heart and respiratory rates,
adjusts blood pressure and body temperature, and increases or decreases gastric secretions
Differences Between the Autonomic Nervous System and the Somatic Nervous System
-both systems have motor fibres but they differ in their effectors, their pathways, and their
target organ responses to their neurotransmitters
1) Effectors
-somatic nervous system stimulates skeletal muscles while the ANS innervates cardiac and
smooth muscle and glands
-differences in physiology of the effector organs account for the major differences between
somatic and autonomic effects
2) Efferent Pathways and Ganglia (a mass of nerve cell bodies)
-in somatic nervous system, the motor neuron cell bodies are in the CNS and their axons
extend in spinal or cranial nerves all the way to the skeletal muscles they activate
-somatic motor fibres are thick and heavily myelinated group A fibres that conduct nerve
impulses rapidly
-in ANS, it uses a two-neuron chain to its effectors
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