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PHSI 208 Final: Comprehensive Final Exam Study Guide

Course Code
PHSI 208
Baillie Landon
Study Guide

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PHSI 208 Final Exam Study
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HOMEOSTASIS the ability to maintain a relatively stable internal environment in spite of
exposure to external variability
Homeostasis and regulation of the internal environment are key principles in physiology
Depends on mass balance
o Law of Mass Balance if the amount of a substance in the body is to remain
constant, any gain must be offset by an equal loss
Does no mean equilibrium between ICF and ECF the body compartments are in a
dynamic stead state ut aet i euiliiu
o Certain ions remain relatively stable at a set point -> e.g. typically very low [Na]
within the cell compared to the ECF
The extracellular fluid (ECF) surrounding the cells is a buffer between cells and the
external environment
Most ells aet toleat to hages i thei suoudigs
Dynamic steady state most variables remain at a fairly steady state
To maintain homeostasis, the body monitors certain key functions
o Regulated variables are kept within a normal range by control mechanisms which
can be local or reflex
Input signal -> integrating center -> output signal -> response
o Local Control
Restricted to tissues or cells involved
E.g. active cells -> reduced O2 levels in tissue -> active cells send local
signals -> O2 levels in tissue restored
Control of variables restricted to a small area
o Reflex Control
Changes widespread throughout the body (systemic) use more complex
control systems to maintain homeostasis
Refers to any long distance pathway that uses the nervous, endocrine
system, or both
Two parts
Response loop: stimulus -> sensor -> input signal -> integrating
center -> output signal -> target -> response
Feedback loop
o Modulates the response loop
o Feeds back to ultimately influence the input
o Negative feedback, positive feedback, feedforward control
E.g. blood pressure
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Need to maintain central blood pressure to ensure that the
pressure is high enough to make it to our head, fingertips, etc.
Large reflex loops that involve different parts of the body that help
to maintain the BP within a homeostatic range
Change in blood pressure -> activation of baroreceptors -> medulla
-> turns on/off the para/sympathetic NS -> tightens/relaxes vessels
to fix BP
Acceptable set point is variable in the body depending on the parameter
Can be antagonistic there can be dual control of any given variable
Types of Feedback
Negative Feedback pathway in which the response opposes or
removes the signal
o Stabilizes a system; homeostatic; can restore the initial
state ut at peet the iitial distuae
o Most common reflex loop
Positive Feedback response reinforces the stimulus to drive the
system further away from a normal value rather than
decreasing/removing it
o Not homeostatic; requires intervention outside the loop to
cease the response
o A few reflexes have evolved that allow the body to predict
a change is about to occur
o Keeps most variables within a tight range
Variables are regulated within a normal range around a setpoint but setpoints vary from
person to person, or may vary within an individual over time
o Can be due to genetics, or constant exposure to new conditions
o Biorhythms variables that change predictable and create repeating patterns or
cycles of changes (e.g. body temperature)
May create an anticipatory response to predictable environment changes
Biological Membranes
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