The$$discipline$$of$$pathophysiology$$seeks$$to$explain$$how$$the$$various$$physiological$$
processes$$are$altered$$in$$diseases$$or$$injury.
Negative feedback control of arterial
pressure by the arterial baroreceptors.
Signals from the sensor (baroreceptors)
are sent to medulla of the brain,
where they are compared with a
reference set point. When arterial
pressure increases above normal, this
abnormal pressure increases nerve
impulses from the baroreceptors to
the medulla of the brain, where
the input signals are compared with
the set point, generating an error
signal that leads to decreased
sympathetic nervous system activity.
Decreased sympathetic activity causes
dilation of blood vessels and reduced
pumping activity of the heart, which
return arterial pressure toward normal.
Figure$$1-4$$shows$$an$$example$$in$$which$$
death$$can$ensue$$from$$positive$$feedback.$$
This$$figure$$depicts$$the$pumping$$
effectiveness$$of$$the$$heart,$$showing$$that$$
the$heart$$of$$a$$healthy$$human$$being$$
pumps$$about$$5$$liters$$of$blood$$per$$minute.$$
If$$the$$person$$is$$suddenly$$bled$$2$$liters,$the$$
amount$$of$$blood$$in$$the$$body$$is$$
decreased$$to$$such$$a$low$$level$$that$$not$$
enough$$blood$$is$$available$$for$$the$$heart$to$$
pump$$effectively.$$As$$a$$result,$$the$$arterial$$
pressure$$falls$$and$$the$$flow$$of$$blood$$to$$
the$$heart$$muscle$$through$the$$coronary$$
vessels$$diminishes.$$This$$scenario$$results$$in$$
weakening$$of$$the$$heart,$$further$$
diminished$$pumping,$a$$further$$decrease$$in$$
coronary$$blood$$flow,$$and$$still$$more$
weakness$$of$$the$$heart;$$the$$cycle$$repeats$$
itself$$again$$and$again$$until$$death$$occurs.$$
Note$$that$$each$$cycle$$in$$the$$feedback$$
results$$in$$further$$weakening$$of$$the$$heart.$$
In$$other$words,$$the$$initiating$$stimulus$$
causes$$more$$of$$the$$same,$which$$is$$
positive$$feedback.$Positive$$feedback$$is$$
better$$known$$as$$a$$“vicious$$cycle,”$but$$a$$
mild$$degree$$of$$positive$$feedback$$can$$be$$
overcome$by$$the$$negative$$feedback$$
control$$mechanisms$$of$$the$$body,$and$$the$$
vicious$$cycle$$then$$fails$$to$$develop.$$For$$
instance,$if$$the$$person$$in$$the$$
aforementioned$$example$$is$$bled$$only$1$$
liter$$instead$$of$$2$$liters,$$the$$normal$$
negative$$feedback$mechanisms$$for$$
controlling$$cardiac$$output$$and$$arterial$
pressure$$can$$counterbalance$$the$$positive$$
feedback$$and$the$$person$$can$$recover,$$as$$
shown$$by$$the$$dashed$$curve$.
1.##Homeostasis#|#Functional##Organization##of##the##
Human##Body##and##Control##of##the##"Internal##
Environment"#
Thursday,$5$September$2019
9:48$PM
