What is homeostasis?
It’s all about balance…

Homeostasis defined:
The ability to maintain a relatively
stable and balanced internal
environment including when the
external environment is changing.
Homeostasis comes from
the Greek and means
“staying the same”.

The “Goldilocks principle”
 Maintains ‘just right’ levels
 Temperature
 Water
 Blood Pressure
 Blood Glucose Levels
 Heart Rate
 Works by negative
feedback
Increase
Body takes
action
Normal
Normal
Decrease
Decrease
Body takes
action
Increase
Too hot
Normal
Too cold
Remove clothing
Too hot
Vasodilation
Sweating
Normal
Vasoconstriction
Too cold
Goosebumps
Shivering
Negative feedback: reverses the stimulus
and restores it within equilibrium range.
Positive feedback: intensifies the stimulus
and pushes further away from equilibrium
Blood Pressure: A Model of
Homeostasis
How to keep your arteries from exploding or collapsing

Blood Pressure
 Has a normal (‘just right’)
range
 Within normal range
 Higher pressure = more O2 +
nutrients to cells
 Lower pressure = less O2 +
nutrients to cells
 Outside of normal range
 High blood pressure = life
threatening
 Low blood pressure = life
threatening
Measuring Blood Pressure
 Inflate cuff to cut off blood
flow
 Slowly deflate cuff (5-10
mm/sec)
 Note when tapping sounds
start = systolic pressure
 Note when tapping sounds
stop = diastolic pressure
Measuring Blood Pressure
 Sounds are critical, but are hard to hear (a quiet room helps)
 Artery completely closed or completely open = NO Sounds
 Artery partially open = Tapping sounds (‘Korotkoff sounds’)
Practice with Sounds + Pressure
 Watch this video clip and
listen carefully to the audio.
What is the blood pressure?
 Watch this video clip and
listen carefully to the audio.
What is the blood pressure?
 Blood Pressure Practice 1
 Blood Pressure Practice 2
 98/68 mm Hg
 94/60 mm Hg
Practice with Sounds + Pressure
 Watch this video clip and
listen carefully to the audio.
What is the blood pressure?

Blood Pressure Practice 3
 132/90 mm Hg
 Watch this video clip and
listen carefully to the audio.
What is the blood pressure?
 Blood Pressure Practice 4
 112/70 mm Hg
What is normal and abnormal?
 90/60 to 120/80 mm Hg =
‘ Just right’
 Below 90/60 = ‘Hypotension’
 Not enough O2
 Light-headed, fainting, death
 Above 140/90 mm Hg =
‘Hypertension’
 Damage to arteries in heart, brain,
kidneys
 Heart attack, stroke, kidney failure
Blood pressure = cardiac output x peripheral
resistance. BP = CO x PR
 To maintain BP homeostasis, need to regulate both CO and PR
 When BP increases/decreases, baroreceptors (neurons in walls of
aorta and carotid arteries) sense it  they signal medulla oblongata
(at the top of spinal cord)  sends impulses to:
-
For CO: cardioinhibitory or cardioacceleratory center of the heart 
they decrease/increase CO and restore BP homeostasis.
-
For PR: smooth muscles around arterioles (small arteries branches
leading to capillaries)  they decrease/increase PR by
vasodilation/vasoconstriction and restore BP homeostasis.
Blood Pressure + Negative Feedback
 Increased blood
pressure starts
mechanisms to
reverse increase =
lower blood
pressure
 Decreased blood
pressure starts
mechanisms to
reverse decrease =
higher blood
pressure
Homeostasis + Negative Feedback
 Additional examples of homeostasis + negative feedback that
we will consider this year
System
Condition
Additional Info
Skeletal
Calcium in bone and
blood
Bones store calcium
Calcium in blood for muscle and nerve
function
Urinary
Water in blood and urine
Concentration of urine is regulated based on
water levels (hydration) in blood
Digestive
Sugar in blood
Excess sugar is stored as starch/fat when
glucose is high and released when low
Reproductive
Production of sperm and
eggs
Hormone levels stimulate production and
gamete production inhibit hormones