Transcript Human Body Systems and Homeostasis Chapter 8.1 McGraw-Hill Ryerson Biology 12 (2011) Homeostasis • Maintenance of a relatively stable internal, fluid environment (internal milieu) despite a changing.

Human Body Systems and
Homeostasis
Chapter 8.1
McGraw-Hill Ryerson
Biology 12 (2011)
Homeostasis
• Maintenance of a relatively stable internal,
fluid environment (internal milieu) despite a
changing external environment
•
Temperature
37oC (98.6oF)
Blood pH
7.35
Blood sugar level
0.1%
http://www.lionden.com/homeostasis.htm
• Critical for survival since proteins, enzymes,
etc, require optimal conditions for operation
Homeostasis
Example:
• Blood glucose levels rise
after a meal thus
endocrine system works
to lower it to normal
• Blood glucose levels fall
during a fast thus
endocrine system works
to raise it to normal by
resorting to glycogen
supply in the liver
Homeostatic Control Systems
• Homeostasis relies on a feedback system: cycle of
events in which a variable* is monitored assessed
and adjusted
• 3 main components of feedback system
– A sensor -> senses state of system
– A control centre-> receives incoming info about system
and decides what the next steps should be
– A regulator or effector -> carries out the appropriate next
step to keep system at homeostasis
*Variable can be tempearture, blood pH, blood glucose level, etc.
Positive Feedback System
• A mechanism by which an output is enhanced
– Contractions in childbirth: contractions stimulate oxytocin
which stimulates more muscle contractions
Positive Feedback System
• What positive feedback actually does is push the variable
even further away from the normal
– The system is pushed to the extreme until the ordeal is over
– Positive feedback is not very commonly seen
– Negative feedback systems are much more common in organisms
Negative Feedback System
• a mechanism by which an output is
diminished
– The system resists change thus will initiate a response to bring it back
to normal range
– Negative feedback loops require a Receptor, A Control Center, and
Effectors
Negative Feedback System
Negative Feedback System
E.g.
– Blood vessels have receptors which can measure the
resistance of blood flow against vessel walls
– The brain (control center) receives info about change in
body’s internal conditions, sends out signals via nerves
– Effectors - the muscles, organs, and other structures that
receive signals from the control center and respond to
correct the deviation.
Sweating when hot
Shivering when cold
Homework
• Pg. 348 #1, 2, 7, 8, 9