1. Homeostasis: A question of Balance
SBI4U Biology

2. Claude Bernard, 1865:
"La fixité du milieu intérieur est la condition d'une vie libre et indépendente"
"The constancy of the internal environment is the condition for a free and independent life”. This is still the underlying principle of homeostasis today.

3. What’s there to balance?
Every internal factor:
responds to some external stimulus
must stay within certain limits
must be regulated automatically
must adjust quickly
HOMEO = The same STASIS = state
Keeping things within a range of tolerance allows organisms to live

4. Consider your thermostat:
Automatic, responsive, fast… keeps room temperature within an acceptable range….

5. Your body’s responses:
Must be equally responsive
STIMULI are picked up by RECEPTORS which send signals to a COORDINATING CENTRE that sends new signals to an EFFECTOR that will RESPOND to the stimulus
STIMULUS
Receptor
Coordinator
Effector
RESPONSE

6. Feedback Systems: Operate all homeostatic responses
Negative feedback loops eliminate the stimulus
Positive feedback loops reinforce the stimulus.

7. Components of a Feedback Loop:
Receptors: have contact with the external environment
Coordinators: the traffic cops of your homeostasis.
Effectors: many & varied, & carry out a response.
All interconnected

8. Types of Receptors:
Chemoreceptors: respond to chemical stimuli (taste, smell, blood CO2…)
Mechanoreceptors: respond to movement (touch)
Photoreceptors: respond to light (vision)
Thermoreceptors: respond to hot & cold (skin)
Pressure Receptors: respond to pressure (muscle, deep tissue, blood vessels…)
Proprioceptors: respond to muscle tension, joint/limb position

9. Two systems provide connections:
ENDOCRINE consisting of glands & hormones
NERVOUS consisting of CENTRAL brain & spinal cord, and PERIPHERAL nerves & ganglia.

10. The connections work in three ways:
ENDOCRINE is long-distance communication, via blood, between a gland & its TARGET ORGAN
NEURAL is direct connection, neurons to target cells
PARACRINE is close-by communication, between neighbouring cells

11. You rely on both systems constantly
Some responses, like Ovulation, are strictly endocrine
Others, like Pain Reflex, are strictly nervous
Many are a combination, like Sweating.

12. Feedback Example:
Pupillary Reflex of the eye allows for rapid adjustment to even slight changes in light levels.

13. Feedback Example: Maintaining neutral pH in the small intestine
Prevents damage to the intestinal wall
Very localized response: all the cells you need are there, so it’s fast.