Homeostasis: A question of Balance SBI4U Biology
Jan 16, 2016
"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.
Claude Bernard, 1865:
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
Consider your thermostat:
Automatic, responsive, fast… keeps room temperature within an acceptable range….
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
Feedback Systems:
Operate all homeostatic responses
Negative feedback loops eliminate the stimulus
Positive feedback loops reinforce the stimulus.
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
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
Two systems provide connections:
ENDOCRINE consisting of glands & hormones
NERVOUS consisting of CENTRAL brain & spinal cord, and PERIPHERAL nerves & ganglia.
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
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.
Feedback Example:
Pupillary Reflex of the eye allows for rapid adjustment to even slight changes in light levels.