Homeostasis:A question of Balance

SBI4U Biology

"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.

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.