Homeostasis

Friday, June 12, 15

HomeostasisHomeostasis - maintenance of a steady internal state

Body Temperature

Blood Pressure

Water Balance

Blood SugarLevel

pH Balance

Reproductive Cycle

Some examples:

Friday, June 12, 15

Balance is achieved by maintaining dynamic equilibrium Made possible by interaction of three key components:

❶

❷

❸

Sensory receptors

Integrator

Effectors

specialized cells that detect stimuli (specific changes in internal or external environment)

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control centre that receives information from receptors and “decides” on an appropriate response

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brain (and sometimes spinal cord) is integrator in vertebrates-

muscles or glands that carry out response of integrator-

Friday, June 12, 15

For example, going outside in winter would cause cold receptors in skin to send a message to brain

Receptors are continuously “sampling” internal and external environment to detect whether you are within an acceptable range

In order to maintain body temperature, brain may respond by causing your muscles to shiver (thereby creating heat), or it might direct your legs to take you back inside

Friday, June 12, 15

Most regulation mechanisms in your body act in negative feedback loops

Receptor EffectorIntegrator

Response(output)

Stimulus (input)

In negative feedback, response counteracts

original stimulus

Negative feedback loop - response of integrator attempts to cancel or counteract original stimulus, thereby returning body to its steady state

Friday, June 12, 15

Positive feedback - leads to instability in a system and must be reversed at some point

Positive feedback mechanisms are rare in body

Set into motion a chain of events that reinforce and intensify a change away from steady state

One positive feedback system that we have all experienced ends with a sneeze

Friday, June 12, 15

Hyperventilation is a positive feedback mechanism

So what ???

Stress can trigger an increase in breathing rate, due to release of adrenaline

Increased breathing rate eliminates CO2 from bloodstream more quickly than it is being produced

Low blood CO2 levels cause alkalosis (high blood pH), that in turn causes oxygen to bind more tightly to hemoglobin

Friday, June 12, 15

When oxygen is tightly bound to hemoglobin, it is not released to body’s cells

Response compounds problem, and system moves farther from point of equilibriumBrain is deprived of oxygen, so you faint

Cells become deprived of oxygen, so breathing rate increases further, leading to increased alkalosis

Lowers breathing rate and allows blood pH to return to normal

Friday, June 12, 15