Homeostasis
HomeostasisHomeostasis - maintenance of a steady internal state
Body Temperature
Blood Pressure
Water Balance
Blood SugarLevel
pH Balance
Reproductive Cycle
Some examples:
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-
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
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
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
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
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