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Homeostasis &
Feedback
Unit 3
Life Sciences-HHMI Outreach.
Copyright 2009 President and
Fellows of Harvard College.
Homeostasis Video
• https://www.teachertube.com/video/homeostasis-398911
• Process that occurs in all
living things
• All organ systems work
together to achieve
homeostasis
Life Sciences-HHMI Outreach.
Copyright 2009 President and
Fellows of Harvard College.
What is homeostasis?
• Ability of an organism to
maintain its internal
environment, despite changes
to its internal or external
environment
http://en.wikipedia.org/wiki/File:Tightrope_artist_Cologne_1.jpg
How does homeostasis work?
• Feedback pathways
• A cellular relay race!
• Specific organs and structures
must communicate with each
other in response to changes
in the body
http://en.wikipedia.org/wiki/File:Southern_12_stage-02_1988.jpg
• Keeps levels of certain
processes within a normal
range
What things in your body need to be kept
within a range?
• Body Temperature
• Blood pressure
• Blood pH
• O2 and CO2 concentration
• Osmoregulation-Water balance
• Blood glucose
• Ion levels
• Hormone levels
Cellular Relay Race
• Stimulus
• Receptor
• Integrating center
• Effector
• Response
• Reverses the
stimulus
Feedback Loops: Types
• Negative feedback loop
• original stimulus reversed
• most feedback systems in the body are negative
• used for conditions that need frequent adjustment
• Positive feedback loop
• original stimulus intensified/amplified
• seen during normal childbirth
Negative Feedback Loop
Homeostasis – Negative Feedback Loop
• Blood glucose concentrations rise after a sugary meal (the stimulus), the
hormone insulin is released and it speeds up the transport of glucose
out of the blood and into selected tissues (the response), so blood
glucose concentrations decrease (thus decreasing the original stimulus).
Homeostasis of Blood Pressure
• Baroreceptors in walls of
blood vessels detect an
increase in BP
• Brain receives input and
signals blood vessels and
heart
• Blood vessels dilate, HR
decreases
• BP decreases
Thermoregulation
(Regulation of Body Temperature)
• Ectothermy (cold-blooded
animals)
• body temperatures vary
with that of the
environment
• e.g. reptiles, fish,
amphibians
Endothermy(warm-blooded
animals)
keep body temperature
constant even in winter by
increasing metabolic rate
e.g. birds, mammals
Ectotherm vs Endotherm
Endothermy
Advantages
•
Disadvantages
Maintains stable body temp
– Cooling & heating the body Greater food consumption to meet
metabolic needs
cooling and heating the body
• high levels of aerobic metabolism • Human metabolic mate at
• sustains vigorous activity for
200C & at rest
much longer than ectotherms
1,300 to 1,800 kcal
per day.
– Long distance running
• American alligator
– Flight
metabolic rate at 200C
& at rest 60 kcal per
day at 200C.
Mechanisms for thermoregulation
• Insulation
-
Fur
Hair
Feathers
Fat
Blubber
• Evaporative cooling
-
sweating, panting, bathing
• Shivering
• Nonshivering thermogenesis & brown fat
• Circulation adaptations
-
Countercurrent exchange
Vasodilatation (cooling)
Vasoconstriction (heat conservation)
• Behavioral responses
Regulating Body Temp in Humans
• Some ectotherms that experience subzero body
temperatures protect themselves by producing
“antifreeze” compounds (cryoprotectants) that
prevent ice formation in the cells.
– In cold climates, cryoprotectants in the body fluids
let overwintering ectotherms, such as some frogs
and many arthropods and their eggs, withstand body
temperatures considerably below zero.
– Cyroprotectants are also found in some Arctic and
Antarctic fishes, where temperatures can drop
below the freezing point of unprotected body fluids
(about -0.7oC).
• Cells can often make rapid adjustments to
temperature changes.
– For example, marked increases in temperature or
other sources of stress induce cells grown in culture
to produce stress-induced proteins, including heatshock proteins, within minutes.
– These molecules help maintain the integrity of other
proteins that would be denatured by severe heat.
– These proteins are also produced in bacteria, yeast,
and plants cells, as well as other animals.
– These help prevent cell death when an organism is
challenged by severe changes in the cellular
environment.
Plant responses to water
limitations
Life Sciences-HHMI Outreach.
Copyright 2009 President and
Fellows of Harvard College.
• When plants under water stress, they close their stomata and
decrease their transpiration rates (water loss that occurs when
stomata open to get carbon dioxide for photosynthesis)
Operons in gene regulation
• An operon is a cluster of coordinately regulated genes. It
includes structural genes(generally encoding
enzymes), regulatory genes(encoding, e.g. activators or
repressors) and regulatory sites(such as promoters and
operators).
Positive Feedback during Childbirth
•
•
•
•
•
Stretch receptors in walls of uterus send signals to the brain
Brain induces release of hormone (oxytocin) into bloodstream
Uterine smooth muscle contracts more forcefully
More stretch, more hormone, more contraction etc.
Cycle ends with birth of the baby & decrease in stretch
Ripening of Fruit
• First fruit that starts to ripen releases ethylene gas that
stimulates other fruits to ripen and create more ethylene gas
Lactation in Mammals
• Suckling by an infant stimulates sensory neurons in the
nipples, generating signals in the nervous system that trigger
the release of the neurohormone oxytocin
• In response to the oxytocin, the mammary gland secrete milk.
• Milk release leads to more suckling and more stimulation.
Role of Body Systems in
Homeostasis
Control of Homeostasis
• Homeostasis is continually being
disrupted by
• External stimuli
• heat, cold, lack of oxygen, pathogens,
toxins
• Internal stimuli
• Body temperature
• Blood pressure
• Concentration of water, glucose, salts,
oxygen, etc.
• Physical and psychological distresses
• Disruptions can be mild to severe
• If homeostasis is not maintained, death
may result
Alteration in the mechanisms of feedback
often results in deleterious consequences
• Diabetes mellitus
• Response to decreased insulin
• Cannot absorb glucose out of the blood
• Dehydration
• Response to decreased antidiuretic hormone (ADH)
• Grave’s disease (hyperthyroidism)
• imbalance of metabolism caused by overproduction of
thyroid hormone which control the way that every cell
in the body uses energy (metabolism).
• Blood clotting