Transcript homeostasis A

homeostasis
Physiology
• In the distant past, humans thought that good health
was somehow associated with a "balance" among
the multiple life-giving forces ("humours") in the body
– Today we know that living tissue is composed of trillions of
small cells, all are packaged to permit movement of certain
substances, but not others, across the cell membrane.
– also we know that cells are in contact with the interstitial
fluid.
• The interstitial fluid is in a state of flux, with chemicals, gases, and
water moving it in two directions between the cell interiors and the
blood.
Fluid compartments of the body
• most of the common physiological
variables found in normal, healthy
organisms are maintained at relatively
steady states.
– i.e. blood pressure, body temperature,
blood oxygen, and sodium.
– This is true despite external conditions that
are not constant.
Homeostasis defined
• homeostasis is simply defined as a state of
reasonably stable balance between the
physiological variables
– NO variable is constant over time.
• Blood glucose can have dramatic swings.
• Homeostasis is in DYNAMIC balance, not static.
• It is relatively stable, if disturbed mechanisms can restore
it to normal values.
What does it mean to be relatively
constant?
• It depends on what is being monitored.
– Arterial oxygen must be tightly controlled
– Blood glucose can vary wildly
• A person can be in homeostasis for one variable but
not for another.
– You could be in sodium homeostasis but have abnormally
high levels of CO2.
• This is a life threatening condition.
• Just one variable out of homeostasis can have life-threatening
consequences.
Physiology vs. Pathophysiology
• If all your major organ systems are in
homeostasis, then you are in good health.
• diseases take one or more systems out of
homeostasis.
• Physiology:When homeostasis is maintained
• Pathophysiology: homeostasis is not maintained.
How do you know if a variable is
in homeostasis?
• You have to observe a person over time to find
out what is “normal.”
• Not usually possible because you only go to a doctor when
you are sick (out of homeostasis).
– Usually, doctors rely on normal values for large
populations of people.
• Body temperature
– Normal values are useful, but not if a person has been exercising.
– There are rhythms to a person’s body temperature.
Many variables are cyclical
• Examples
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Body temperature,
sleep/wake,
levels of certain hormones
If you took one measurement, they may be
normal, but might not detect when they are
abnormally high or low.
• Measure over 24 hour period to get a better picture
of homeostasis.
Characteristics of homeostatic
control systems
• cells, tissue and organ activity must be
integrated so that changes in the ECF initiate a
reaction to correct the change.
• Homeostasis, then, denotes the relatively
stable conditions of the internal environment
– These conditions result from compensating
regulatory responses controlled by homeostatic
control systems.
Regulation of body temperature
• Man w/ body temp. of 370 C is in room at
200 C
– He is losing heat to the environment
– Chemical reactions in his cells are releasing
heat at a rate = to loss
– Body is in a steady state but state is maintained
by input of energy
• Steady state is not equilibrium
• Steady state temperature is the set-point
Lower room temp to
• This increases loss of heat
from skin and body temp
starts to fall
– What responses will
occur?
• Blood vessels to skin
constrict
• Person curls to reduce skin
surface area
• Shivering occurs producing
large amounts of heat
0
5
C
Negative Feedback
• Defined
– an increase or decrease in the variable being
regulated brings about responses that tend to
move the variable in the direction opposite
("negative" to) the direction of the original
change
– It can occur at the organ, cellular, or molecular
level
negative feedback example
Negative feedback in an enzyme
pathway
• When energy is needed by a cell,
– glucose is converted into ATP.
• The ATP that accumulates in the cell inhibits
the activity of some of the enzymes involved in
the conversion of glucose to ATP
• As ATP levels increase within a cell, production
of ATP is slowed down
Not all feedback is negative
• Positive feedback is less common but does occur
– In nerve cells, when a stimulus is received, pore-like
channels open letting Na+ in
– In childbirth
• The baby’s head presses against the uterus stimulating the
release of oxytocin
• Oxytocin causes uterine contractions, pushing the baby’s head
against the uterine wall releasing more oxytocin.
Feedforward regulation
• While your body can respond to changes in
external temperatures AFTER the body’s internal
temperature changes, it can also respond to
changes BEFORE your body temp. starts to fall.
– Nerve cells in the skin detect changes and send
information to the brain.
– Often this response is a result of LEARNING
Parts of homeostatic control
systems- Reflexes
• reflex is a specific
involuntary,unlearned "built
in" response to a particular
stimulus
– The stimulus is a detectable
change in the internal or
external environment.
– Detected by a nerve receptor
– The stimulus causes the
receptor to send a signal to
the integrating center
(afferent)
Reflex Arc
Reflex part 2
• Integrating center
receives signals from
many receptors
– Receptors may be for
different kinds of
stimuli
– Output from center
(efferent) goes to
effector to alter its
activity
Reflex for minimizing decrease in
body temperature
Reflexes are not just part of the
nervous system
• We usually think of reflexes are part of the
nervous system (hand on a hot stove), but
now we include many other systems as part
of reflexes.
– Hormone-secreting glands serve as integrating
centers
– Chemical messengers travel through the blood.
Intercellular chemical messengers
• reflexes and other responses depend on the ability of
cells to communicate w/ each other.
– Most often occurs with chemical messengers.
• Hormones- allow hormone secreting cell to communicate
with target cells.
– Blood delivers the hormone to the cell.
• Neurotransmitters- allow nerve cells to communicate with
each other
– One nerve cell can alter the activity of another cell.
– Neurotransmitters released into the area around effector cells can
alter their activity.
• Paracrine agents- chemical messengers in local responses
Categories
of chemical
messengers
Paracrine/autocrine agents
• Paracrine agents are made by cells (given a
stimulus) and released into the ECF.
– Agents diffuse to neighboring cells which are their
target cells.
• Autocrine agents are made by a cell, released
and the target cell is the one that released it. (?)
Why do you care about these
agents?
• We are finding many different paracrine/autocrine
agents that have many diverse effects.
– They are not just proteins.
– Secreted by many cell types in many kinds of tissues
– So many that they can be organized into families
• i.e. Growth factor family has 50 distinct molecules that can
cause cells to divide/differentiate.
Processes related to homeostasis
• Some seemingly unrelated processes have
implications for homeostasis
– Adaptation and acclimatization
– Biological rhythms
– Apoptosis
Adaptation/ acclimatization
• Adaptation is a characteristic that favors
survival in specific environments.
– Your ability to respond to a specific
environmental stress isn’t fixed, but it can be
enhanced by prolonged exposure to the stress.
– Acclimatization: A specific type of adaptationthe improved functioning of an existing
homeostatic system.
Acclimatization is reversible
(usually)
• If daily exposure to the
stress is eliminated, then
acclimatization is
reversible…
• Some acclimatizations that
happen early in life may
become permanent.
– Natives of the Andes
Mountains
• Low oxygen levels cause
increased chest sizes, wide
nostrils, broad dental arches
Biological rhythms
• Many body functions are rhythmic
– Occur in 24 hour (circadian rhythm) cycles
– Sleep/wake, body temp., hormone levels, etc…
– Are anticipatory (kind of like feedforward systems
without detectors)
Rhythms allow responses to
occur automatically
• Remember that most homeostatic responses
are corrective, they occur after homeostasis
is perturbed
– Rhythms cause responses to occur when a
challenge is likely but before it actually does.
• Urinary excretion of potassium is high during the
day and low at night.
Body rhythms are internally driven
• Environmental factors don’t drive the
rhythms, but provide timing cues.
– Sleeping experiment (no light cues)
– Sleep/wake cycle is a free-running rhythm
– Sleep/wake cycles can vary between 23-27
hours but not more or less than that.
Other environmental cues
• Light/dark cycle is very
important, but not the only
one.
• External environmental
temperature
• Meal timing
• Social cues
– Sleep experiment people are
separated, their cycles are
each different.
– Put them together and their
cycles synchronize
Jet Lag
• Environmental time cues can phase-shift
rhythms.
– Going from LA to Atlanta and staying for a
week.
– Circadian rhythm will adjust, but it takes time
– In the meantime, you suffer jet lag
• Sleep disruption, gastrointestinal trouble, decreased
vigilance and attention span, general malaise
Neural basis of body rhythms
• In the hypothalamus
– A group of nerve cells (suprachiasmatic nucleus)
– Acts as the pacemaker for rhythms
• Pacemaker receives input from the eyes and other senses.
• Then it sends signals to other parts of the brain that control
other systems, activating some and inhibiting others.
• Not well understood
Sleep and the Pineal gland
• Pacemaker sends signal to pineal gland
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Gland releases melatonin
Pineal secretes during darkness, not daylight
Melatonin influences other organs
Makes you sleepy
Apoptosis
• Defined– The ability to self-destruct by activation of an
intrinsic program within the cell
• Important for
– sculpting a developing organism or
– Eliminating undesirable cells (cancerous)
Importance of Apoptosis
• Crucial for regulating the number of cells in
a tissue or organ.
– Control of cell number is determined by a
balance between cell proliferation (addition of
new cells by mitosis) and cell death (apoptosis)
• Neutrophils (cells alive)
How does it occur?
• Controlled autodigestion of cell organelles.
– Enzymes breakdown the nucleus and then other
organelles
• The cell membrane isn’t digested.
• The cell sends out chemical signals that recruit
phagocytic cells (cells that “eat” other cells).
– This is different than what happens when a cell
is injured (necrosis)
How is it kept off?
• Virtually all cells have the apoptosis enzymes.
– Why aren’t they turned on?
• A large number of molecules called “survivor signals” keep
the cell from activating the enzymes.
• So most cells are programmed to commit suicide UNLESS
they receive a signal to stay alive.
– Prostate gland cells will die if testosterone is not present
What about cancer? Degenerative
diseases?
• Cancer cells undergo uncontrolled cell
proliferation.
– So the apoptosis enzymes are always turned off.
• In degenerative diseases (osteoporosis)
– The rate of cell death is higher than that of cell
proliferation.
• Drugs that reduce rate of apoptosis
Balance in the homeostasis of
chemicals
• Most homeostatic systems control the balance of
specific chemicals.
3 states of total body balance
• Negative balance
– Loss exceeds gain, total amount of substance in
body is decreasing.
• Positive balance
– Gain exceeds loss
• Stable balance
– Gain equals loss
Water, sodium balance
• Water
– Stable balance is upset with excessive sweating.
– Restored by?
• Sodium (Na+)
– Kidneys excrete Na+ into urine in approx. = amounts of ingested
daily.
– If intake were to increase dramatically, kidneys will excrete more
in urine, but only so much can be excreted.
– If the increase is continued, it can have effects on other systems
– A small change in blood sodium has been linked to hypertension.
A quick summary
• Homeostasis is a complex, dynamic process.
– It regulates the adaptive responses of the body to changes
in external and internal environments.
– homeostatic systems require a sensor to detect changes
and a means to produce a response.
• Responses can include: muscle activity, synthesis of chemical
messengers (hormones) and behavioral changes.
• All responses require energy.
• You get energy to respond from the food you eat.