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Homeostasis
General Education Program
Biology
Presented by: Dr. Shaimaa Nasr Amin
Lecturer of Medical Physiology
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HOMEOSTASIS
Homeostasis, from the Greek words
for "same" and "steady," refers to
any process that living things use
to actively maintain fairly stable
conditions necessary for survival…
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Homeostasis
• A person threatened by the environment (or informed
of an approaching pleasure or danger) prepares for
action. The body mobilizes reserves of energy and
produces certain hormones such as adrenalin, which
prepare it for conflict or flight. This mobilisation can be
seen in familiar physiological reactions. In the presence
of emotion, danger, or physical effort the heart beats
faster and respiration quickens.
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Homeostasis
• The face turns red or pales and the body perspires.
The individual may experience shortness of breath,
cold sweats, shivering, trembling legs. These
physiological manifestations reflect the efforts of the
body to maintain its internal equilibrium. Action can be
voluntary--to drink when one is thirsty, to eat when
hungry, to put on clothing when cold, to open a window
when one is too warm--or involuntary--shivering,
sweating.
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Internal components of homeostasis
1.Concentration of oxygen
and carbon dioxide
2.pH of the internal
environment
3.Concentration of
nutrients and waste
products
4.Concentration of salt and
other electrolytes
5.Volume and pressure of
extracellular fluid
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Control Systems
Open systems are linear
and have no feedback,
such as a light switch.
Closed Systems has two
components: a sensor and
an effector, such as a
thermostat (sensor) and
furnace (effector). Most
physiological systems in
the body use feedback to
maintain the body's internal
environment.
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Extrinsic
Most homeostatic systems are extrinsic: they are
controlled from outside the body. Endocrine and
nervous systems are the major control systems in
higher animals.
The nervous system depends on sensors in the
skin or sensory organs to receive stimuli and
transmit a message to the spinal cord or brain.
Sensory input is processed and a signal is sent to an
effector system, such as muscles or glands, that
effects the response to the stimulus.
Control Systems
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Extrinsic
The endocrine system is the second type of extrinsic
control, and involves a chemical component to the
reflex. Sensors detect a change within the body and
send a message to an endocrine effector (parathyroid),
which makes PTH. PTH is released into the blood when
blood calcium levels are low. PTH causes bone to
release calcium into the bloodstream, raising the blood
calcium levels and shutting down the production of
PTH.
Control Systems
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Extrinsic
Some reflexes have a combination of nervous and
endocrine response. The thyroid gland secretes
thyroxin (which controls the metabolic rate) into the
bloodstream. Falling levels of thyroxin stimulate
receptors in the brain to signal the hypothalamus to
release a hormone that acts on the pituitary gland to
release thyroid-stimulating hormone (TSH) into the
blood. TSH acts on the thyroid, causing it to increase
production of thyroxin.
Control Systems
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Intrinsic
Local, or intrinsic, controls usually involve only
one organ or tissue. When muscles use more
oxygen, and also produce more carbon dioxide,
intrinsic controls cause dilation of the blood
vessels allowing more blood into those active
areas of the muscles. Eventually the vessels
will return to "normal".
Control Systems
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Negative feedback control mechanisms (used by
most of the body's systems) are called negative
because the information caused by the feedback
causes a reverse of the response. TSH is an
example: blood levels of TSH serve as feedback for
production of TSH.
Feedback Systems in Homeostasis
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Positive feedback control is used in some cases.
Input increases or accelerates the response. During
uterine contractions, oxytocin is produced. Oxytocin
causes an increase in frequency and strength of
uterine contractions. This in turn causes further
production of oxytocin, etc.
Homeostasis depends on the action and interaction of
a number of body systems to maintain a range of
conditions within which the body can best operate.
Feedback Systems in Homeostasis
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Negative feedback control mechanisms
• negative feedback The stopping of the synthesis of an enzyme by the accumulation of the products of the enzyme-mediated reaction.
• negative feedback control Occurs when information produced by the feedback reverses the direction of the response; regulates the secretion of most hormones.
• negative feedback loop A
biochemical pathway where
the products of the reaction
inhibit production of the
enzyme that controlled their
formation.
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Positive feedback control
• Biochemical control
where the
accumulation of the
product stimulates
production of an
enzyme responsible
for that product's
production.
• positive feedback
control Occurs when
information
produced by the
feedback increases
and accelerates the
response.
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Body Systems and Homeostasis
• Eleven major organ systems are present within
animals, although some animals lack one or more
of them. The vertebrate body has two cavities:
the thoracic, which contains the heart and lungs;
and the abdominal, which contains digestive
organs. The head, or cephalic region, contains
four of the five senses as well as a brain encased
in the bony skull. These organ systems can be
grouped according to their functions.
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THERMOREGULATION
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Overview of Thermoregulation
•Mechanisms of Thermoregulation
•Exercise in Heat Stress
•Heat Illness
•Exercise in Cold Stress
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Thermal Balance
• Core temperature (TCO) is in dynamic equilibrium as a
result of balance between heat gain and heat loss.
• Mean body temperature (Tbody) represents an average of
skin and internal temperatures.
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Hypothalamus Regulation of Temperature
• Hypothalamus acts as “thermostat” that makes thermoregulatory adjustments to deviations from temperature norm in the brain (37 C ± 1 C or 98.6 ±1.8 F ).
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Hypothalamus Regulation of Temperature
• Mechanisms are activated in two ways: • Thermal receptors in
skin provide input to central command
• Direct stimulation of hypothalamus through changes in blood temperature perfusing area
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Thermoregulation in Cold
• Vascular adjustments: constrict peripheral blood vessels.
• Muscular activity: exercise energy metabolism and
shivering.
• Hormonal output: epinephrine and norepinephrine
increase basal heat production; prolonged cold – thyroxin.
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Thermoregulation in Heat
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Thermoregulation in Heat
• Heat Loss by Radiation (~ 10%)
• Objects emit electromagnetic heat waves without molecular contact
with warmer objects.
• When temperature of things in environment exceeds the skin
temperature, radiant heat energy is absorbed from the
surroundings.
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Thermoregulation
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Thermoregulation in Heat
• Heat Loss by Conduction
• Direct transfer of heat through a liquid, solid, or gas from one
molecule to another.
• A small amount of body heat moves by conduction directly through
deep tissues to cooler surface. Heat loss involves the warming of
air molecules and cooler surfaces in contact with the skin.
• The rate of conductive heat loss depends on thermal gradient.
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Thermoregulation in Heat
• Heat Loss by Convection (+ conduction 35%) • Effectiveness depends on how rapidly the air (or water) adjacent to the
body is exchanged.
• Air currents at 4 mph are about twice as effective for cooling air
currents at 1 mph.
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Thermoregulation in Heat
• Heat Loss by Evaporation (~ 55%)
• Heat transferred as water is vaporized from respiratory passages
and skin surfaces.
• For each liter of water vaporized, 580 kcal transferred to the
environment.
• When sweat comes in contact with the skin, a cooling effect occurs
as sweat evaporates.
• The cooled skin serves to cool the blood.
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Heat Loss at High Ambient Temperatures
• Effectiveness of heat loss via conduction, convection, and
radiation decreases.
• When ambient temperature exceeds body temperature,
heat is gained.
• The only effective mechanism is evaporation of sweat and
respiratory tract vaporization of water.
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Heat Loss in High Humidity
• Total sweat vaporized from skin depends on:
• Surface area exposed to environment
• Temperature and humidity of ambient air
• Convective air currents about the body
• Most important factor is relative humidity.
• When relative humidity is high, the ambient water vapor
pressure approaches that of the moist skin and evaporation
is impeded.
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Integration of Heat-Dissipating
Mechanisms
• Circulation.
Superficial venous
and arterial blood
vessels dilate to divert
warm blood to the
body shell.
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Integration of Heat-Dissipating
Mechanisms
• Evaporation. Sweating begins within 1.5 s after start of vigorous exercise.
• Hormonal adjustments. Certain hormonal adjustments are initiated in heat stress as body attempts to conserve fluids and sodium.
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Hormones in Heat Stress
• Antidiuretic hormone
(ADH) is released to
increase water re-
absorption from kidneys.
• Aldosterone is released
to increase the re-
absorption of sodium.
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Effects of Clothing
Cold Weather Clothing
provide an air barrier
to prevent convection
and conduction.
• Layers provide more
trapped air
• Allow water vapor to
escape
Warm Weather Clothing
loose fitting to permit
free convection.
• The less surface
covered the more
evaporative cooling.
• Clothing should be
loosely woven to allow
skin to breathe.
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Exercise in Heat Stress
• Circulatory Adjustments.
• Cardiovascular drift – fluid loss reduces plasma volume
(about 10% of fluid lost comes from plasma. About 50%
comes from intracellular water).
• Visceral vascular constriction and skin & muscle
vascular dilation.
• Maintaining blood pressure. Circulatory regulation and
maintenance of muscle blood flow take precedence
over temperature regulation often at the expense of
spiraling core temperature during exercise in heat.
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Exercise in Heat Stress
• Core temperature
• More than likely a modest
increase in core temperature
reflects favorable internal
adjustments.
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Water Loss in the Heat
• Magnitude of Fluid Loss in Exercise.
• Consequences of Dehydration. • plasma volume peripheral blood flow & sweat rate
• Water Replacement • Primary aim of fluid replacement during exercise is to maintain
plasma volume
• The most effective defense against heat stress is adequate hydration
• Electrolyte Replacement.
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Acclimatization to Heat
Acclimatization refers to physiological changes
that improve heat tolerance.
2 – 4 hours daily heat exposure produce complete
acclimatization 5-10 days.
o Rectal temperature
• HR
∆ Sweat rate
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Factors that Improve Heat Tolerance:
Acclimatization
Improved cutaneous blood flow Transports metabolic heat from deep tissues to body’s shell
Effective distribution of cardiac output
Appropriate circulation to skin & muscles to meet demands.
Lowered threshold for start of sweating
Evaporative cooling begins early in exercise.
More effective distribution of sweat over skin surface
Optimum use of surface for effective evaporative cooling.
Increased rate of sweating Maximize evaporative cooling.
Decreased salt concentration of sweat
Dilute sweat preserves electrolyte in fluids.
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Factors that Improve Heat Tolerance
• Fitness Level
• Age (see FYI)
Aging delays the onset of sweating and blunts the magnitude of
sweating response
• Gender
• Body fatness
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Evaluating Heat Stress
• Prevention remains most effective way to manage heat-stress injuries
• Wet bulb-globe temperature relies on ambient temperature, relative humidity, and radiant heat.
• Heat stress index
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Heat Illness
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Heat Illness
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Prevention of Heat Illness
• Allow adequate time for acclimatization.
• Exercise during cooler parts of day.
• Limit/defer exercise if heat stress index is in high risk zone.
• Hydrate properly prior to exercise and replace fluid loss
during and after exercise.
• Wear clothing that is light in color and loose fitting.
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Exercise in the Cold
• Cold strain
• Exposure to cold produces physiological & psychological
challenges
• Body fat differences effect physiological function in cold
• Acclimatization to the Cold
Humans adapt more successfully to chronic heat than cold exposure.
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Exercise in the Cold
• Evaluating Environmental
Cold Stress
• Wind chill index
• Respiratory tract in Cold
• Cold air does not damage
respiratory passages.
• Air warms to between 80° F to
90° F as it reaches bronchi.
• Humidification of inspired cold
air produces water & heat loss
from respiratory tract.
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