Homeostasis
Chapter 9
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/homeo/homeosts.shtml
Homeostasis
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Physiological state of the body
Internal physical and chemical conditions are maintained
within a tolerable range
Includes
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Temperature, hormone levels, pH, pressure, concentrations of
glucose and other solutes in the blood
Internal Environment
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Extracellular fluid
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Interstitial fluid – fills the spaces between cells and tissues (e.g. plasma)
Consists of water, sugars, salts, FA, AA, coenzymes, hormones,
neurotransmitters, waster products
Regulates flow of chemicals and allows cells to function properly
Lymphatic system transports fluid throughout the body
Internal Environments
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Changes in Extracellular Fluid has negative effects on cellular
function
Body uses organ systems to regulate internal conditions
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Nervous system
Endocrine system
Muscular system
Integumentary system
Excretory system
Reproductive system
Nervous System
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Brain, spinal cord, peripheral nerves, sensory organs
Receives sensory data from the environment
Informs body of external conditions
Transmits signals throughout the body
Endocrine System
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Pituitary, thyroid, pancreas, adrenal (glands)
Regulates levels of hormones and other chemicals
Excretory System
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Kidneys, bladder, urethra, ureters
Rids the body of waste
Maintains clean internal environment
Integumentary System
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Skin, sweat glands, hair, nails
Maintains a constant body temperature
Immune System
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White blood cells
Protects/fights infection
Digestive System
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Liver
Breaks down amino acids
Detoxifies harmful chemicals (alcohol)
Manufactures important proteins
Homeostatic Mechanisms
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Respond to internal and external conditions
Feedback systems – Positive/Negative
Help bring the body back into balance
Breathing rate, heart rate, internal temperature, blood
glucose levels
Negative Feedback
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Reduces the output or activity of an organ or system back to its
normal range
Include 3 elements
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Sensor
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Integrator - hypothalamus
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tissues or organs - detects change
control centre – compares conditions from environment with to optimal
conditions in the body
Set points – ranges of values which need to be maintained
Effector
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returns measured condition back to set point – response
Antagnositc effectors – produce opposite effect of change detected
Hypothalamus
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Body’s thermostat
Maintains body temperature
Optimal body temperature – 35⁰ - 37.8⁰
Body temp falls → vasoconstriction in skin/shivering→ reduced
blood flow→ less thermal energy lost to environment → body
temp increases
Body temp rises → blood vessels dilate/induce
vasodilation/sweating → increase blood flow→ increase thermal
energy loss to environment→ body temp decreases
Signals from hypothalamus make us aware of our own
temperature
Positive Feedback Mechanisms
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Increases change in environmental condition
Does not result in homeostasis
Cause system to become unstable
“fight or flight” response
reproduction
fever
Positive feedback mechanisms operate within negative
feedback mechanisms
Allows body to be brought back into balance
Thermoregulation
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Internal temperature
regulation
Negative feedback mechanism
Thermoreceptors – compare
external temp with internal
set point
Trigger responses (2)
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Rate of exothermic reactions in
body (metabolism)
Rate of thermal energy
exchange through surface of
body
Mechanisms of Thermal Energy Exchange
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Occurs at the surface where body comes into contact with the
external environment
Exchange of thermal energy occurs through 1 of 4 mechanisms
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Conduction
Convection
Radiation
Evaporation
All of these mechanisms act
simultaneously
Conduction
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Flow of thermal energy between molecules that are in
direct contact
Convection
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Transfer of thermal energy within a fluid (liquid or gas)
Radiation
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Thermal energy is transferred electromagnetically
Evaporation
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Absorbs thermal energy from skin via water/sweat
Homeotherms
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Animals that maintain a stable internal temperature
regardless of external conditions
Includes
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Poikilotherms
Endotherms
Ectotherms
Poikilotherms
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Fish, amphibians, reptiles, and most invertebrates
Body temperature varies with and often matches the
temperature of the external environment
Endotherms
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Warm blooded animals (mammals, birds)
Homeotherms that use internal physiological
mechanisms (metabolism) to generate thermal
energy and maintain body temp
Remain fully active over a wide range of
temperatures
Need a constant supply of energy
Ectotherms
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Cold blooded animals (reptiles, amphibians, fish)
Homeotherms that use external sources of energy to absorb
thermal energy and regulate body temperature
Temperature fluctuates with environmental temperature
Inactive when temp are too low
Undergo thermal acclimatization
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Gradual adjustment to seasonal temp
Torphor, Hibernation, Estivation
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Adaptations to survive extreme
climates by conserving energy
Torphor
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Hibernation
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Sleeplike state
Metabolic rate and body temperature
drop in response to daily temp
(nocturnal animals, hummingbird)
State of inactivity over an extended
period of time
Estivation
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Seasonal torphor – environment is hot
and water is scarce
Water Balance
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Extracellular fluid needs to maintain a constant volume
(~15L) of water and balance of solute within the body
Mechanism
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Osmosis
Osmosis
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Water molecules move from a high concentration to a region of
lower concentration across a selectively permeable membrane
Osmotic pressure
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Results from a difference in water concentration gradient between the
two sides of the selectively permeable membrane
Hyperosmotic
Hypoosmotic
Isoosmotic
Hyperosmotic
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Solution with higher concentration of solute molecules
Water tends to move to this side
Hypoosmotic
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Solution with lower concentration of solute molecules
Water tends to move from this solution
Isoosmotic
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Solution with the same solute and water concentrations
Osmoregulation
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Process of actively regulating the osmotic pressure of
bodily fluids
Extracellular fluid = intracellular fluid (isoosmotic)
[solute] remains the same
[water] remains the same