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Homeostasis and Response (The Endocrine System (Hormones vs Reflexes…
Homeostasis and Response
Homeostasis
Cells in body need right conditions in order to function properly, including right conditions for enzymes
Homeostasis regulates the internal environment and conditions inside the body to maintain a stable environment in response to changes in both internal and external conditions
All automatic control systems (eg. nervous and hormonal systems) contain receptors, coordination centres and effectors
Negative feedback
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Coordination centre receives and processes information, then coordinates a response
Effector produces a response which counteracts the change and restores the optimum level - the level decreases
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Coordination centre receives and processes information, then coordinates a response
Effectors produce a response which counteracts the change and restores the optimum level - the level increases
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Coordination centre - areas that receive and process information from the receptors, the coordinate the response of the body by sending out signals eg. brain, spinal cord, pancreas
Effectors - muscles or glands that bring about responses to the stimuli received, so restore conditions to the optimum level
The effectors will carry on producing the response for as long as they are stimulated - this could make the level too high or too low, but then negative feedback will start again
The Nervous System
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Receptors can form parts of larger, more complex organs such as light receptor cells cover the retina
Effectors - all muscles and glands - muscles contract in response to a nervous impulse and glands secrete hormones
Synapses and Reflexes
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Reflexes
Rapid, automatic responses to certain stimuli that don't involve conscious part of brain - reduce chances of being injured
eg. a bright light is shone into the eye, pupils automatically get smaller so less light is allowed into the eye
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Reflex arc
When a stimulus is detected by receptors, impulses are sent along a sensory neurone to a relay neurone in the CNS
When impulse reaches synapse between relay and sensory neurone, they trigger chemicals to be released which cause impulses to be sent along relay neurone
When impulses reach synapse between relay and motor neurone, chemicals released and impulses sent along motor neurone
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The Brain
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Parts of brain
Cerebral cortex - outer bit. Controls consciousness, memory, intelligence and language
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The Eye
Parts of the eye
Sclera - tough, supporting wall of the eye
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Iris - contains muscles that allow it to control the diameter of the pupil, so controls how much light enters the eye
Lens - focuses light onto the retina (which contains rods - low light levels no colour, and cones - higher light levels with colour) - which is covered with light receptors
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Iris reflex
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When light receptors in the eye detect very bright light, a reflex is triggered which makes the pupil smaller - the circular muscles in the iris contract and the radial muscles relax, which reduces the amount of light which can enter the eye
In dim light the radial muscles contract and the circular muscles relax, making the pupil wider
Focus of the eye
To look at near objects
Ciliary muscles contract, so suspensory ligaments relax, so lens becomes becomes more curved and thicker
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To look at far objects
Ciliary muscles relax, so suspensory ligaments contract - lens goes thin and less curved
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The Endocrine System
Hormones are chemical molecules released from glands in the bloodstream to particular cells in organs (target organs) to control aspects such as temperature and concentration of blood sugar
Produced and secreted by various glands - endocrine glands, which make up the endocrine system
Hormones, unlike reflexes, have a relatively long lasting effect
Pituitary gland
Produces many hormones that regulate body conditions - often referred to as the 'master gland,' as these hormones act on other glands, causing them to release hormones which cause certain changes
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Thyroid gland
Produces thyroxine, which regulates things like metabolism, heart rate and temperature
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Ovaries (females)
Produce oestrogen, which is involved in the menstrual cycle
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Adrenal glands
Produced adrenaline, which is used to prepare the body for fight or flight response
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Testes
Produce testosterone, which controls puberty and sperm production in males
Pee is produced in the balls, just like testosterone
Pancreas
Produces insulin, which is used to regulate blood sugar level
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Hormones vs Reflexes
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Hormones mainly affect organs, while reflexes mainly affect muscles and glands
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The Kidneys
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Selective re-absorption - useful substances like glucose, some ions and the right amount of water are then absorbed back in to the blood
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Urea
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Deamination - where any excess amino acids are converted into fats and carbohydrates, which can be stores. which occurs in the liver
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Ammonia is toxic so it is converted to urea in the liver - then transported to the kidneys, where it is filtered out of the blood and excreted from the body in urine
Ions
Ions such as sodium are taken into the body in food, then absorbed into the blood
If the ion content of the body is wrong, this could upset the balance between ions and water - too little or too much water is drawn into the cells by osmosis
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Some ions are lost in sweat - this amount isn't regulated so the right balance of ions must be maintained by the kidneys - right amount of ions is reabsorbed into the blood after filtration and the rest if removed from the body in urine
Water
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Cannot control water loss through sweat and breathing, so water content of the body is balanced by what we take in, and the amount removed by the kidneys in urine
Concentration of urine
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Brain monitors the water content of the blood, and stimulates the pituitary gland to release ADH into the body according to how much is needed
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If content is too high, less ADH released, so less water reabsorbed from kidneys, and if too low, then more released, so more water reabsorbed from kidneys
Kidney Failure
If kidneys don't work properly waste substances build up in the blood and you lose the ability to control the levels of ions and water in your blood - results in death
Dialysis
Has to be done regularly to keep concentrations of dissolved substances in blood at normal levels, and removes waste substances
The person's blood flows between a partially permeable membrane, surrounded by dialysis fluid - permeable to things like ions and waste substances, but not big molecules like proteins
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Useful dissolved ions and glucose aren't lost from the blood - only waste substances like urea, excess ions and excess water diffuse across the barrier
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Can cause blood clots or infections, and is very expensive
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Kidney Transplant
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Kidneys can also be transplanted from someone who is alive, but slight risk to person donating the kidney
Risk that the kidney will be rejected by the immune system - patient has to be treated with drugs to prevent this from happening
Cheaper than dialysis in long run and put an end to dialysis, but long list for kidney transplants and not many donors
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Controlling fertility
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Barrier methods
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Condoms - only contraception that protects against STI's, and femidoms also
Diaphragm - shallow plastic cup that fits over cervix to form a barrier - has to be used with a spermicide (substance which kills / disables sperm - only 70-80% effective)
Other methods
Sterilisation - cutting or tying the fallopian tubes, or the sperm duct - permanent, however small chance tubes can rejoin
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Adrenaline and Thyroxine
Adrenaline
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Released in response to scary or stressful situations - brain detects fear or stress and sends nervous impulses to adrenal gland, causing them to secrete adrenaline
Gets body ready for fight or flight by allowing the body to increase supply of oxygen and glucose to cells in the brain and muscles - eg. increases heart rate
Thyroxine
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Important in regulating basal metabolic rate - speed at which chemical reactions occur while body is at rest
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Negative feedback keeps amount of thyroxine in blood at right level - when high levels of thyroxine, TSH no longer released, and when low levels thyroxine, TSH released
Plant Hormones
Auxin
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Produced in tips and moves backwards to stimulate the cell elongation process which occurs in the cells just behind the tips
If the tip is removed, no auxin produced and shoot may stop growing
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Roots grow towards gravity, shoots away
When a shoot is growing sideways, gravity produced an unequal distribution of auxin in the tip - more auxin on the lower side
Causes the lower side to grow faster, bending the shoot upwards
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In roots extra auxin inhibits growth, so the cells on top elongate faster, and the root bends downwards
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