Homeostasis & Response


Principles Of Homeostasis

  • The regulation and control of the internal conditions of a cell/organism.
  • Maintains the optimum conditions for enzyme action in all cell functions. 
  • Detects and responds to internal & external changes
  • Is an automatic control system which may involve nervous responses or chemical responses - It is made up of receptors, coordination center & effectors.
  • Receptors (cells) - detect changes in the internal or external environment. These are known as stimuli
  • Co-ordination Centre (Brain & Spinal cord) - areas that receive and process information from the receptors, they send out signals and coordinate responses
  • Effectors (Muscles or Glands) - bring about responses to the stimulus that has been received. these responses restore conditions in the body to the optimum levels
  • In the human body, homeostasis includes control of blood glucose concentration, body temperature & water levels
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The Nervous System

  • Nervous System - Allow us to react to our surrounding and coordinate our behavior. The response is fast, short lasting & on a very precise area.
  • Sense Organs - contain receptor cells
  • The skin detects stimuli - Temperature, Touch, Pain & Pressure
  • The tongue detects stimuli - Chemicals in the food
  • The nose detects stimuli - Chemicals in the air
  • The eyes detect stimuli - Light
  • The ears detect stimuli - Position of the head & Sound
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Structure of the Nervous System

  • Stimulus
  • Receptor
  • (Electrical Impulse)  - Electrical Impulses pass back and forth
  • Sensory Neurone
  • Relay Neurone (CNS)
  • Motor NEurone
  • Effector
  • Response
  • The CNS = Central Control System
  • Consists of the Brain & Spinal Cord
  • Is a coordination center
  • Processes information received from the receptors
  • Coordinates effectors response
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Reflex Actions

These are faster & automatic. The relay neuron passes through the CNS, these are needed for: Avoiding danger/harm & Basic body function

  • Stimulus 
  • This is detected by sensory receptors in sense organ
  • Electrical Impulse
  • Impulse sent along a seneory neuron
  • Impulse carried through relay neuron in CNS
  • Impulse sent along the motor neuron
  • Effector (muscle or gland) responds
  • In response; muscle contracts/glad secrets chemical
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  • Where two neurons meet, there is a tiny gap called a synapse.
  • Electrical impulse ross this gap using chemicals.
  • One neuron releases the chemical into the gap
  • The chemical diffuses across the synapse and makes the next neuron transmits an electrical impulse
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The Brain

  • Receives information, from sensory neurons all over your body
  • Processes this information
  • Sends out impulses along motor neurons to coordinate your response

Structure of the Brain

  • Protected inside the bones of your skull
  • Encased in protective membranes called meninges
  • Made up of billions of interconnected neurons that form different regions, which carry out a different function
  • Cerebral cortex - Memory, Intelligence, Language & Consciousness - Remeber MILC
  • Cerebellum - Coordination of muscular activity
  • Medulla - Unconcious activities  - heartbeat and breathing
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The Eye - Bright & Dim Light

Dim Light

  • Circular Muscles relax
  • Radial Muscles contract
  • Pupil Dilates
  • Amount of light Increases

Bright Light

  • Circular Muscles contract
  • Radial Muscles relax
  • Pupil Constricts
  • Amount of light decreases
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The Eye - How Do We See An Image?

  • LIght enters the eye
  • The cornea and jelly in the eyeball change the direction of the light rays onto the retina
  • This always refracts light by the same amount
  • The ciliary muscles may contract or relax
  • This changes the shape of the lens
  • This changes the amount that light is refracted
  • Light is focused on the retina, which helps you see close and distant objects clearly
  • The image formed on the retina is upside down
  • When the light-sensitive cells in the retina are stimulated, they send impulses to the brain along sensory neurons in the optic nerve
  • When the brain receives the impulses it interprets the image the right way up
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Near & Distant Vision

Accommodation = The process of changing the shape of the lens of the eye to focus on near or distance objects

Near Vision - Accommodation

  • Light is spreading out or diverging. Ciliary Muscles contract. Suspensory Ligaments loosens. The lens becomes more rounded and relatively thick. To focus light rays on the retina, they are strongly refracted

Distant Vision

  • Light is traveling in parallel rays. Ciliary Muscles relax. Suspensory Ligaments are pulled tight. The lens becomes flat and thin - less convex. To focus light rays on the retina, they are refracted slightly.
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Myopia & Hyperopia

Myopia - Short-Sightedness

  • Close objects are seen clearly but distant objects are blurry - the light is focused in front of the retina so the image is out of focus and blurry. This is caused by the lens being too curved or a long eyeball. It can be treated by a Concave Lens - This helps spread out the light rays before they reach the retina

Hyperopia - Long Sightedness

  • Distant objects are seen clearly but close objects are blurry - the light is focused behind the retina so the image is blurred. This is caused by the lens being too flat or a short eyeball. It can be treated by a Convex Lens - This helps bring the light rays together before they reach the retina

Development of new materials/technologies in recent  years has resulted in new ways to overcome myopia and hyperopia:

  • Hard & Soft Lenses, Laser Surgery & Replacement Lenses
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Type 1 & 2 Diabetes

Type 1 Diabetes

  • Your pancreatic cells do not make enough insulin - your blood glucose concentration is not controlled - has a genetic link
  • Your kidneys excrete glucose in your urine, you produce lots of urine and feel thirsty all the time.
  • Without insulin, glucose cannot get into the cells of your body, so you lack energy & feel tired
  • Can be treated by Pancreatic Transplant, Pancreatic Cells Transplant, Genetic Engineering & use stem cells from human embryos

Type 2 Diabetes

  • When your cells no longer respond to the insulin made by your pancreas. - it is linked to obesity, lack of exercise and old age
  • Can be treated by losing weight, regular exercise, eating a balanced diet
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Negative Feedback


  • The thyroid gland in your neck uses iodine to produce the hormone thyroxine
  • This controls the basal metabolic rate of your body- how quickly substances are broken down and built up, how much oxygen the tissues use and how the brain of a growing child develops
  • This is a Negative Feedback System


  • Your adrenal glands, located at the top of your kidneys secrets adrenaline
  • If you are stressed, angry, excited or frightened your body need to be ready for action. The adrenaline that is carried rapidly around the body in your blood.
  • Your heart rate and breathing rates increase
  • Stored glycogen in the liver to be converted to glucose for respiration
  • The pupils dilate
  • Blood to be divided away from your digestive system to the big muscle of limbs
  • This is NOT a Negative Feedback System
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Contraception - Non-hormonal & Hormonal


  • Condom (Barrier)
  • Diaphgram (Barrier)
  • Intrauterine Device
  • Sterilisation
  • Abstinece


  • Oral Contraceptive Pill
  • Contraceptive Patch
  • Contraceptive Injection
  • Contraceptive Implant
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